5. Ring-Opening Polymerizations

Various Lewis and protonic acids are capable of initiating cationic polymerization of epoxies. Among them, the following metal salts are effective in polymerizations of ethylene and propylene oxides [3, 4]: ZnCl₂, AlCl₃, SbCl₅, BF₃, BCl₃, BeCl₂, FeCl₃, SnCl₄, and TiCl₄. Often these polymerizations can be carried out in bulk without any solvent, particularly in the laboratory. The mechanism of these reactions can be complex, however, depending upon the particular Lewis acid used. In fact, not all of these polymerizations can even be treated in general terms as cationic. For instance, ferric chloride initiated polymerizations of epoxides initially proceed by a mechanism that has all the superficial features of cationic polymerization. After the initial stages, however, the polymerizations proceed by a coordination mechanism. This is discussed further in this section.

Stannic chloride yields only low molecular weight poly(ethylene oxide) from ethylene oxide (molecular weight below 5,000) when the reaction is carried out in ethylene chloride at room temperature. Some dioxane and dioxolane also form in the process. Following reaction scheme was proposed [2–6]:

Initiation

Propagation

Termination

The initiation step depends upon formation of oxonium ions. Because a carbon cation intermediate is indicated, it was suggested [4] that the propagation probably occurs by ether exchange that results from a nucleophilic attack by the monomer on the oxonium ion.

Boron trifluoride forms complexes with oxygen-containing compounds, like water, alcohols, and ethers. When it initiates the polymerization of epoxides, it can associate simultaneously with several different moieties. These are the monomeric cyclic ethers, as well as the open-chain polymeric ether groups, and the hydroxy groups on the chain ends. In addition it can also associate with the hydroxy groups of water. The following illustration shows the type of equilibrium that can take place [2]:

The alcohols and open-chain ethers have comparable basicities toward the coordinated acid ROH:BF₃. Ethylene oxide, on the other hand, is much less basic than the open-chain ethers [6]. In the initiation step, therefore, the monomer reacts with the coordinated acid [1]:

During propagation three different reactions can occur [2]:

This reaction is also accompanied by formation of dioxane. It is actually a step of depolymerization:

The ring-opening reaction, a nucleophilic substitution, usually takes place with an inversion of configuration at the carbon atom that undergoes the nucleophilic attack [8, 9, 11]. This can be illustrated as follows:

Alkyl substituents on the ethylene oxide ring enhance the process of cationic polymerization. For instance, ethylene oxide yields only low molecular weight oils with strong Lewis acids. Tetramethylethylene oxide, on the other hand, is converted readily by BF₃ into high molecular weight polymers that are insoluble in common solvents [10].

When proton donors initiate the polymerizations of epoxides, only low molecular weight products result. The reaction is quite straightforward. Oxonium ions form during the initiation step as follow:

Propagation is the result of a ring-opening attack by a monomer:

Chain-growth can terminate by a reaction with water:

In cationic polymerizations of propylene oxide the ring-opening step involves a direct attack on the oxonium ion at the carbon that bears a more labile bond to the oxygen:

Cationic ring-opening polymerization of oxiranes can also be carried out photochemically (photochemical reactions are discussed in Chap. 10). Yagci and coworkers reported polymerizations of cyclohexene oxide with the aid of highly conjugated thiophene derivatives [12]. The reaction is illustrated as follows:

The cationic polymerization was initiated at room temperature upon irradiation with light in the visible region in CH₂Cl₂ solutions in the presence of diphenyliodonium hexafluorophosphate and the thiophene derivative. According to the suggested mechanism, Ayodgan et al. [12] discuss the formation of exciplex (see Chap. 10) by the absorption of light. Subsequent electron transfer from excited Photosensitizer to iodonium ion yields radical cations of the thiophene derivatives. The resulting strong Bronsted acid derived from this process catalyzes the cationic polymerization.

Anionic polymerizations of ethylene oxide were originally observed as early as 1894 [13]. A step-growth mechanism for these polymerizations was proposed later [14]. This mechanism is now well established [14]. The conversion increases linearly with time and the molecular weight also increases with conversion. Reactions with bases like sodium or potassium hydroxides or alkoxides yield only low molecular weight polymers. The initiation, an S_N2 displacement, results in formation of an alkoxide ion:

Subsequent propagation may occur by nucleophilic displacement involving a new alkoxide ion formation in each addition:

Termination takes place by a transfer to a hydroxy group of another molecule. It can also take place to a terminal hydroxyl group of a formed polymer, which starts new chain-growth from the second unit:

The chain ends remain active and higher molecular weights can be obtained by further additions of the monomer.

The reaction is often more complex than is shown above, because many such polymerizations, catalyzed by alkali hydroxides or alkoxides, are carried out in the presence of alcohols. This is done to achieve a homogeneous system. Such conditions, however, lead to exchange reactions:

Of course, the exchanges, as shown above, affect the kinetics of the process. The extent of these reactions is subject to the acid strength of the alcohols present, including the terminal hydroxyl groups of formed polymers. If their acidities are approximately equal, the exchange reactions take place throughout the whole course of the polymerization [2].

One of the reasons for the relatively low molecular weights of the products is the low reactivity of the epoxide ring toward anionic propagation. Another reason is the tendency to chain transfer to monomers, particularly in polymerizations of substituted ring structures, like, for instance, in propylene oxide:

The newly formed species rearranges rapidly:

Such transfer reactions are E-2 type eliminations. This was shown on tetramethylethylene oxide that undergoes the reaction when treated with catalytic amounts of potassium t-butoxide [5]:

In propylene oxide polymerization, therefore, the E-2 type elimination reaction is in competition with propagation:

There are both allyl and propenyl ether end groups in the products, according to the infra-red spectra [5]. This suggests that in addition to the E-2 type elimination, an intramolecular transfer takes place by allylic hydrogen.

Raynaud et al. reported carrying out ring-opening polymerizations of ethylene oxide initiated by heterocyclic carbene [15]. The reaction yields high molecular weight polymers. It is illustrated as follows: where R can be a propyl or a tertiary butyl group.

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler–Natta type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic μ-oxoalkoxides, such as [(RO)₂AlO₂]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 5.1).

From propylene oxide these catalysts yield crystalline, isotactic polymers [16]. Living polymerizations with metalloporphyrin derivatives are difficult to terminate and are, therefore, called by some immortal [18]. Catalysts like, (C₆H₅)₃–SbBr₂–(C₂H₅)₃N in combination with Lewis acids also yield crystalline poly(propylene oxide). Others, like pentavalent organoantimony halides are useful in polymerizations of ethylene oxide [19].

Polymerizations of epoxides by coordination mechanism result in high molecular weight products. The details of the reaction mechanism have not been fully resolved yet, but it is commonly believed to involve coordination of the monomers to electrophilic centers of the catalyst. This is followed by activation for an attack by the anion [2]. Such mechanism [1] can be illustrated by the following reactions: where Me means metal. where, Me represents the metal catalyst.

