Polymerizations with contribution of covalent and ionic species

Abstract

AbstractThe similarities and dissimilarities between the ring‐opening and vinyl cationic polymerizations are discussed for systems with simultaneous presence of covalent and ionic species. The common denominator in comparing kinetics of polymerization of these systems is the comparison of the total times of polymerization with the time elapsing when a given macromolecule is active (i.e. ionic) and inactive (e.g. covalent in cationic vinyl and ring‐opening polymerization). In the cationic ring‐opening polymerization the reactivation of covalent species proceeds mostly by intramolecular nucleophilic dipole‐dipole reaction (conformationally assisted) involving the chain unit adjacent to growing ester chain end. This reaction is fast (e.g. in THF polymerization) and is responsible for covalent ion‐pair interconversion. In the cationic vinyl polymerization this path does not exist but covalent species (esters) can be faster ionized by using of the nucleophile, increasing thus the rate of exchange between esters and ions. This process provides one of the possible ways for converting the non‐living polymerization into the living‐dormant process. The cationic polymerizations are then compared with covalent (pseudoanionic) polymerization of cyclic esters, initiated with covalent metal alkoxides like R2AlOR' or Al(OR)3.