Controlled and highly effective ring-opening polymerization of α-chloro-ε-caprolactone using Zn- and Al-based catalysts

The present study details the highly effective and controlled ring-opening polymerization (ROP) of alpha-chloro-epsilon-caprolactone (1, alpha Cl epsilon CL), a cyclic ester that has been little explored thus far in ROP catalysis, using Zn- and Al-based catalysts [Zn(C6F5)(2)(toluene)] (4), [N,N '-bis(3,5-di-tert-butylsalicylidene)1,3-diaminopropanato]aluminium(III)benzyloxide (5) and [N,N '-bis(3,5-di-tert-butylsalicylidene)1,3-diamino-2,2 '-dimethylpropanato]aluminium(III)benzyloxide] (6). Chain-length-controlled P alpha Cl epsilon CL material is produced under solution ROP conditions, as deduced from GPC, NMR, MALDI-TOF, and kinetic data. In contrast, the ROP of 1 is ill-defined under bulk ROP conditions due to partial thermal degradation of the polymer chain (presumably through C-Cl cleavage), reflecting the limited stability of P alpha Cl epsilon CL. The Al Catalysts 5 and 6 are highly active ROP catalysts of alpha Cl epsilon CL at room temperature (TOF up to 2,400 hr(-1)) to afford well-defined P(alpha Cl epsilon CL). In the case of Catalyst 6, carrying out the ROP of alpha Cl epsilon CL under immortal conditions (with BnOH as chain transfer agent) is clearly beneficial to ROP activity and control, with no apparent side-reaction of chloro-functionalized PCL chains as the ROP proceeds. The controlled character of these ROPs was further exploited for the production of chain-length-controlled PLLA-b-P alpha Cl epsilon CL diblocks through sequential ROP of l-lactide and alpha Cl epsilon CL, affording copolymers with improved thermal and biodegradable properties.