Cationic activated monomer polymerization of heterocyclic monomers

Abstract

In the first part of this review the meaning of activation is discussed and selected examples of polymerizaton processes in which activation of monomer is required prior to actual propagation are presented. In some systems, activation of monomer proceeds with such a strong interaction between an activator and monomer that a new chemical entity is derived from the monomer. To describe the mechanism of such a process, the term ‘Activated Monomer Mechanism’ has been coined. The main part of the review is concerned with cationic Activated Monomer (AM) polymerization of cyclic ethers. In this process, cyclic ether is activated by formation of protonated species in the presence of a protic acid. Reaction of the protonated (activated) cyclic ether with hydroxyl group containing compounds leads to ring opening reforming the hydroxyl group. Several repetitions of such a reaction constitute a chain process. Thus, in AM polymerization of cyclic ethers hydroxyl group containing compounds act as initiator, protic acid is a catalyst, growing chain end is fitted with hydroxyl group and the charged species is a protonated monomer. The important feature of such a polymerization mechanism is that due to the absence of charged species at the growing chain end, back-biting leading to the formation of macrocyclics can be eliminated. The mechanism and kinetics of AM polymerization of cyclic ethers is discussed and the approach allowing one to determine the rate constant for propagation involving activated monomer species is outlined. The application of the AM concept to the copolymerization of cyclic ethers as well as to the polymerization of monomers containing both initiating (hydroxyl groups) and propagating (cyclic ether) functions within one molecule are presented. In the subsequent parts of the review, examples of cationic AM polymerization of other types of heterocyclic monomers, including cyclic acetals, cyclic esters (lactones), amines and amides (lactams), are given. Finally, the polyaddition of oxiranes to derivatives of phosphoric acid is discussed. Although this system does not conform to the AM polymerization scheme, it bears formal resemblance to earlier systems in such a sense that the activation of the cyclic ether is required for the reaction to occur.