Living and apparently living carbocationic polymerizations

Abstract

AbstractSome years ago, the occurrence of living carbocationic polymerization had not generally been expected to be possible, since it is well known that most carbocationic species are quite unstable and have very short lifetimes, and since transfer to monomer had been shown to be important, particularly near room temperature. However, during the last years, many reports of living carbocationic polymerizations have been made. They were based on the observation of various features usually linked to living polymerizations, such as a linear increase of mol. wts. with conversion, sometimes even after several monomer additions, which was attributed to the absence of termination and transfer. In some cases, narrow mol. wt. distributions were also obtained.There seems to be now a general agreement that in these polymerizations a reversible termination occurs, making eventually further growth possible on all macromolecules. Another general feature of those apparently living systems is that the ratio of propagation rate and initiation rate is not too high, so that the concentration of macromolecules is approximately equal to that of the initiator.But the experimental data do not necessarily imply, as this has been generally assumed, that transfer is absent and that the nature of active sites is completely different from those in more classical systems. It is shown that the values of transfer constants already measured in these last ones are compatible with the results obtained in the apparently living systems. A perfectly linear relationship between number‐average degree of polymerization (D̄P̄n) and polymer yield may be observed even up to mol. wts. of about 2.104 with transfer constants ktrM/kp as high as 5.10−4 in apparently living systems. Termination and transfer might be, however, reduced in some cases by various means that are examined, such as the presence of polar additives, a lowering of temperature and the presence of excess monomer. The distinction between systems obeying all the main criteria for living polymerization and those which are only apparently living is discussed.