Molecular Structure-Catalytic Activity Relationship in the Ring-Opening Polymerization of (Macro)lactones

A kinetic analysis on the aluminum salen-catalyzed ring-opening polymerization (ROP) of (macro)lactones is presented, which focuses on how chain transfer agents and the steric hindrance at the alpha-methylene groups of both the growing chain and the (macro)lactones affect the polymerization rate. It is shown that for branched macrolactones the choice of initiator does not only influence the initiation rate but surprisingly also the apparent rate of polymerization. The increased polymerization rate, when an unbranched initiator was used, was ascribed to transesterification reactions taking place at the polymer chain end, which effectively results in an unprecedented chain growth at the other end of the polymer chain compared to normal ROP. Furthermore, application of a kinetic model including initiation, propagation, and transesterification led to the ability to accurately quantify the effect of steric hindrance for various alkoxide initiators by determining individual rate constants for initiation. It appeared that secondary alcohols not only have a lower reactivity but also are less likely to be bonded to the metal center than primary alcohols when an excess of the two types of alcohols (i.e., chain transfer agents) is used. For the strained seven-membered lactone it was shown that branching mainly has a retarding effect when present at the metal-bonded alkoxide and not on the monomer, which was ascribed to the cisoid conformation of the ester group of the monomer. On the other hand, the transoid conformation of the ester group in macrolactones resulted in a similar decrease in reactivity for branching on the metal-bonded alkoxide and the monomer. Next to this, it was shown that an excess of alcohol chain transfer agent had a retarding effect on the reaction, which was ascribed to coordination of the alcohol to the metal center.