A proposal on the steric course of propagation in the homogeneous cationic polymerization of vinyl and related monomers

Abstract

A stereochemical scheme of propagation was proposed for polymerizations of vinyl and related monomers by Friedel‐Crafts catalysts. For the cationic propagation proceeding via the simple carbonium ion pair, the following two factors were considered to be of primary importance in determining the steric course of propagation: (1) the conformation of the last two units of the propagating polymer segment and the direction of approach of the incoming monomer; (2) the tightness of the growing ion pair. Thus, the front‐side (less hindered site) attack to the carbonium ion gives rise to a syndiotactic placement and the back‐side attack an isotactic placement. The present model can satisfactorily explain the effects of substituents, catalysts, polymerization media, and polymerization temperature on the steric structure of polymers in cationic polymerization of vinyl ethers. Extension of the scheme to polymerization of the β‐substituted vinyl ethers in nonpolar solvents predicts formation of the diisotactic structures consistent with the experimental result. The influences of the polymerization condition on the steric structure of polymer were studied for cationic polymerizations of α‐methylstyrene at low temperatures. Highly syndiotactic polymers were obtained for homogeneous reactions in toluene‐rich media. The isotactic unit increased by increasing the content of methylcyclohexane in the solvent mixture. The effect of catalysts, though insignificant in toluene‐rich media, was clearly noted in methylcyclohexane‐rich media, less active catalysts (e.g., SnCl4) yielding higher amounts of the isotactic unit than more active catalysts (e.g., AlCl3). These results can be readily accommodated in the present model.

Partitioned partial least squares regression with application to a batch fermentation process

“…End‐group analysis via 1 H NMR spectroscopy of a representative Pα‐MeSt sample (Figure 4) shows the presence of halogen atom as the end group, instead of the indan and the olefinic groups. Again, the above results also suggest that the electron donor (nBu 4 NBr) and/or its complex with the Lewis acid (FeCl 3 ) interacts with the propagating polymeric cationic end to reduce its cationicity and thus to convert highly reactive species into less‐reactive species, as suggested previously by Kaszas et al68 On the basis of the above experimental results, the results of model reactions of our previously published paper47 and the literature reports, we can propose the following reaction sequence (Scheme ) as a possible pathway for this stereospecific living cationic polymerization: The steric course of propagation in cationic polymerization is mainly determined by two factors: 1) the conformation of the last two units of the propagating polymer segment and the direction of approach of the incoming monomer; 2) the tightness of the interaction of the counteranion with the growing carbocation based on the steric hindrance of the counteranions and the monomer 18, 69. It is known that the non‐polar solvents presumably favored contact ion pair formation, which usually resulted in the formation of polymers with less syndiotacticity.…”

“…It is known that the non‐polar solvents presumably favored contact ion pair formation, which usually resulted in the formation of polymers with less syndiotacticity. In contrast, polar solvents such as dihloromethane and chloroform allow the formation of the less sterically crowded syndiotactic units, presumably because of sufficient ion pair separation 31, 69, 70. Thus, the formation of syndiotactic polymers in cationic polymerization reactions of α‐methylstyrene involves greater free ion character in the active end group, as explained on the basis of mechanistic considerations and kinetic effects 27.…”

Cationic polymerization of p-methylstyrene in selected ionic liquids and polymerization mechanism

“…8) indicated a mixture of polymer triad sequences (isotactic, atactic, and syndiotactic). 84 A possible mechanism for the stereospecific poly( p-MeSt) with triad sequences is shown in Fig. 9).…”

Poly(vinyl ethers)

“…Mechanisms explaining their formation suggest that the stereoregularity depends on the association of the growing cation and counterion and also by the steric hindrance of monomer approach by side-chain substituents. Another factor is thought to be the solvent that determines the tightness of the growing ion pair [29,30]. A summary of more recent caionic polymerization of vinyl ethers is given in Table III.…”