Abstract
Catalytic ring-opening polymerization (ROP) of cyclic esters (lactides, lactones) and cyclic ethylene phosphates is an effective way to process materials with regulated hydrophilicity and controlled biodegradability. Random copolymers of cyclic monomers of different chemical nature are highly attractive due to their high variability of characteristics. Aryloxy-alkoxy complexes of non-toxic metals such as derivatives of 2,6-di-tert-butyl-4-methylphenoxy magnesium (BHT-Mg) complexes are effective coordination catalysts for homopolymerization of all types of traditional ROP monomers. In the present paper, we report the results of density functional theory (DFT) modeling of BHT-Mg-catalyzed copolymerization for lactone/lactide, lactone/ethylene phosphate and lactide/ethylene phosphate mixtures. ε-Caprolactone (ε-CL), l-lactide (l-LA) and methyl ethylene phosphate (MeOEP) were used as examples of monomers in DFT simulations by the Gaussian-09 program package with the B3PW91/DGTZVP basis set. Both binuclear and mononuclear reaction mechanistic concepts have been applied for the calculations of the reaction profiles. The results of calculations predict the possibility of the formation of random copolymers based on l-LA/MeOEP, and substantial hindrance of copolymerization for ε-CL/l-LA and ε-CL/MeOEP pairs. From the mechanistic point of view, the formation of highly stable five-membered chelate by the products of l-LA ring-opening and high donor properties of phosphates are the key factors that rule the reactions. The results of DFT modeling have been confirmed by copolymerization experiments.
Highlights
Biodegradable and biocompatible polymers have currently received great attention in industrial, biomedical and pharmaceutical applications [1,2,3,4,5,6,7,8]
As was demonstrated in our previous work [31], binuclear reaction mechanism was preferable for homopolymerization of both ε-CL and l-LA
For the simulations of ε-CL/l-LA copolymerization, we took into account the relatively high stability of binuclear chelates formed by (BHT)Mg and lactate fragments that was experimentally proved by the X-ray diffraction method [31]
Summary
Biodegradable and biocompatible polymers have currently received great attention in industrial, biomedical and pharmaceutical applications [1,2,3,4,5,6,7,8]. Catalytic ring-opening polymerization of lactones, lactides, cyclic carbonates and phosphates (Scheme 1a) is the main synthetic approach to these materials with their given hydrophilicity, biodegradability and mechanical properties [2,9,10,11,12]. Applied in thehomopolymerization mechanistic study ofofcoordination homopolymerization lactones [21,22,23,24,25,26,27,28,29,30,31,32,33], lactides and phosphates [51]. Polymers 2019, 10, x FOR PEER REVIEW coordination lactones [21,22,23,24,25,26,27,28,29,30,31,32,33], lactides [31,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48], of cyclic carbonates [30,42,49,50]
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