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 (BPLORs) by yttrium‐bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPLORs of opposite configurations, namely (R)‐BPLOR1/(S)‐BPLOR2 [R1, R2=OMe, OAllyl, OCH2Ph (=OBn), OSiMe2tBu (=OTBDMS)], by the syndioselective Y{ONOOcum}/iPrOH catalyst/initiator system affords P(HBOR1‐alt‐HBOR2) copolymers with high alternation degrees (altern.=89–94 %), as determined by comprehensive kinetic, 13C{1H} NMR spectroscopy, MALDI‐ToF MS and ESI MS/MS fragmentation studies. The ROCOP of the (R)‐BPLOMe/(S)‐BPLOTBDMS 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{ONNOCl}/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)‐BPLOAll/(S)‐BPLOMe into a 1:1 mixture of the corresponding isotactic iso‐(R)‐PHBOAll and iso‐(S)‐PHBOMe homopolymers; almost no copolymerization defects are observed. This new approach has been extended successfully to the ROCOP of equimolar mixtures of racemic monomers, rac‐BPLOAll/rac‐BPLOMe. That′s Y! Highly alternated copolymers or mixtures of isotactic enantiopure homopolymers can be synthesized on purpose from equimolar mixtures of two chemically different β‐lactone monomers with opposite configuration, upon using a syndioselective or an isoselective yttrium‐based catalyst, respectively.