Comparative Experimental and Theoretical Study of Mg, Al and Zn Aryloxy Complexes in Copolymerization of Cyclic Esters: The Role of the Metal Coordination in Formation of Random Copolymers

Polymers. 2020. Vol. 12. P. 2273.

Nifantiev I., Komarov P., Ovchinnikova V., Киселев А. В., Minyaev M., Ivchenko P.

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 (ВНТ)Mg(THF)2nBu (1), (ВНТ)2AlMe (2) and
[(ВНТ)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.

Keywords: catalysis, Copolymers