Ferric chloride polymerizes propylene oxide, a monomer with an asymmetric carbon atom, with retention of asymmetry in the backbone [3]. The products of polymerization contain either optically active polymers or racemic mixtures, depending upon the monomers used. When only a pure optical isomer monomer is used the products are crystalline polymers composed of the same optically active units:

The polymers are fairly high in molecular weight, approximately 100 times greater than the products from KOH initiations. Propylene oxide initially reacts with ferric chloride to form an oligomer, a chloropolyalkoxide. The material contains approximately four or five propylene oxide repeat units. This forms two different halogen sites. It can be illustrated as follows:

The above compound may be the catalyst or one closely related to it for forming stereoregular polymers. Water appears to play a role, because the proportion of crystallinity increases with addition of water. When water is added in a molar ratio of 1.8:1.0 of water to iron, the proportion of crystalline to amorphous fraction increases from 0.13 to 0.86. Price and Osgen [17] suggested that the polymerization proceeds in a step-growth mechanism as follows:

The solid surface of the catalyst causes the transition state to be more compressed. Steric repulsions between the incoming monomer and the ultimate unit are minimized if the incoming monomer molecule is forced to be trans to the methyl group of the previous unit. Such a conformational approach also results in minimum repulsion between the incoming monomer and the bulky growing polymer chain [18–20]. Also, ferric alkoxides are associated in nonpolar solvents. A dimer may have the following structure:

By comparison, intramolecular chelation can be expected to reduce the degree of association of the catalyst. Addition of water results in increased association after hydrolysis of the ferric alkoxide. This may explain the effect of promoting stereoregularity by addition of water [20]. The ferric alkoxide catalyst can also be made highly stereospecific by partial hydrolysis and still remain soluble in ether, the polymerization medium [21]. This led to a suggestion [22] that the catalyst may contain active Fe–O–Fe bonds. Such bonds would be formed from condensation of partially hydrolyzed alkoxide derivative. The monomer insertion between the iron–oxygen bonds can be illustrated as follows:

The forces of interaction between the iron atoms and the various oxygen atoms as shown above assure a cis opening of the epoxide ring. The mechanism of the reaction of the ferric alkoxide is an S_N2 type. There is, therefore, increased restriction on the conformation of the monomer unit as it approaches the reaction center [22].

Many other coordinated anionic catalysts that are metal alkoxides or metal alkyls are also much more reactive in the presence of water or alcohols. The function of these co reactants is to modify the catalyst itself. For instance, diethylzinc combined with water in a ratio of 1:1 yields a very reactive species. The exact nature of the catalyst is still not fully established, however, the reaction product is pictured as follows [23, 24]:

Several reaction mechanisms were proposed. One suggested pathway for propylene oxide polymerization pictures an initial coordination of the monomer with a cationically active center [25]:

The propagation is preceded by an intramolecular rearrangement:

Another mechanism is derived from the structure of the diethylzinc–water catalyst [25] that is visualized as a dimer:

A similar structure pictured can be shown for diethylzinc–alcohol. The asymmetric induction is suggested to takes place during coordination of the monomer to the catalyst site. This is a result of indirect regulation that results form interactions between the monomer and the penultimate unit [25].

In yet another mechanism the initial coordination and subsequent propagation steps are pictured as follows [26]

While the detailed structures of most catalyst sites are still unknown, it was established that stereoselectivity does not come from the chirality of the growing chain end. Rather it is built into the catalyst site itself [27, 28]. Normal preparations of the catalysts give equal numbers of (R) and (S) chiral catalyst sites. These coordinate selectively with (R) and (S) monomers respectively in the process of catalytic-site control [23].

Cationic polymerizations of oxiranes are much less isospecific and regiospecific than are anionic polymerizations. In anionic and coordinated anionic polymerizations, only chiral epoxides, like propylene oxide, yield stereoregular polymers. Both pure enantiomers yield isotactic polymers when the reaction proceeds in a regiospecific manner with the bond cleavage taking place at the primary carbon.

In all polymerizations of oxiranes by cationic, anionic, and coordinated anionic mechanisms, the ring-opening is generally accompanied by an inversion of the configuration at the carbon where the cleavage takes place. A linear transition state mechanism involving dissociated nucleophilic species has been proposed [15]. Yet, there are some known instances of ring-opening reactions of epoxies that are stereochemically retentive. For instance, ring opening of 2,3-epoxybutane with AlCl₃ results in formation of 3-chloro-2-butanol, where the cis and trans epoxides are converted to the erythro and threo-chlorohydrins. Inoue and coworkers [19] found, however, that polymerizations of cis and trans 2,3-epoxybutanes take place with inversion of configuration when aluminum 5,10,15,20-tetraphenylporphine and zinc 5,10,15,20-tetraphenyl-21-methylporphine catalysts are used. To explain the inversion, Inoue and coworkers proposed a linear transition state mechanism that involves a simultaneous participation of two porphyrin molecules [19]. One porphyrin molecule accommodates a coordinative activation of the epoxide and the other one serves as a nucleophile to attack the coordinated epoxide from the back side.

Potassium hydroxide or alkoxide polymerizes racemic propylene oxide with better than 95% regioselectivity of cleavage at the bond between oxygen and the carbon substituted by two hydrogens. The product, however, is atactic. Both (R) and (S) propylene oxides react at the same rate. This shows that the initiator is unable to distinguish between the two enantiomers of propylene oxide. When t-butyl ethylene oxide is polymerized by KOH it yields a crystalline product. This product is different in its melting point, X-ray diffraction pattern, and solution-NMR spectra from the typical isotactic polymers. It contains alternating isotactic and syndiotactic sequences [31]. It was suggested [34] that this may be a result of the configuration of the incoming monomer being opposite to that of the penultimate unit. Chelation of the paired cation (K⊕) with the last and the next to the last oxygen is visualized. Geometry of such a chelate is dictated by the requirement that the penultimate t-butyl group be in an equatorial conformation. This makes it reasonable to postulate that the necessary preference for the incoming monomer is to be opposite to that of the penultimate unit [31]:

When phenyl glycidyl ethers are polymerized under the same conditions, the steric arrangement is all isotactic rather than isotactic–syndiotactic [31]. Price explained that on the basis of the oxygen in C₆H₆–O–CH₂ seeking to coordinate potassium ions in the transition state [31]. In the case of t-butylethylene oxide, on the other hand, the tertiary butyl group tends to be as far as possible away from the potassium ion [34]. This is supported by the observation that p-methoxy and p-methyl groups on phenyl glycidyl ether increase the crystalline portion of the polymer, while the p-chloro substituent decreases it [31].

