Biodegradable polyesters represent an advanced alternative to polyolefin plastics in various applications. Polybutylene adipate terephthalate (PBAT) can compete with polyolefins in terms of their mechanical characteristics and melt processing conditions. The properties of PBAT depend on the molecular weight, dispersity, and architecture of the copolymer. Long-chain branching (LCB) of the PBAT backbone is an efficient method for the improvement of the copolymer characteristics. In the present work, we studied branching agents (BAs) 1–7 of different structures in the two-stage polycondensation of 1,4-butanediol, dimethyl terephthalate, and adipic acid and investigated the composition and melt rheology of the copolymers. According to the results of the research, 1,1,1-tris(hydroxymethyl)ethane 2 and 3-hydroxy-2-(hydroxymethyl)-2-methylpropanoic acid 5 outperformed glycerol 1 as BAs in terms of shear thinning behavior and viscoelasticity.

We report here that the heteroleptic BHT-Mg(OR) complex, i.e. [(BHT)Mg(OBn)(THF)]₂ (Mg3), represents an effective and versatile ring-opening polymerisation (ROP) catalyst for several cyclic ethylene phosphate monomers (CEPMs), such as ethylene phosphates with methoxy- (MeOEP, 1), isopropoxy- (ⁱPrOEP, 2), and tert-butoxy- (^tBuOEP, 3) substituents, N,N-diethyl ethylene phosphoramidate (Me₂NEP, 4) as well as ethyl (EtPPn, 5) and tert-butyl (^tBuPPn, 6) ethylene phosphonates. Mg3 retains its catalytic activity over a broad temperature range from −50 to 100 °C and efficiently carries out the fast and controlled polymerisation of CEPMs with sterically unhindered alkoxy groups with less than 1% chain branching even at near-complete conversion. Compared with ROP catalysed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), Mg3 performs much better in the polymerisation of CEPMs with bulky groups and of monomers sensitive to strong nucleophiles, such as ^tBuOEP.

Homogeneity of copolymers is a general problem of catalytic coordination polymerization. In ring-opening polymerization of cyclic esters, the rational design of the catalyst is generally applied to solve this problem by the equalization of the reactivities of comonomers—however, it often leads to a reduction of catalytic activity. In the present paper, we studied the catalytic behavior of BnOH-activated complexes (BHT)Mg(THF)₂ⁿBu (1), (BHT)₂AlMe (2) and [(BHT)ZnEt]₂ (3), based on 2,6-di-tert-butyl-4-methylphenol (BHT-H) in homo- and copolymerization of L-lactide (lLA) and ε-caprolactone (εCL). Even at 1:5 lLA/εCL ratio Mg complex 1 catalyzed homopolymerization of lLA without involving εCL to the formation of the polymer backbone. On the contrary, Zn complex 3 efficiently catalyzed random lLA/εCL copolymerization; the presence of mono-lactate subunits in the copolymer chain clearly pointed to the transesterification mechanism of copolymer formation. Both epimerization and transesterification side processes were analyzed using the density functional theory (DFT) modeling that confirmed the qualitative difference in catalytic behavior of 1 and 3: Mg and Zn complexes demonstrated different types of preferable coordination on the PLA chain (k² and k³, respectively) with the result that complex 3 catalyzed controlled εCL ROP/PLA transesterification, providing the formation of lLA/εCL copolymers that contain mono-lactate fragments separated by short oligo(εCL) chains. The best results in the synthesis of random lLA/εCL copolymers were obtained during experiments on transesterification of commercially available PLLA, the applicability of 3/BnOH catalyst in the synthesis of random copolymers of εCL with methyl glycolide, ethyl ethylene phosphonate and ethyl ethylene phosphate was also demonstrated.