Phenoxide and alkoxide complexes of Mg, Al and Zn, and their use for the ring-opening polymerization of ℇ-caprolactone with initiators of different natures

Functionalized Biodegradable Polymers via Termination of Ring-Opening Polymerization by Acyl Chlorides

Aliphatic polyesters are an important class of polymeric materials for biomedical applications due to their versatile and tunable chemistry, biocompatibility and biodegradability. A capability of direct bonding with biomedically significant molecules, provided by the presence of the reactive end functional groups (FGs), is highly desirable for prospective polymers. Among FGs, N-hydroxysuccinimidyl activated ester group (NHS) and maleimide fragment (MI) provide efficient covalent bonding with -NH- and -SH containing compounds. In our study, we found that NHS- and MI-derived acyl chlorides efficiently terminate living ring-opening polymerization of ε-caprolactone, L-lactide, ethyl ethylene phosphonate and ethyl ethylene phosphate, catalyzed by 2,6-di-tert-butyl-4-methylphenoxy magnesium complex, with a formation of NHS- and MI-functionalized polymers at a high yields. Reactivity of these polymers towards amine- and thiol-containing model substrates in organic and aqueous media was also studied.

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

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.

Synthesis of a mononuclear magnesium bis(alkoxide) complex and its reactivity in the ring-opening copolymerization of cyclic anhydrides with epoxides

Synthesis of a new mononuclear magnesium complex with a bulky bis(alkoxide) ligand environment and its reactivity in ring-opening polymerization (ROP) and ring-opening copolymerization (ROCOP) are reported. Reaction of n-butyl-sec-butylmagnesium with two equivalents of HOR (HOR = di-tert-butylphenylmethanol, HOCtBu2Ph) formed Mg(OR)2(THF)2. The reaction proceeded via the Mg(OR)(sec-Bu)(THF)2 intermediate that was independently synthesized by treating n-butyl-sec-butylmagnesium with one equivalent of HOR. Mg(OR)2(THF)2 led to active albeit not well-controlled ROP of rac-lactide. In contrast, well-controlled ROCOP of epoxides with cyclic anhydrides was observed, including efficient and alternating copolymerization of phthalic anhydride with cyclohexene oxide as well as rare copolymerization of phthalic anhydride with limonene oxide and terpolymerization of phthalic anhydride with both cyclohexene oxide and limonene oxide. In addition, novel copolymerization of dihydrocoumarin with limonene oxide is described.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

Ring-opening polymerization (ROP) of cyclic esters (lactones, lactides, cyclic carbonates and phosphates) is an effective tool to synthesize biocompatible and biodegradable polymers. Metal complexes effectively catalyze ROP, a remarkable diversity of the ROP mechanisms prompted the use of density functional theory (DFT) methods for simulation and visualization of the ROP pathways. Optimization of the molecular structures of the key reaction intermediates and transition states has allowed to explain the values of catalytic activities and stereocontrol events. DFT computation data sets might be viewed as a sound basis for the design of novel ROP catalysts and cyclic substrates, for the creation of new types of homo- and copolymers with promising properties. In this review, we summarized the results of DFT modeling of coordination ROP of cyclic esters. The importance to understand the difference between initiation and propagation stages, to consider the possibility of polymer-catalyst coordination, to figure out the key transition states, and other aspects of DFT simulation and visualization of ROP have been also discussed in our review.

DFT Visualization and Experimental Evidence of BHT-Mg-Catalyzed Copolymerization of Lactides, Lactones and Ethylene Phosphates

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.

(1R,2S,4r)-1,2,4-Tri-phenyl-cyclo-pentane-1,2-diol and (1R,2S,4r)-4-(2-meth-oxy-phen-yl)-1,2-di-phenyl-cyclo-pentane-1,2-diol: application as initiators for ring-opening polymerization of ∊-caprolactone

Reductive cyclization of 1,3,5-triphenyl- and 3-(2-meth-oxy-phen-yl)-1,5-di-phenyl-pentane-1,5-diones by zinc in acetic acid medium leads to the formation of 1,2,4-tri-phenyl-cyclo-pentane-1,2-diol [1,2,4-Ph3C5H5-1,2-(OH)2, C23H22O2, (I)] and 4-(2-meth-oxy-phen-yl)-1,2-di-phenyl-cyclo-pentane-1,2-diol [4-(2-MeOC6H4)-1,2-Ph2C5H5-1,2-(OH)2, C24H24O3, (II)]. Their single crystals have been obtained by crystallization from a THF/hexane solvent mixture. Diols (I) and (II) crystallize in ortho-rhom-bic (Pbca) and triclinic (P ) space groups, respectively, at 150 K. Their asymmetric units comprise one [in the case of (I)] and three [in the case of (II)] crystallographically independent mol-ecules of the achiral (1R,2S,4r)-diol isomer. Each hydroxyl group is involved in one intra-molecular and one inter-molecular O-H⋯O hydrogen bond, forming one-dimensional chains. Compounds (I) and (II) have been used successfully as precatalyst activators for the ring-opening polymerization of ∊-caprolactone.

Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters

This review focuses on recent developments concerned with the use of well-defined main group complexes as (pre-)catalysts for the ROP of cyclic esters to give aliphatic polyesters; factors influencing catalytic activity, selectivity and polymer properties are all discussed.