Most stereoselective coordination catalysts polymerize propylene oxide to yield polymers that contain high ratios of isotactic to syndiotactic sequences. Large portions of amorphous materials, however, are also present in the same materials. These amorphous portions contain head to head units that are imperfections in the structures [29, 30]. For every head to head placement, one (R) monomer is converted to an (S) unit in the polymer [23]. This shows that at the coordination sites abnormal ring openings occur at the secondary carbon with an inversion of the configuration and results in head to head placements [23, 31]. Also, erythro and threo isomers units are present. The isotactic portion consists almost exclusively of the erythro isomer while other amorphous fraction contains 40–45% erythro and 55–60% threo [31]

All the above information is indirect evidence that a typical catalyst, such as (C₂H₅)₂Zn–H₂O contains isotactic and amorphous sites. The isotactic sites are very selective and coordinate either with (R) or with (S) monomers. The amorphous sites, on the other hand, coordinate equally well with both (R) and (S) monomers. In addition, there is little preference for attack on either the primary or the secondary carbons during the ring-opening reactions [23].

According to a Tsuruta mechanism [36] the first step in propylene oxide polymerization, with catalysts like zinc alcoholates, is the coordination of the ether oxygen onto a zinc atom. The second step is a nucleophilic attack at the oxirane ring by the alkoxy ion. Almost all the bond cleavage takes place at the CH₂–O bond. This results in retention of the steric configuration of the carbon atom at the C–H group. The next oxirane molecule repeats the process, coordinates with the same zinc atom and then undergoes the ring-opening reaction to form a dimer. Repetition of this process many times yields a high molecular weight polymer [36]:

The catalyst can also be ZnR₂–CH₃–OH.

Special catalyst complexes, like [Zn(OCH₃)₂⋅(C₂H₅OCH₃)₆], form through carefully control of reaction conditions by adding 16 moles of methyl alcohol to 14 moles of diethylzinc in heptane under an argon atmosphere. X-ray analysis shows that two different structures [36]. One of them is a centrosymmetric complex of two enantiomorphic distorted cubes that share a corner Zn atom. The two would be equivalent if they were not distorted. Another structure, also centrosymmetric, consists of two enantiomorphic distorted structures that resemble “chairs without legs,” where the surfaces share a common seat. Both types of complexes are active initiators for polymerization of propylene oxide. Each has two enantiomorphic sites for polymerization. Based on that knowledge, NMR spectra and GPC curves, Tsuruta suggested the following mechanism of a monomer coordinating with the catalyst [36] (see Fig. 5.2). The bonds at the central zinc atom are loosened and coordination takes place with methyl-oxirane molecule at the central atom. Cleavage at the O–CH₂ bond of the oxirane takes place by a concerted mechanism. If the bond loosening takes place at the d cube and the nucleophilic attack takes place at one of the methoxy groups on that cube then chirality around the central zinc will favor L monomer over the D monomer. This is the origin of the l* catalyst site. If the bond loosening takes place in the l cube the catalyst site will have d* chirality. Because the probability of bond loosening in the d cube is exactly the same as in l cube, an equal number of l* and d* sites should be expected to form. These two cubes become a source of d* and l* chiral nature [35].

Theoretically, any Lewis acid can catalyze oxetane polymerizations. However, these acids differ considerably in their effectiveness. Boron trifluoride and its etherates are the most widely reported catalysts. Moisture must be excluded, as it tends to be detrimental to the reaction [35].

Chlorinated hydrocarbon solvents, like methylene chloride, chloroform and carbon tetrachloride, are common choices. The reactions are usually conducted at low temperatures and there are indications that the lower the reaction temperature the higher the molecular weight of the product

It was reported that when oxetane polymerizations are carried out with boron trifluoride catalyst in methylene chloride at temperatures between 0 and −27.8°C a cocatalyst is not required [32]. The product, however, is a mixture of linear polymer and a small amount of a cyclic tetramer. This is in agreement with an earlier observation that the polymerizations of oxetane are complicated by formations of small amounts of cyclic tetramers [33]. Other catalysts, like protonic acids, capable of generating oxonium ions, will also polymerize oxetane. Such acids are sulfuric, trifluoroacetic, and fluorosulfuric. The initiation reaction can be illustrated as follows:

The adduct reacts with cyclic ether:

When complexes of Lewis acids with active hydrogen compounds initiate the polymerizations, such complexes acts as protonic acids. On the other hand, etherates initiate by forming oxonium ions and may involve alkyl exchange reactions with the monomer:

The propagation takes place via tertiary oxonium ions [37, 38]:

A cyclic oligomer forms in some instances in addition to the polymer [40]. For instance, in polymerizations with BF₃ in methylene chloride at low temperatures a cyclic tetramer forms, probably by a backbiting process [40].

The oxonium exchange reactions may occur with the polymer ether linkages as well as with cyclic tetramers that form, as shown above. The concentrations of the oxonium ions of the ether group on the polymer and on the cyclic tetramers, however, are very small [42]. Polymerizations with PF₅, on the other hand, or with (C₂H₅)₃OPF₆ either in bulk or in methylene chloride solutions, yield no significant amounts of cyclic oligomers [43].

The activation energy of polymerizations of oxetane monomers is higher that that of tetrahydrofuran (see next section). This indicates that the orientation of the cyclic oxonium ion and the monomer is looser in the S_N2 transition state [42]:

In principle, stereospecificity should be possible in substitute polyoxycyclobutanes, such as 2-methyl, 3-methyl, and others. The 2-methyl derivative however, yields amorphous polymers. This is due to the monomer’s unsymmetrical structure [33] NMR studies of the microstructure of polymers from 3,3-dimethyloxetane [44] and 2-methyloxetane [43] led to no conclusions about the manner of ring opening. The predominant head to tail structures may result from attacks at either the methylene or the methine carbons next to the oxonium ions of the propagating species.

Oxetane compounds also polymerize with the aid of aluminum trialkyl–water acetylacetone catalysts [45, 46]. The reactions can take place at 65°C in heptane and yield very high molecular weight polymers. These polymerizations, however, are ten times slower that similar ones carrier out with propylene oxide, using the same catalyst. The reaction conditions and the high molecular weights of the products led to assumption that coordinated mechanisms of polymerizations take place [46].

Also, it was reported [218] that quaternary onium salts coupled wit bulky organoaluminum diphenolates initiate controlled (living) coordinate anion polymerizations of oxetane to give narrow molecular weight distribution polyethers The catalyst system consists of onium salts, such as quaternal ammonium phosphonium halides that are combined with sterically hindered methylaluminum diphenolates [47]

Crivello reported frontal photopolymerization of oxetane [48]. He proposed a mechanism for the polymerization that is shown in the following scheme:

Crivello reported that only diaryliodonium salts were used as cationic photoinitiators in this study. Photo activation was carried out by UV irradiation prior to thermal initiation. Initiation was carried out using an electrically heated wire immersed in the monomer. The velocity of the resulting propagating front is quite high with the temperature of the front reaching 110°C.

