SELECTIVITY AS A MEASURE OF LIVINGNESS OF THE POLYMERIZATION OF CYCLIC ESTERS

The selectivity parameter, defined as the ratio of the rate constant o f desired elementary reaction (i.e. initiation or propagation) to the rate constant of the competing side reaction, should be used as a quan titative measure of the livingness of polymerization. The most importa nt side reactions in the anionic and pseudoanionic polymerization of c yclic esters are presented and the importance of selectivity in initia tion and propagation is discussed, taking into account the relation be tween selectivity and the structure of monomers, initiators, and activ e species. Reasons for the low selectivity of ions and the high select ivity of aluminium-based initiators, namely dialkylaluminium alkoxides (R(2)AlOR') and aluminium trialkoxides (Al(OR)(3)) are given. The rel atively low reactivity and steric hindrance, created by the presence o f bulky substituents at the Al atom, are responsible for the enhanced selectivity. Thus, Al(OR)(3) found recently to carry three chains, pro vides particularly crowded and therefore selective active species. The structure of the aluminium alkoxide active species is described, as e stablished on the basis of H-1 NMR, Al-27 NMR, and molecular weight me asurements by Multi Angle Laser Light Scattering (MALLS) of the living polymers.