A general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation ($) to the rate constant of the bimolecular transfer ( k f ) ) from the dependence of the MWD on monomer conversion. Numerical values of k,/kf) equal to =lo2 and 25 were thus determined for the polymerization of L,L-lactide (L,L-dilactide) initiated with aluminium tris(isopropoxide) trimer ({ Al(O'Pr),) 3) and tributyltin ethoxide ("Bu$nOEt), respectively.

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Chain transfer processes (ktr) taking place in the polymerization (anionic and pseudoanionic) of cyclic esters (lactones) are reviewed. These reactions are mostly responsible for the departure of these systems from the fully controlled (living) polymerizations. The ratios of kp/ktr have been determined for a number of initiating systems and the structures of the growing species are related to their selectivity, expressed by kp/ktr. It has been shown that the less reactive and more sterically crowded active species polymerize more selectively.

Polymerization of cyclic esters leads to (bio)degradable polymers of the increasing industrial importance. These polymerizations are of the living nature, although chain transfer to polymer with chain scission may cause deviations from the livingness and introduce structural differences (e.g. in end‐groups), important for physical properties. Two different systems are discussed. In the first one two living macromolecules react one with another and reproduce two living macromolecules, retaining the same reactivities and the same end‐groups. Polymerizations of ϵ‐caprolactone and lactide belong to this category. On the other hand, polymerization of cyclic carbonates proceeds with chain transfer, in which disproportionation of the living chains takes place: from two living macromolecules one “dead” and one “doubly active” can be formed. Conditions of retaining the livingness in terms of the ratios of the rate constants of transfer, reinitiation, and propagation are discussed.

Polycondensation of diols with phosphonic bis(dialky1amide)s leads to high-molecular-weight polyphosphites (a, up to 4 . lo4). Model reactions of 1-butanol, 2-butanol and phenol with phosphonic bis(diethy1amide) (1) were studied and it was established, that these reactions exhibit characteristic induction periods. Linearization of the kinetic curves after addition of amines prior to reaction enabled us to postulate the mechanism of esterification, involving activation of 1 by hydrogen-bonding to amines. However, the major side reaction lowering the degree of polymerization was found to be dealkylation of the formed ester units by added and/or evolved amine. Therefore, in order to increase the rate of polycondensation and to prevent the formed polymer chain from breaking down by dealkylation, the process was conducted at increased temperature (90°C) in a nonsolvent for the polymer of higher molecular weight. Thus, the polymer separates from the major solution and becomes less prone to dealkylation.

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