Polymerization of cyclic amines

The cyclic amines or imines (aziridines) polymerize only with acidic catalysts. reaction can be illustrated as follows  

The termination mechanism is still not fiilly explained. It is believed that it may take place by proton abstractions from the iminium ions by the counterions, or by any nitrogen in the polymer chains, or by the nitrogens of the monomer units. It was also suggested that backbiting and ring expansion terminate the reactions. Such ring expansions result in formations of relatively unreactive piperazine end groups  

Substitution on the ethylene imine ring hinders polymerization. The 2,3- and 1,2-substituted aziridines fail to polymerize. Only low molecular weight linear and cyclic oligomers form from 1-and 2-substituted ethylene imines. 

The catalysts, that they call cis- and trans-Caz-l, promote a difficult tosylamine ring-closing with 100% conversion, compared with about 60% achieved by existing catalysts. And a considerably smaller amount of Caz-1 is needed to promote ring-closing metathesis of hindered dienes than is required for current catalysts. The Caz-1 catalysts also show unusually good stability and longevity in reactions [184]. 

Wathier, Stoddart, and Grinstaff reported using the Grubs catalyst to form high molecular weight polymers, poly(ethyl-5-norbomene-2-carboxylate) and poly(methyl-5-oxanorbomene-2-carboxylate) carrying ester functions. The preparations were illustrated as follows [185]  

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The problem of water in ionic ring-opening polymerization is much less critical than in vinyl polymerization, because, if present, water in the former system has to compete with relatively nucleophilic monomer present in large excess. Thus, certain polymerizations (e.g., polymerization of cyclic amine-conidine) can be conducted even in alcohols as solvents. 

The other limitation stems from very different structure of heterocyclic monomers and thus very different reactivity of resulting active species. As already discussed, oxonium ions may initiate the polymerization of cyclic amines, but ammonium ions would not initiate the polymerization of cyclic ethers. Thus, the sequential polymerization is possible only when the first monomer is not a stronger nucleophile than the second monomer. 

In the random copolymerization process, both types of active species should be able to participate in the cross-propagation reactions. This imposes certain limitations on the choice of comonomers in the cationic polymerization of heterocyclic monomers. Onium ions, being the active species of these polymerizations, differ considerably in reactivity thus, as already discussed, oxonium ions initiate the polymerization of cyclic amines, whereas ammonium ions do not initiate the polymerization of cyclic ethers and the corresponding cross-propagation reaction would not proceed ... 

The formation of additkm products and further ionization according to Eq. (17) w e shown directly also in the polymerization of cyclic amines. 

Enikolopyan studied the polymerization of cyclic amines such as conidine ... 

The data collected by Goethals on the kp/k ratio in the polymerization of cyclic amines (determined from Eq. (153) permits to show (Fig. 16) that there is a linear relationship between In (kp/kf) and the effective volume of substituents. These volumes were calculated according to Ref. 262. Thus, although the linearity itself has little importance, this dependence is in good agreement with the discussed earlier influence of substituents on the rate constants of termination and fuopagation. 

Cationic polymerization of cyclic amines proceeds usually with a high activation energy and often requires relatively high temperatures, even for the strained four-membered rings, e.g. for 1,3,3-trimethylazetidine AH = 73 kJ mol-1, kp = 1.4 xl0 4mor1 -1-s"1 at 78 °C 5). 

The application of protonic acids in the polymerization of cyclic amines gives rise to a higher yield of cyclic oligomers due to the formation of more basic secondary amine end-groups and rapid end-to-end cyclization. For instance, in the polymerization of 1-t-butyl aziridine, 25% of cyclic pentamer was formed with HS03CF3 initiator 13). 

Cationic polymerization of cyclic amines and sulfides proceeds by onium ions as active species ... 

Polymerization of cyclic amines proceeds usually with high activation energies, due to the high strength of the C—N bond. To attain significant rates (k 10-3 mol-1 1 s 1) requires temperatures from 30 to 100 °C for azetidines and about 200 °C for the less strained bicyclic six-membered amines quinuclidine and triethylenediamine 9). Cyclic sulfides polymerize more rapidly than amines but much slower than ethers of a comparable structure (e.g. for 3,3-dimethylthietane k, = 2 10-2 mol-1 1 s at 35 °C, which is at least 104 times lower than for 3,3-dimethyloxetane (cf. Adv. Polymer Sci. 37 (1980)). 

The cationic polymerization of cychc amines is well known [98-100]. Low-molecular-weight initiators such as ethyltosylate induce the polymerization of cyclic amines, such as 1-fert-butylaziridine. The concept of using a macroinitiator bearing a tosylate end group to polymerize cyclic amines prompted Kazama etal. [101] to attempt the polymerization of 1-tert-butyl aziridine, using PDMS with a terminal tosylate group. The fact that no polymerization occurred when the macroinitiator was used provided a clear demonstration of the initiative behind studying the transformation reaction between anionic and cationic polymerizations. 