The initiations result from coordination of the cation catalysts with the oxygen of the monomers to form oxonium ions [48, 49]. This weakens the oxygen–carbon bonds and leads to ring openings after reactions with a second molecule of the monomer. New oxonium ions are generated in the process:

Some active oxonium salts are [48–50]: [(C₂H₅O)₃O]⁺ BF4⁻, [(C₂H₅O)₃O]⁺ SbCl₆⁻, [(C₂H₅O)₃O]⁺ FeCl₄⁻, and [(C₂H₅O)₃O] ⁺AlCl₄⁻

Examples of carbon cations that can initiate polymerizations of tetrahydrofuran, as well as some other cyclic ethers are:

The initiation mechanisms, however, by many carbon cations as, for instance, by triphenylmethyl cations, are not straightforward. Initially, hydride ions are abstracted from the monomers to form triphenylmethanes [51–53]. Simultaneously, acids are released from the counterions. The acids become stabilized by complexing with monomers. After that, the complexes react slowly with additional monomers to form the propagating oxonium ions. This makes the acids the real initiators::

Other initiators for tetrahydrofuran polymerizations also include Lewis acids in combinations with “promoters.” These are complexes of Lewis acids, like BF₃, SnCL1, or C₂H₅AlCl₂ with epirane compounds like epichlorohydrin [42]. The small ring compounds are mote reactive toward many Lewis acids, or protonic acids, then tetrahydrofuran and act as promoters of the initiation reactions. The initiations in the presence of small quantities of oxirane compounds, for instance, can be illustrated as follows:

Strong Bronsted acids form when diaryliodonium salts. like BF₄⁻, AsF₆⁻, PF₆⁻ and SbF₆⁻ are reduced with compounds like ascorbic acid in the presence of copper salts. such acids also initiate the polymerizations of tetrahydrofuran, cyclohexene, and s-trioxane [54].

The propagation process is a succession of nucleophilic attacks by fn electrons on the oxygens of the monomers upon the α-carbons of the heteroatoms of the ultimate polymerizing species [1]:

The products of these reactions are linear. Actually, this is common to polymerizations of many heterocyclics. The propagation reactions proceed by stepwise additions of monomer by S_N2 mechanism to the growing ends of the propagating chains. The NMR spectra of the growing chains only shows a presence of the oxonium ions [55, 56]:

The oxonium ions could, in principle, be in equilibrium with minute quantities of carbon cations, –CH₂⁺ that are more active. All evidence, to date, however, shows that in tetrahydrofuran polymerizations the presence of carbon cations is negligible in the propagation process [57]. Also, the rate constant for propagation of free macroions with the counterions is equal, within experimental error, to the rate constant for macroions–counterion pairs. This does not appear to depend upon the stricture of the anion studied. The above information, however, was obtained on large anions. With smaller anions, differences in the rates of propagation of macrocations and those of macroion–counterion pairs has not be ruled out.

An S_N2 attack requires that the reaction occur at the oxygen carbon bond. In such an attack steric requirements are less restricted than they are in an anionic polymerization. In addition, positive and negative charges in the macroion-pairs that contain the oxonium ions are dispersed and the anions are large. This means that the electrostatic interactions are less important in cationic polymerizations of this type than they are in anionic ones.

When the polymerization of tetrahydrofuran is carried out with the aid of CF₃SO₃H, both covalent and ionic species are present They can be detected during propagation by means of NMR spectroscopy. Both species exist in a mobile equilibrium. Solvent polarity, apparently, influences the position of such equilibria. In nitromethane, 95% of the growing chains are macroions. In carbon tetrachloride 95% of them are macroesters. In methylene chloride both species are present in the reaction mixture, approximately in equal amounts [58–62]. The propagation rate of macroions, however, is 10² times faster than that of the macroesters. As a result; chain growth even in carbon tetrachloride is still by way of the ions. The macroesters, therefore, can be considered as dormant species [59], or, as some suggest, even cases of temporary termination [59]. The much higher reactivity of the macroions is attributed to the contribution of the partially released strain in ionic species [49]. Macroions and macroesters can be illustrated as follows:

The termination reactions in tetrahydrofuran polymerizations can depend upon the choice of the counterion, particularly if the reaction is conducted at room temperature [60]. In many reactions, the chains continue to grow without any considerable termination or transfer [63, 64]. Some refer to this process as “living” polymerization. thus in polymerizations of tetrahydmfuran [65] with PF₆⁻ or SbF₆⁻ counterions the molecular weights of the products can be calculated directly from the ratios of the initiators to the monomers. The molecular weight distributions of the polymers from such polymerization reactions with PF₆⁻ and, SbF₆⁻ however, start out as narrow, but then broaden. This is believed to be due to transfer reactions with ether oxygen. It is supported by evidence that with SbF₆⁻ initiation, both termination and transfer reactions take place [65]. In addition, polymerizations of tetrahydrofuran, like those of the epoxides, can be accompanied by formations of some macrocyclic oligomers. This is often the case [66, 67] when strong acids are used as initiators. The proposed mechanism involves backbiting and chain coupling and results in linear polymers with hydroxyl groups and oxonium ions at opposite chain ends as well as some macrocycles.

The absence of linear oligomers is due to rapid reactions of the hydroxyl and oxonium ion end groups. This mechanism is quite general [67, 68] for ring-opening polymerizations of cyclic ethers initiated with string protonic acids. Substituted tetrahydrofurans generally resist polymerizations.

Trioxane is unique among the cyclic acetals because it is used commercially to form polyoxymethylene, a polymer that is very much like the one obtained by cationic polymerization of formaldehyde. Some questions still exist about the exact mechanism of initiation in trioxane polymerizations. It is uncertain, for instance, whether a cocatalyst is required with strong Lewis acids like BF₃ or TiCl₄.

The cationic polymerization of trioxane can be initiated by protonic acids, complexes of organic acids with inorganic salts, and compounds that form cations [70]. These initiators differ from each other in activity and in the influence on terminations and on side reactions. Trioxane can also be polymerized by high-energy radiation [70]. In addition, polymerizations of trioxane can be carried out in the solid phase, in the melt; in the gas phase, in suspension, and in solution. Some of these procedures lead to different products, however, because variations in polymerization conditions can cause different side reactions.

Polymerizations in the melt above 62°C are very rapid. They come within a few minutes to completion at 70°C when catalyzed by ten moles of boron trifluoride. This procedure, however is only useful for preparation of small quantities of the polymer, because the exothermic heat of the reaction is hard to control.

Typical cationic polymerizations of trioxane are characterized by an induction period. During that period only oligomers and monomeric formaldehyde form. This formaldehyde, apparently, results from splitting the carbon cations that form in the primary steps of polymerization. The reaction starts after a temperature dependent equilibrium concentration of formaldehyde is reached [70].

