Selectivity as a measure of ''livingness'' of the polymerization of cyclic esters

The selectivity parameter, defined as the ratio of the rate constant of desired elementary reaction (i.e. initiation or propagation) to the rate constant of the competing side reaction, should be used as a quantitative measure of the livingness of polymerization. The most important side reactions in the anionic and pseudoanionic polymerization of cyclic esters are presented and the importance of selectivity in initiation and propagation is discussed, taking into account the relation between selectivity and the structure of monomers, initiators, and active species. Reasons for the low selectivity of ions and the high selectivity of aluminium-based initiators, namely dialkylaluminium alkoxides (R(2)AlOR') and aluminium trialkoxides (Al(OR)(3)) are given. The relatively low reactivity and steric hindrance, created by the presence of bulky substituents at the Al atom, are responsible for the enhanced selectivity. Thus, Al(OR)(3) found recently to carry three chains, provides particularly crowded and therefore selective active species. The structure of the aluminium alkoxide active species is described, as established on the basis of H-1 NMR, Al-27 NMR, and molecular weight measurements by Multi Angle Laser Light Scattering (MALLS) of the living polymers.