From syndiotactic homopolymers to chemically tunable alternating copolymers: highly active yttrium complexes for stereoselective ring-opening polymerization of β-malolactonates.

Alternating copolymers constitute an attractive class of materials. It was shown previously that highly alternated poly(β-hydroxyalkanoate)s (PHAs) can be prepared by ring-opening polymerization (ROP) of mixtures of two different enantiomerically pure 4-alkyl-β-propiolactones. However, the approach could not be extended to PHAs with chemically tunable functional groups, which is highly desirable to access original advanced materials. Reported herein is the first highly syndioselective and controlled ROP of racemic allyl and benzyl β-malolactonates (MLA(R); R=allyl, benzyl) using an yttrium complex supported by a tetradentate dichloro-substituted bis(phenolate) ligand. This highly active catalyst allows the nearly perfect alternating copolymerization of MLA(Allyl) and MLA(Benzyl). Hydrogenolysis of the benzyloxycarbonyl or functionalization of the allyl pendant groups opens a route towards a new class of functional alternating copolymers.