Several reaction mechanisms were proposed. One of them is based on the concept that Lewis acids, like BF₃ coordinate directly with an oxygen of an acetal. This results in ring opening that is induced to form a resonance stabilized zwiter ion [71]:

Resonance stabilizations of the adjacent oxonium ions lead to formations of carbon cations that are believed to be the propagating species. Propagations consist of repetitions of the sequences of addition of the carbon cations to the monomer molecules and are followed by ring opening. The above mechanism has to be questioned, however, because rigorously dried trioxane solutions in cyclohexane fail to polymerize with BF₃⋅O(C₄H₉)₃ catalyst [72]. The same is true of molten trioxane [73]. It appears, therefore, that BF₃-trioxane complexes don’t form as suggested and do not result in initiations of the polymerizations. Additions of small quantities of water, however, do result in initiations of the polymerizations.

Another mechanism, is based on a concept that two molecules of BF₃ are involved in the initiation process [69]. This also appears improbable since without water BF₃ fails to initiate the reaction. the following mechanism, based on water as the cocatalyst was developed [73]:

Chain growth in the reaction is accompanied by formations of tetraoxane and 1,3-dioxalane because of backbiting [71, 74].

Complex molybdenyl acetylacetones also act as catalysts for trioxane polymerization. The mechanism that is visualized involves formation of a coordinated intermediate [75]:

The termination mechanism and the catalyst requirement have not yet been fully explored.

Some transfer to water takes place during the reaction. As a result the polymer contains at least one terminal hydroxyl group [76]. Besides water, methyl alcohol and low molecular weight ethers also act as transfer agents [77].

The new cation can initiate chain growth:

Polymerization of this cyclic monomer yields polymers that consists of strictly alternating oxymethylene and oxyethylene units [76]. the polymerization reaction can be induced by acidic catalysts, like sulfuric acid, boron trifluoride, p-toluenesulfonic, acid and phosphorous pentafluoride [76]:

The polymers of molecular weight 10,000 or higher are tough solids that can he cold drawn. The following mechanism was proposed for the polymerizations that are initiated by reaction products of acetic anhydride with perchloric acid [78]:

Initiation

Propagation

Termination

Acetate groups are present at both end of the polymer molecules as shown above [78]. This was confirmed by analytical evidence. The initiation of dioxolane polymerization is pictured differently [79, 80]:

Chain cleavage can occur as a result of BF₃ complexation with an oxygen in a chain [80]:

There is some disagreement about the nature of the end groups. and there are some speculations that the polymers might possess large cyclic structures. Nevertheless, polymerizations initiated with benzoyllium hexafluoroantimonate (C₆H₅CO⁺SbF₆⁻) and conducted at −15°C in nitromethane or methylene chloride result in mostly linear polymers. The terminal end groups come from terminating agents that are deliberately added [81]. These polymerizations proceed without any appreciable amounts of transfer reactions, affecting the DP.

Polymerization of six membered cyclic formals has, apparently not been explored [1]. Polymerization of 1,3-dioxopane can be initiated by camphor sulfonic acid [82, 83]:

1,3-trioxopane, a product of condensation of trioxane with ethylene oxide, can be polymerized by cationic mechanism both in solution and in bulk [84]:

Polymerizations, carried out with boron trifluoride catalyst in dichloroethane solvent result in several reactions that occur simultaneously [84]:

At low temperatures the amount of dioxolane that forms in the above reaction decreases considerably and can become zero.

Ring-opening polymerizations of trioxocane result in the following polymer [84, 85]:

So far, the nature of the end groups has not been established. Nor has it been shown that a macrocyclic structure does not form.

1,3,6,9-tetraoxacycloundecane (triethylene glycol formal) can be polymerized can be polymerized by several cationic initiators in solution or in bulk at varying temperatures from −30 to +150°C [84]:

Cationic polymerizations of lactones has been carried out with the help of alkylating agents, acylating agents, Lewis acids, and protonic acids. Various reaction schemes were proposed to explain the cationic mechanism. They tend to resemble the schemes suggested for the polymerization of cyclic ethers [86, 87]. The initiation step involves an equilibrium that is followed by a ring-opening reaction:

The propagation consists of many repetitions of the above step:

The polymerization of propiolactone in methylene chloride with an antimony pentachloride-dietherate catalyst was investigated [88]. The results show that the concentration of the active centers is dependent upon catalyst concentration and upon the initial concentration of the monomer. They also support the concept that opening of the lactone rings includes initial formation of an oxonium ions [88]:

Because the carbonyl oxygen is the most basic of the oxygens in the lactone molecule, a reverse reaction is

Conductivity measurements during polymerizations of β-propiolactone with antimony pentafluoride-etherate or p-toluenesulfonic acid show [89] that ion triplets form during the reaction. These are:

The triplets appear to be active centers throughout the course of the polymerizations. In addition, most of the growing chain ends exist as ion pairs, depending upon the concentration of the monomer [89].

Bourissou et al. reported recently controlled cationic polymerization of lactones using a combination of triflic acid with a protonic reagent as the initiators [90]. The reaction was carried out in CH₂Cl₂. Results indicated that the process is controlled is a linear relationship between the molecular weight of the product and the monomer to initiator ratio as well as to monomer conversion. The process is believed to proceed by protonation of the lactone by triflic acid and then followed by a nucleophilic attack by the initiating alcohol:

It is believed that the controlled cationic ring-opening polymerization proceeds by an “activated anionic mechanism” as suggested by Penczek [91]. According to his suggestion, the acid activates the cyclic ester and the alcohol subsequently initiates the polymerization.

Kakuchi and coworkers reported controlled/living cationic ring-opening polymerizations of δ-valerolactone and ε-caprolactone with the aid of diphenyl phosphate [89]. The reaction was illustrated as follows:

The ring-opening polymerization of δ-valerolactone and ε-caprolactone was carried out using 3-phenyl-1-propanol as the initiator and diphenyl phosphate as the catalyst in toluene at room temperature. They reported that the reaction proceeded homogeneously to yield poly(δ-valerolactone) and poly(ε-caprolactone) with narrow polydispersity indices. Analyses indicated a presence of residues of the initiator.

In anionic polymerizations the initiations result from attacks by bases upon the carbonyl groups:

Common initiators are Li and K alkoxides. In addition to that, it was reported that phosphazene bases can be used to carry out polymerizations of cyclic esters [92]. Also, commercially available materials, like tert-butoxybis(dimethylamino)methane and tris(dimethylamino)methane yield high molecular weight polylactic acid by ring-opening polymerization with narrow molecular weight distribution:

The propagations take place by a similar process:

These steps repeat themselves until the chains are built up. Anionic polymerizations can yield optically active polymers. This was observed in formations of poly(α-methyl, α-ethyl-β-propiolactone) [193] that contains asymmetric carbon atoms.

The mechanism of coordination polymerization was pictured by Yong, Malzner, and Pilato [90] as being an intermediate between the above two modes of polymerization (a cationic and anionic one):

Initiation where, M_T means metal.

