Chain-growth polycondensation:The living polymerization process in polycondensation

摘要

The historical development of research on the living polymerization process in polycondensation is reviewed.Classical polycondensation is a step-growth process,but a living polymerization polycondensation must proceed by a chain-growth rather than a step growth mechanism.Early work demonstrated that some polycondensations do not obey Flory's statistical treatment:for example,high molecular weight polymer may be obtained,even at low conversion.This means that a chain-growth mechanism must be involved,with or without a step-growth mechanism.Recent years have seen dramatic development in understanding of polycondensations that proceed only by chain-growth(chain-growth polycondensation).Several possible mechanisms are:(1)activation of the polymer end group by changed substituent effects between the monomer and the polymer,as with aromatic polyamides,polyesters,polyethers,poly(ether sulfone)s and poly(ether ketone)s;(2)activation of the polymer end group by transfer to it of the catalyst,as with polythiophenes;(3)transfer of the reactive species,derived from the initiator,to the polymer end group,as with polymethylenes and polyphosphazenes;and(4)phase-transfer polymerization in a biphase composed of a monomer storage phase and a polymerization phase,as with aliphatic polyesters.These chain-growth polycondensations have been applied to the synthesis of condensation polymers with various architectures:block copolymers,star polymers,graft copolymers,etc.