For aliphatic monoamine [43], it is shown that secondary amines R2NH always possess a higher promoting effect for the polymerization of AAM and even the primary amine PA will enhance the polymerization with Rr = 1.47 and Ea = 36.4 kJ/mol, while the tertiary aliphatic amine TPA will not provide the polymerization due to some steric hindrance (Table 6). All of the data of cyclic amines listed in Table 7 are effective, i.e., NMMP with Rr = 1.81 and Ea = 29.9 kJ/mol showing the absence of steric hindrance. 

Lithium hexafluoroarsenate is thermally stable [54, 55] but shows environmental risks due to possible degradation products [56-58], even though it is itself not very toxic. Its LD 50 value is similar to that of lithium perchlorate [55]. Just like lithium hexafluorophosphate, it can initiate the polymerization of cyclic ethers. Polymerization may be inhibited by tertiary amines [59], or 2-methylfuran [60], yielding highly stable electrolytes. 

Discuss by means of equations the occurrence of backbiting, ring-expansion reactions in the polymerizations of cyclic ethers, acetals, and amines. 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

Covalent compounds, which are strong alkylating or acylating agents may initiate the cationic polymerization of heterocycles. Again, as in the case of acids, classification of these agents is relative for example, alkyl bromide will efficiently alkylate strongly nucleophilic monomers like cyclic amines or oxazolines and thus initiate their polymerization whereas it will be uneffective in the polymerization of cyclic ethers. 

Again, these reactions, depending on the structure of monomer, may be reversible or irreversible. If resulting branched or macrocyclic onium ions are not reactive, i.e., they can not re-form the original active species either by intramolecular cyclization or by reaction with the next monomer molecule, then these reactions lead to termination (such a situation exists in the polymerization of cyclic sulfides or amines cf., Section 1II.D.E.). 

The factors, governing the reactivities of branched onium ions are of complex nature and will not be discussed here. It has been shown, however, that in the polymerization of cyclic sulfides and cyclic amines the formation of branched onium ions of the type ... 

The other onium salts (sulfonium or ammonium salts) are less often applied due to their lower reactivity. They can neither initiate the polymerization of cyclic ethers or acetals nor be displaced by less nucleophilic ligands (cyclic sulfides and amines respectively). 

It has been shown in the previous sections that at least the polymerization of cyclic ethers, sulfides and amines proceeds via onium ions. The large majority of authors have agreed on this point (the mechanism of propagation of disubstituted cyclic ethers like isobutylene oxide is, however, still in dispute) . ... 

The choice of counterions (anions) in the cationic polymerization of heterocyclic monomers can be almost as wide as in anionic polymerization, but only for the most nucleophilic monomers (i. e. cyclic amines). Unfortunately, in the polymerization of cyclic ethers, this choice is much more restricted. Thus, the small anions like F or OH cannot be used because, due to their high nucleophilicity and ability to form covalent bonds, they give rise to fast termination. In order to suppress or even to eliminate termination by collapse within an ton pair (cf. Sect. 5.1.), it is necessary to use complexed anions having large ionic radii. These are shown below (rctyst)-... 

The polymerization of cyclic sulfides and amines provided, although tentatively, some information on kp and kp. The equality k kp was found (the Kq value measured for an initiator used was further applied in the determimtion of a in polymerization studies) in the polymerization of 3,3-dimethylthietane in CgHsNOa and qualitatively for l-phenylmethyl-2-methyl-aziridine Small differences were found between kp and k (the macrocation being less... 

Table 12. Rate constants of propagation and activation parameters in the cationic polymerization of cyclic sulfides, amines and iminoethers at 0 °C... 

In some systems, mostly in the polymerization of cyclic sulfides and amines, the formation of polymeric onium ions is irreversible, i.e. the reactivity of these structures is so low that further propagation on these species does not proceed ... 

Polymerization of cyclic sulfides and amines can be initiated not only by the usual cationic and cationogenic compounds like Lewis and protonic acids, carbenium and oxonium salts and esters of strong acids, but also by alkyl halides, which are active enough to induce polymerization of some azetidines 9 10). 

As already mentioned and analysed in some detail in Vol. I (Adv. Polymer Sci. 37 (1980)), the cationic polymerization of cyclic sulfides and amines is accompanied by chain transfer to polymer. This type of transfer could become a termination, if the heteroatoms in the chain are more nucleophilic than those in the monomer ... 

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 Exceptions are polymerizations of pivalolactone in the presence of cyclic amines that yield high molecular weight polyesters at high conversion. The initiating steps result from formations of adducts, amine-pivalate betaines ... 

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