Propagation

The above shown mechanism, however, is incorrect when caprolactone is polymerized with tin compounds [95]. Yet, it appears to be correct for polymerizations of propiolactones with an ethylzinc monoxide catalyst [95].

The bimetallic oxoalkoxides are useful catalysts for the polymerizations of ε-caprolactone. The general course of the reaction is quite similar to one for oxiranes. A typical coordination mechanism is indicated from kinetic and structural data [97]. The molecular weight increases with conversion and the reaction exhibits a “living” character, because there is a linear relationship between DP and conversion. When the monomer is all used up, addition of fresh monomer to the reaction mixture results in increases in DP. By avoiding side reactions it is possible to achieve high molecular weights (up to 200,000) with narrow molecular weight distribution (M _w/M _n ≥ 1.05) [97]. The reaction proceeds through insertion of the lactone units in the Al–OR bonds. The acyl–oxygen bond cleaves and the chain binds through the oxygen to the catalyst by forming an alkoxide link rather than a carboxylate one:

There are potentially four active sites per trinuclear catalytic molecule. The number of actual sites, however, depends upon the aggregation of the oxoalkoxides. Two different types of OR groups exist, depending upon the bridging in the aggregates. Only one is active in the polymerization. This results in a catalytic star-shaped entity. The fact that the dissociated catalysts generate four growing chains per each Al₂(CH₂)₅CO₂(OR)₄ molecule [97] tends to confirm this.

The commercially available aluminum triisopropoxide was reported to be a very effective initiator for the “living” ring-opening polymerizations of ε-caprolactone, lactides, glacolide, and cyclic anhydrides [98]. Based on kinetic and structural data, the ring-opening polymerization is believed to take place by a coordination-insertion mechanism. While the molecules of aluminum triisopropoxide are coordinatively associated in toluene, in the presence of lactones single isolated monomeric species form and are believed to remain unassociated during the propagation reaction [98].

Actually, ring-opening polymerizations of ε-caprolactone were achieved by various catalysts. Only a few, however, initiate “living” polymerizations. Among these are the aluminum alkoxides described above, bimetallic μ-alkoxides [99], porphynatoaluminum [100], mono(cyclopentadienyl)titanium complexes [101], and rare earth alkoxides [102, 103]. Examples of rare earth alkoxides are Ln, Nd, Y, or Nd isopropoxy diethyl acetoacetates and (C₅H₅)₂LnOR and [C₅(CH₃)₅]₂LnCH₃ (donor) complexes. It was suggested that the steric effect of bulky groups of these catalysts is to suppress an interfering transesterification reaction by screening linear polymeric chains from the active centers during the reactions and yield “living” polymerizations [104]. These catalysts also are useful in formation of various block copolymers of lactones with other monomers [104, 105]. Among other lactones that were polymerized with the help of such rare earth catalysts are lactide [106–108], δ-valerolactone [109], β-propiolactone [109], and β-butyrolactone [107].

Polymerization of ε-caprolactone with a catalyst system consisting of tris(2,6-di-tert-butylphenoxy)yttrium and 2-propanol is first order with respect to the monomer and initiator [105]. This led to the conclusion that the reaction proceeds via a three-step mechanism that can be illustrated as follows [105]:

It was also reported that “living” ε-caprolactone polymerization can be carried with bis(acryloxy-)lanthanide (II) complexes based on samarium [110]. Thus, (ArO)₂Sm(THF)₄, (where ArO = 2,6-di-tert-butyl-4-methyl-phenoxy) yielded 98% conversion in toluene at 60°C in 1 h. The central ions and ligands appear to have an effect on the activity of the catalyst [110].

Some lactones can polymerize in the presence of compounds like alcohols, amines, and carboxylic acids without additional catalysts. The reactions, however, are slow and yield only low molecular weight polymers [95]. Exception is polymerizations of pivalolactone in the presence of cyclic amines that yield high molecular weight polyesters at high conversion [111]. The initiating steps result from formations of adducts, amine-pivalate betaines:

The above reaction appears to be restricted to highly strained lactones and may not work with larger lactones [95], For instance, when polymerization of δ-valerolactone is initiated with ethanolamine at temperatures up to 200°C there is initially a rapid reaction between the amine group and the monomer:

The subsequent reactions, however, are slow:

It was suggested that initiators, like dibutylzinc, that lack active hydrogens should be placed into a special category [96]. They can initiate polymerizations of some lactones. One of them is ε-caprolactone. Polymers form that are inversely proportional in molecular weights to the catalyst concentrations [112]. The same is true of stannic tetraacrylate. High molecular weight poly(ε-caprolactone), as high as 100,000 forms. Addition of compounds that may serve as source of active hydrogens is not necessary [95]. This group of initiators also includes dimethylcadmium, methylmagnesium bromide, and a few others that are effective in polymerizations of δ-valerolactone, ε-caprolactone, and their alkyl substituted derivatives. The polymers that form are high in molecular weight, some as high as 250,000 [113].

Another group consists of zinc and lead salts, stannous esters, phosphines, and alkyl titanates. This group does require additions of compounds with active hydrogens. Such additives can be polyols, polyamines, or carboxylic acid compounds [95]. Molecular weight control is difficult with the catalysts belonging to the first group. This second group, on the other hand, not only allows control over the molecular weights, but also over the nature of the end groups [95].

Weymouth and coworkers carried out kinetic and mechanistic studies of heterocyclic carbene mediated zwitterionic polymerization of cyclic esters [96]. Based on their results they proposed the following ring-opening mechanism:

From the kinetic studies they were able to conclude that in the heterocyclic carbene initiated polymerization of lactide, the rate of initiation is slower than the rate of propagation. Also, the rate of propagation is much faster than chain termination via cyclization.

The catalysts for cationic polymerization can be strong anhydrous acids, Lewis acids [115], salts of primary and secondary amines, carboxylic acids, and salts of amines with carboxylic acids that split off water at elevated temperatures [114]. The initiators react by coordinating with and forming rapid pre equilibrium lactam cations. These cations are the reactive species in the polymerizations. Initiations of this type are also possible with weakly acidic compound, but such compounds are not able to transfer protons to the lactam. They are capable, however, of forming hydrogen bonds with the lactams. The high reactivity of the lactam cations may be attributed to the decreased electron density at the carbonyl carbon atoms. This makes them more subject to nucleophilic attacks [114].

