Poly(hydroxyalkanoate) Block or Random Copolymers of β‑Butyrolactone and Benzyl β‑Malolactone: A Matter of Catalytic Tuning

The controlled copolymerization of racemic β-butyrolactone (BL) and racemic benzyl β-malolactone (MLABe) has been achieved under mild operating conditions (in bulk at 60 °C) with various systems derived either from a metal-based (pre)catalyst associated with isopropanol ( i PrOH) acting as a co-initiator and a chain transfer agent or more simply from a neat basic organocatalyst. Among the metallic systems evaluated, only the neodymium triflate-based catalyst system, Nd(OTf)3/ i PrOH, enabled the preparation, upon simultaneous addition of the two monomers, of poly(benzyl β-malolactone-ran-β-butyrolactone) random copolymers (P(MLABe-ran-BL)) with M n,NMR up to 5800 g mol–1 (Đ M = ca. 1.4) as evidenced by 1H and 13C NMR. Simultaneous copolymerization of the comonomers mediated by the zinc β-diketiminate [(BDI)Zn{N(SiMe3)2}]/ i PrOH system only afforded PMLABe, leaving BL unreacted. Also, the sequential copolymerization with this zinc catalyst proceeded effectively only when BL was introduced prior to MLABe. In contrast, both the Nd-based system and basic organocatalysts of the guanidine (1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD), amidine (1,8-diazabicyclo[5.4.0]-undec-7-ene, DBU), and phosphazene (2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2diazaphosphorine, BEMP) type effectively copolymerized MLABe and BL in a sequential approach, regardless of the order of comonomers addition, forming the corresponding P(MLABe-b-BL) block copolymers, with segments of significant length (ca. 88 BL and 360 MLABe units; M n,NMR up to 73 500 g mol–1 with Đ M = 1.44). Remarkably, BEMP afforded P(MLABe-b-BL) from either a simultaneous or a sequential approach and regardless of the order of the comonomers addition. Kinetic and microstructural control in copolymerization of MLABe and BL can thus be achieved via catalytic tuning.