Propagation of heterocycles

EXAMPLE. PROPAGATION OF HETEROCYCLES. PERIODIC CHANGES OF GROWING CENTRE REACTIVITY... 

For less reactive anionic chain ends such as those involved in propagation of heterocyclic monomers, a wider range of solvents can be utilized. For example, dipolar... 

As for the propagation of heterocycles, it proceeds via onium -type cationic species, but actually involve transient carbonium ions ... 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

In conclusion, it has been shown that use of cryptates for the anionic polymerization of heterocyclic monomers leatis to a tremendous increase of the rates of polymerization. There are two main causes to the higher reaction rates observed with cryptates. The first one is a suppression of the association between ion pairs in the non polar media, and the second one is the possibility of ion pairs dissociation into free ions in ethereal solvents like THP or THF. By this way, it has been possible to make detailed studies of the propagation reaction for propylene sulfide, ethylene oxide, and cycloslloxanes. 

The coordination polymerisation of heterocyclic and heterounsaturated monomers consists in the nucleophilic attack of the metal initiating substituent (or the growing polymer chain) on the carbon atom of the coordinated monomer. Scheme 2.6 shows initiation and propagation steps in the coordination polymerisation of epoxides, as the most representative heterocyclic monomers with an endocyclic heteroatom, with catalysts containing an Mt-X active bond [68,114,115] ... 

Thus, it must be remembered, that in the polymerization of heterocycles, species originally formed by interaction of initiator with monomer, may differ significantly (both in structure and reactivity) from the propagating active species. Initiation may involve the sequence of at least two reactions and the second one may be the slow, thus rate-determining step in initiation. 

The collection of rate constants of propagation of different heterocyclic monomers are given in Ref. 2. 

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 ... 

In many systems, however, the analysis of the cationic copolymerization of heterocyclic monomers is complicated by two factors (1) at least some of the homo- and cross-propagation reactions may be reversible (2) redistribution of the sequences of comonomers within the chain may occur as a result of chain transfer to polymer. Therefore, the conventional treatment involving four irreversible propagation steps is rarely applicable in cationic ring-opening copolymerization. Instead, the diad model should involve four reversible reactions, i.e., eight rate constants... 

There are two groups of reports describing the determination of the rate constants of propagation of macrocations (kp) and macroion pairs (kp) in the polymerization of heterocycles. In the first one, k and k were found to be indistinguishable and in the second group, kp and differ a few times. 

The initiation and propagation steps in the cationic polymerization of heterocycles have been described in previous sections. The chemistry of these steps has been reasonably well established, mostly by spectroscopic (particularly NMR) methods, and a number of corresponding rate constants have been determined. 

The equaibrium according to Eq. (92) involving oxazolinium cations was apparently the first observation of the equilibrium between ionic and covalent species in the cationic polymerization of heterocyclic monomers A similar collapse, being however irreversible, has earlier been postulated for the cationic polymerization of oxetane propagating with C(N02)3 anions and of styrene propagating with CIO anions ... 

Thus, in general, in cationic polymerization, and rticularly in the cationic polymerization of heterocyclic monomers, propagation can involve both ionic and covalent species ... 

In the majority of cationic polymerizations of heterocyclic monomas, chain propagation immlves a nucleophilic attack of a monomer molecule on the strained onium ions, e.g. ... 

Closely related to the ionic polymerization of heterocyclic monomers is, what we can call, pseudoionic polymerization (or sometimes, perhaps, cryptoionic). We use the preffix pseudo- in the same meaning as it was first used in the vinyl cationic polymerization. It means that propagation actually proceeds on the covalent species that could have been in equilibrium with their ionic counterparts. Several systems falling to this category have recently been described for both anionic and cationic polymerization of heterocyclics. In the anionic processes derivatives of Zn or A1 alkyls or alcoholates are believed to function this way. However, for none of these systems the absence of ionic contribution was shown. Two catalytic systems are of particular interest, namely the -Zn-O-AK systems (20) and>Al-alkyl modified by bulky porphyrin derivatives (21). Both are discussed in this volume and both have been clearly shown to produce living systems. The former with e-caprolactone and the latter... 

Table IV. Rate constants of propagation in anionic polymerization of heterocyclic compounds...

The Sj 2 mechanism of propagation in polymerization of heterocyclic monomers was generally accepted. 

Heterocyclic Monomers.—Reviews of the polymerization of tetrahydrofuran (THF) were published. Rate constants of propagation of THF on macroesters and macroions were measured. In the polar solvent nitromethane, where macroesters are not important, it was shown that k and k t are identical within experimental error, and are not influenced by the nature of the counterion. It was postulated that the active centres are so highly solvated by monomer that free ions and ion-pairs are indistinguishable in terms of reactivity. 

Investigations of CROP of cyclic ethers (mainly THE) provided the first thoroughly studied examples of polymerization with reversible deactivation of growing species involving equilibria between ionic and covalent (dormant) species. Studies of polymerization kinetics led to determination of rate constants of elementary reactions and in several cases equal reactivity of ions and ion-pairs in propagation was observed, which seems to be a general phenomenon in CROP of heterocyclic monomers. 

Scheme 1.2 Comparison of cationic and anionic propagation pathways in the ROP of heterocyclic monomers.

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