Protonations of the amides occur at the oxygens [116], but small fractions of N-protonated amides are also presumed to exist in tautomeric equilibrium. To simplify the illustrations, all lactams will be shown in this section as:

So, while the above structure commonly represents propiolactam, in this section it can mean any lactam, like a caprolactam, valerolactam, etc. Thus, the equilibrium can be shown as follows:

In a reaction mixture where the initiators are strong acids the strongest nucleophiles are the monomers. Acylations of the monomers with the amidinium cations result in formations of aminoacyllactams [113]:

Acylation of these amine groups by molecules of other protonated lactams results in the monomers becoming incorporated into the polymers [117]. The growth centers are preserved and a molecule of lactam is protonated. This occurs in two steps [117]:

These reactions attain equilibrium quickly. Aminolyses of acyllactams, that are the reverse of the initiation reactions, precede rapidly [117–119]. Aminolyses of aminoacyllactams actually contribute to the propagation process [120, 121]:

These reactions attain equilibrium quickly. Aminolyses of acyllactams, that are the reverse of the initiation reactions, precede rapidly [117–119]. The reaction, therefore, proceeds as follows [120, 121]:

The above reaction results in the destruction of the equilibrium and a regeneration of the strongly acidic amide salt. Total lactam consumption results from repetitions of the above sequences and formations of new aminoacyllactam molecules [113–121]. Initiations of polymerizations with acid salts of primary and secondary amines result in chain growths that proceeds predominantly through additions of protonated lactams to the amines [113]:

The rate at which the initiating amines are incorporated is proportional to the basicity. As the conversion progresses the concentration of protonated lactams in the reaction mixture decreases while that of the protonated polymer amide groups increases. The latter takes part in the initiation reactions with lactam molecules and in exchange reactions with polymer molecules [113]:

In each of the initiation steps the strongest nucleophile present reacts with the lactam cation. When strong anhydrous Bronsted acids initiate the polymerizations, the free lactams are acylated first with the formation of aminoacyllactams. When the polymerizations are initiated by amine salts, the initial steps are conversions of the amines to the amino acid amides. On the other hand, hydrolytic polymerizations start formations of unsubstituted amino acids [122]:

When weak carboxylic acids or acids of medium strength initiate lactam polymerizations at anhydrous conditions, there is an induction period [123]. In addition, the rates of these reactions are proportional to the pK_a of the acids [105]. It appears that different reaction mechanisms are involved, depending upon the acid strengths [113]. The nucleophiles are present in equilibrium:

The acylation of the carboxylate anions is assumed to lead to formations of mixed anhydrides of the acids with amino acids [124] and subsequent rearrangements:

When strong acids, however, initiate the polymerizations, the strongest nucleophiles present are the lactam amide groups that undergo acylations. As a result, the acids are not incorporated into the polymers.

The propagation steps in cationic polymerizations of lactams occur by transamidation reactions between lactam rings and the ammonium groups formed during the steps of initiation. It is believed that during the reaction proton transfers take place first from the amine salts to the lactams or to the acyllactams to form cations. These in turn acylate the free amines that form with the regeneration of ammonium groups:

The propagation step is very rapid when aminolysis takes place at the carbonyl group of the activated acid derivative (like acyllactam or an acid chloride). It is slower, however, if it involves an amide group of the monomer [114]. As is typical of many carbonyl reactions, acylations are followed by eliminations [125]:

The above water elimination reaction results in formations of amidines. Acylamidinium ions can also result from dehydration of the tetrahedral intermediates during the reactions of amino groups with acyllactams. Such groups could also be present within the polymer molecules. The water that is released in these reactions hydrolyzes the acyllactams, acylamidine salts, and lactam salts to yield carboxylic acids [114].

In the cationic polymerization of lactams the ammonium and amidinium groups form N-terminal chain ends. The C-terminal chain ends are in the form of carboxylic acid groups or alkylamide residues. This is important, because the nature of the end groups and their reactivity determine the steps that follow in the polymerizations. This means that the different types of cationic polymerizations of lactams are the results of the different end groups that form during the initiation steps. Formation of amidines increases with increasing acidity and concentration of the initiator and with an increase in the temperature:

When strong acids or amine salts initiate the polymerizations, almost all amine salt groups become converted to amidine salts shortly after the start of the initiation reaction [108]. Formation of amidinium salts leads to a decrease in the reaction rate because they initiate polymerizations of lactams less effectively than do ammonium salts [125, 126]. Lewis acids act in a similar manner, unless a co-reactant is present, like water. In that case, the Lewis acids are transformed into protonic acids and the polymerizations proceed as if they were initiated by protonic acids [114].

N-substituted lactams can generally not be polymerized. Some exceptions, however, are known when cationic mechanisms are employed [122] and when strong carboxylic or inorganic acids are used as initiators. In such cases the anions of the initiating acids, like Cl⁻, react with the lactam cations to yield amino acid chlorides [114]:

Only the more strained four-, eight- and nine-membered N-substituted lactams have so far been shown to be capable of polymerizations [113]. The 2,2-dimethylquinuclidone is highly strained and undergoes polymerizations at room temperature [127]. The propagation reaction of substituted lactams can be illustrated as follows [122]:

The anionic polymerizations of lactams are initiated by strong bases [128] capable of forming lactam anions:

Such bases can be alkali metals, metal hydrides, organometallic compounds, and metal amides. The initiation step of ring-opening amidation can be shown as follows:

The primary amine anions abstract protons very rapidly from other molecules of lactams to form amino-acyllactams [130]:

In these reactions, the propagation centers are the cyclic amide linkages of the N-acylated terminal lactam rings. Acylation of the amide nitrogens have the effect of increasing the electron deficiencies of these groups. This in turn increases the reactivities of the cyclic amide carbonyls toward attacks by the nucleophilic lactam anions [115]:

Very rapid proton exchanges follows. This results in equilibrium between the lactam and the polymeric amide anions [129]:

The polymer amide anions can undergo acylation by acyllactam groups with accompanying ring opening or with formation of lactam anions. In the first instance, it is an alternate path of propagation with formation of imide groups:

The acylation reactions shown above are much faster than the initiation reactions [129, 131] As a result, there are induction periods in anionic polymerizations of lactams [113]. In addition, steep increases in molecular weights take place at the beginning of the polymerizations. Bimolecular aminolyses may contribute to that, though their contributions to the total conversions are negligible [113].

The overall rates of polymerizations depend on the concentrations of acyllactams and diacylamine groups as well as on the lactam anions. The latter result form dissociations of the lactam salts, depending upon the nature of the metal: where Me means metal The alkali metals can be rated in the following order with respect to rates of initiations and propagations [113, 132]:

Additions of activators or cocatalysts, such as acyl halides, anhydrides, or isocyanates, can result in elimination of the induction period. These additives insure formations of stabilized adducts:

The structures of the activators can determine the rates of addition to the first lactam anion [113]. If, for instance, the acyl group is large, as in pivaloylcaprolactam, the decrease in the rate can be merely due to steric hindrance [113, 129]. On the other hand, substituents like the benzoyl group increase the rates of additions to the first lactam anions [113, 129]. In addition, the structures of the activators can also affect the course of the polymerization. This is because they become incorporated at the end of the polymeric molecules and may influence the basicity during the polymerization reactions.

