Departure from Livingness Kinetics of Selected Side Reactions

3 Departure from Livingness Kinetics of Selected Side Reactions 

1 Cationic Polymerization The kinetics of termination, chain transfer and macrocyclization processes in cationic ROP have been extensively reviewed in Chapter 5 of Ref. [3a] and also in Ref. [74]. Of particular value is Table 18 in Ref. [3a], which details the basic equations in nonstationary kinetics with chain transfer. Although, since these two reviews were published few new phenomena have been identified, we will at this point provide a brief description of activated monomer cationic polymerization. 

In polymerization proceeded by an AM mechanism, the active centers are located on the monomer molecule (thus activated monomer), while the growing polymer chain is neutral, despite terminating with a reactive hydroxyl group. Because in the cationic AM mechanism there is no active species at the chain end, the back-biting is greatly reduced. 

Even if the AM mechanism operates in a cationic polymerization of oxiranes in the presence of hydroxyl groups, it does not eliminate the possible contribution of a conventional active chain end (ACE) mechanism (active center oxonium ion located at the macromolecular chain end). In order for an AM-type propagation to prevail, the instantaneous concentration of monomer should be kept as low as possible (e.g. via continuous slow monomer addition). 

The kinetic studies, which were based exclusively on the determination of monomer consumption, allowed only an apparent rate constant to be determined (Equation 1.39). 

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