Cyclic ethylene phosphates with (CH2)nCOOR and CH2CONMe2 substituents: synthesis and mechanistic insights of diverse reactivity in aryloxy-Mg complex-catalyzed (co)polymerization

Abstract

A novel pathway is proposed for the formation of branched poly(ethylene phosphate)s during the coordination ring-opening polymerization (ROP) of ethylene phosphates catalyzed by a 2,6-di-tert-butyl-4-methylphenoxy (BHT) magnesium complex, [(BHT)Mg(μ-OBn)(THF)]₂ (Mg1). Electron-donating fragments in the substituents at the P atom of the ethylene phosphates and the stability of chelate Mg complexes were hypothesized to drive the formation of ethylene phosphate macromonomers. This hypothesis was validated through a comparative experimental and theoretical study of the ROP reactivity of the ethylene phosphates with –O(CH₂)_nCOOMe (n = 1–3, 5), –CH₂COO^tBu, –OCHMeCOOMe, and –OCH₂CONMe₂ substituents (1–7). In good agreement with the results of theoretical modeling, glycolate- and lactate-substituted ethylene phosphates 1, 5, and 6 formed highly branched polymers, and the participation of the BHT-Mg glycolate intermediate in ROP was confirmed by the presence of –P(O)(OCH₂COOMe)₂ fragments in the homopolymer of 1. Notably, the reaction of amide-substituted 7 with Mg1 quantitatively afforded benzyl ethylene phosphate without subsequent ROP. The copolymerization of 1 with ethyl ethylene phosphonate furnished branched polymers, the architectures of which were determined by spectral and rheological studies. Consequently, new –OCH₂COX substituted cyclic phosphates can be used as efficient branching (X = OR) or chain termination (X = NR₂) agents.