Polymerizations in the presence of acylating agents are often called activated polymerization. If the acylating agents are absent from the reaction mixture the reactions may be called nonactivated. Sometimes the terms assisted and nonassisted are used instead.

Several reaction mechanisms were offered to explain the mechanism of anionic ring-opening polymerizations of lactams. One mechanism is based on nucleophilic attacks by the lactam anions at the cyclic carbonyl groups of N-acylated lactams. This leads to formations of intermediate symmetrical mesomeric anions that rearrange with openings of the rings [133, 134]:

Champetier and Sekiguchi concluded that the intermediate anions are neutralized first by protons from the lactams or from the polymer amide groups. The neutral molecules subsequently rearrange with the openings of the penultimate units [135, 136].

They also felt that the acylating strength of the acyllactams is enhanced by coordination of the cations with the imide carbonyl groups [135, 136]. This is based on an assumption that the incorporations of the lactam units proceed through additions of lactam anions. Protonations and subsequent rearrangements follow [135, 136]. This type of chain growth is termed lactomolytic propagation [137]:

The mechanism implies that the alkaline cation is fixed to the imide group and that a nucleophilic attack (that is the rate-determining step) by the lactam anion takes place on the endocyclic carbonyl group of the imide to give a “carbinolate” anion. Proton exchange takes place between this intermediate and a lactam monomer. Intramolecular rearrangement results in ring opening of the unit that is now in the penultimate position.

In anionic activated polymerization of ε-caprolactam chain growth involves both free anions and ion pairs [138]. Quantum-chemical calculations suggest that in the alkali metal lactamate molecule the negative charge is delocalized between the oxygen and the nitrogen heteroatoms. This led to a suggestion by Frunze et al [138]. that the acts of initiation are formations of activated intermediate chelate type complexes between the activators and the catalyst molecules [138]: where Me is a metal like Li, K, Cs, etc.

The carbinol fragment of the resulting complex, shown above, undergoes an intramolecular rearrangement. It leads to opening of the heterocyclic ring and to growth of the polymer chain by one unit:

The Frunze et al. mechanism [138] has much in common with the “alkali lactomolytic” mechanism of Champetier and Sekiguchi [135], except for the formation of the above shown complex. Frunze et al. also believe that probably a single mechanism exists for the anionic polymerization of lactams that they describe as ion-coordinative [138]. The contributions of various mechanisms via ion pairs or via free ions depend upon the nature of the alkali metal counterion and upon their capacity to coordinate with electron donating compounds (activator and monomer). The growth of ion pairs may mainly be expected from a lithium counterion, while growth by free anions may be expected from potassium or cesium

The products of anionic lactam polymerizations can vary, depending upon reaction conditions such as the temperatures, and upon the structures of the lactams themselves. Thus, five-, six-, and seven-membered lactams polymerize at different temperatures and the products differ in molecular weights. For instance, α-pyrrolidone polymerizes readily at 30°C to a polymer of a molecular weight of 15,000, while ε-caprolactam requires 178°C to form polymers of that size or larger [137]. A third lactam, α-piperidone is hard to polymerize to a high molecular weight polymer in good yields [137]. The regular molecules that form in activated anionic polymerizations, as already shown, are [139]: while those activated by lactam anions, are:

At higher polymerization temperatures, however, side reactions occur. Among them are Claisen type condensations. They lead to two type of N-acylated β-keto imide structures and take place readily above 200°C. Formation of these imides decreases the concentration of lactam anions:

Cyclic keto imides as well as linear ones can yield active species through acylation of lactam anions. This results in formations of growth centers and keto amides:

The acidity of keto amides with α-hydrogen atoms is much greater than that of the monomers or of polymer amide groups. Any formation of such structures, therefore, decreases the concentration of lactam anions.

Side reactions give rise to a variety of irregular structures that may be present either in the backbones, or at the ends of the polymer molecules, or both. Formation of branches in anionic polymerizations occurs in polymerizations of ε-caprolactam [140, 141]. This lactam and higher ones polymerize at temperatures greater than 120°C. Above 120°C the β-keto-amide units and possibly the n-acyl-keto-amide structures are preserved. They may, however, be potential sites for chain splitting later during polymer processing that takes place at much higher temperatures [142].

A new group of catalysts, metal dialkoxyaluminum hydrides, for anionic polymerizations of lactams, were reported recently [143]. A different anionic mechanism of polymerization apparently takes place. When ε-caprolactam is treated with sodium dialkoxyaluminum hydride, a sodium salt of 2(dialkoxyaluminoxy)-1-azacycloheptane forms:

Such compound differs in nucleophilicity from activated monomers. These salts are products of deprotonation of lactam monomers at the amides followed by reduction of the carbonyl functions. It is postulated that during lactam polymerizations, after each monomer addition, the active species form again in two steps [143]. In the first one proton exchanges take place: in the second step hydrogen and dialkoxyaluminum group are transferred:

The third step is propagation. It consists of addition of a unit to the chain end and takes place upon reaction of the terminal acyl lactam with a sodium salt of 2(dialkoxyaluminoxy)-1-azacycloheptane shown above [143].

This polymerization can be looked upon as a special case of cationic polymerization. It is particularly true when weak acids are added to the reaction mixture, as is often the case in industrial practice. In hydrolytic polymerizations of lactams initiated by water, the hydrolysis–condensation equilibria determine the concentrations of the amine and the carboxylic acid groups. Both functional groups participate in the propagation reactions:

The concentration of these groups also determines the molecular weights of the final products [118–128]. This type of equilibria also occurs in polymerizations initiated by amino acids or by salts of carboxylic acids formed with primary and secondary amines. In the hydrolytic polymerizations of caprolactam the above reactions involve only a few percent of the total lactam molecules present [144, 145]. The predominant propagation reaction is a step-growth addition of lactam molecules to the end groups. It is acid catalyzed [144, 145]:

The exact mechanism of this addition is uncertain. It was postulated that the addition steps are through reactions of neutral lactam molecules with ammonium cations [146–148]. Others felt, however, that the lactam molecules add to the undissociated salts [149].

Hydrolytic polymerizations are the smoothest of all three types of polymerization reactions because the growing species are less activated than in either cationic or anionic polymerizations. Many commercial processes utilize it in ε-caprolactam polymerizations. Formation of irregular structures, however, and even crosslinked material was detected. In addition, at elevated temperatures deamination and decarboxylation of polycaprolactam can take place [150] Such reactions can result in formations of ketones and secondary amine groups. The ketones, in turn, can react with amines and form Schiff bases. This leads to branching and crosslinking.

In industrial preparations most of the water used to initiate the polymerizations is removed after conversions reach 80–90% in order to attain high molecular weights. The final products contain about 8% of caprolactam and about 2% of a cyclic oligomer [150]. These are removed by vacuum or hot water extraction. The material is then dried under vacuum at 100–200°C to reduce moisture to about 0.1%.