Chain-growth polycondensation: The living polymerization process in polycondensation

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.