Yttrium-Mediated Ring-Opening Copolymerization of Oppositely Configurated 4-Alkoxymethylene-β-Propiolactones: Effective Access to Highly Alternated Isotactic Functional PHAs.

The ring-opening copolymerization (ROCOP) of functional 4-alkoxymethylene-β-propiolactones (BPL s) by yttrium-bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPL s of opposite configurations, namely (R)-BPL /(S)-BPL [R , R =OMe, OAllyl, OCH Ph (=OBn), OSiMe tBu (=OTBDMS)], by the syndioselective Y{ONOO }/iPrOH catalyst/initiator system affords P(HB -alt-HB ) copolymers with high alternation degrees (altern.=89-94 %), as determined by comprehensive kinetic, C{ H} NMR spectroscopy, MALDI-ToF MS and ESI MS/MS fragmentation studies. The ROCOP of the (R)-BPL /(S)-BPL combination, featuring a large difference in the substituents' bulkiness, constitutes the only observed exception to this trend (altern.=64 %). On the other hand, the isoselectivity of the Y{ONNO }/iPrOH catalyst/initiator system has been exploited to generate, in a one-pot/one-step procedure, original mixtures of isotactic poly(hydroxyalkanoate)s (PHAs). This system efficiently transforms equimolar mixtures of (R)-BPL /(S)-BPL into a 1:1 mixture of the corresponding isotactic iso-(R)-PHB and iso-(S)-PHB homopolymers; almost no copolymerization defects are observed. This new approach has been extended successfully to the ROCOP of equimolar mixtures of racemic monomers, rac-BPL /rac-BPL .