ABSTRACT
Chain polymerization of olefinic monomers can also be effected via active centres which possess an ionic charge. There are two types of ionic polymerization: those in which the active centre is positively charged are termed cationic polymerizations and those in which it is negatively charged are termed anionic polymerizations. Because the active centre has an ionic charge, these polymerizations are more monomer-specific than radical polymerization and will proceed only with monomers that have substituent groups which can stabilize the active centre (e.g. by inductive and/or meso-
, polymerization will proceed if the substituent group X is able to donate electrons and/or delocalize the positive charge. However, for polymerization via anionic active centres (e.g. CH2 CHX), the substituent group must be able to withdraw electrons and/or delocalize the negative charge. Thus, although most monomers are susceptible to radical polymerization, cationic and anionic polymerization tend to be mutually exclusive for a given monomer (see Table 5.1). Only when the substituent group has a weak inductive effect and is capable of delocalizing both positive and negative charges will a monomer undergo both cationic and anionic polymerization (e.g. styrene and 1,3-dienes).
