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 α-chloro-ε-caprolactone ( 1 , αClεCL), 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 PαClεCL 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 PαClεCL. The Al Catalysts 5 and 6 are highly active ROP catalysts of αClεCL at room temperature (TOF up to 2,400 hr⁻¹) to afford well-defined P(αClεCL). In the case of Catalyst 6 , carrying out the ROP of αClεCL 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-PαClεCL diblocks through sequential ROP of l -lactide and αClεCL, affording copolymers with improved thermal and biodegradable properties.