Polymerization of ε-Caprolactone Initiated by Aluminum Isopropoxide Carried Out in the Presence of Alcohols and Diols. Kinetics and Mechanism

The mechanism of polymerization of ε-caprolactone (εCL) initiated with aluminum isopropoxide (Al(O i Pr) 3 ) trimer (A 3 ) and/or tetramer (A 4 ) carried out in the presence of ethanol, 2-propanol, 1,2-ethanediol, 1,5-pentanediol (PD), and poly(ethylene glycol) as chain-transfer agents was studied. It has been established, by the kinetic studies, that the low-molecular-weight alcohols and diols, introduced purposely into the polymerization mixture, operate not only as the chain-transfer agents but also either as inhibitors, in polymerization initiated with A 3, or as accelerators, in polymerizations initiated with A 4 . Thus, the kinetic curves are practically identical for different compositions of the A 3 /A 4 mixture in the feed, under the otherwise comparable conditions. At the early stages of polymerization the relative polymerization rate ( r p = −d ln [εCL]/d t ) increases, and at higher conversions, after complete consumption of the starting alcohol or diol, r p becomes constant; i.e. polymerization becomes first-order (internally) in monomer. These kinetic effects were explained by assuming the complete disruption of any aggregate present, followed by a coordination of the resulting nonaggregated (monomeric) Al−tris(macroalkoxide) active species with the low-molecular-weight alcohol or diol. Analysis of the 1 H and 27 Al NMR spectra of the polymerizing mixtures confirmed that both A 3 and A 4 are consumed quantitatively. This is in contrast to the polymerization conducted without alcohols, in which only A 3 is reacted fast, whereas A 4 mostly remains unreacted when the polymerization is over. Moreover, the 1 H NMR spectra show that the equilibrium >Al−O i Pr + HO(CH 2 ) 5 OH ⇌ >Al−O(CH 2 ) 5 −OH + i PrOH is shifted to the right-hand side. The kinetic scheme taking into account both propagation, P n * + εCL → and the temporary deactivation of the growing species, P n * + PD ⇌ P n *·PD (where the complex P n *·PD does not propagate), was proposed. A set of the resulting differential equations was solved numerically. The computed kinetic plots, ln([εCL] o /[εCL]) and [PD] vs time, reproduce well the respective plots obtained experimentally.