Cyclic and Linear Polyhydroxylbutyrates from Ring-Opening Polymerization of β-Butyrolactone with Amido-Oxazolinate Zinc Catalysts

Abstract

A series of amido-oxazolinate zinc complexes have been employed for the ring-opening polymerization of rac-β-butyrolactone (BBL). Experimental results show that these complexes efficiently catalyze the reactions, yielding cyclic poly­(3-hydroxybutyrate)­s (PHBs) with high molecular weights (M n up to 197 kg/mol) and relatively low dispersity. In comparison, in the presence of alcohol cocatalysts, the zinc-catalyzed ROP reactions lead to the formation of linear PHBs end-capped by the alcohol initiator and hydroxylbutyrate. A possible mechanism for cyclic PHBs is proposed, in which the zinc catalysts function as a loose Lewis pair at elevated temperature, followed by a fast propagation through a zwitterionic intermediate. Use of diols such as 1,4-cyclohexanediol and 1,4-benzenedimethanol results in the formation of telechelic polyester diols. The thermal analysis by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) shows higher thermal resistance of cyclic PHBs than linear ones.

Cyclic Polyester Formation with an [OSSO]‐Type Iron(III) Catalyst

“…[8] However,t he formation of cyclic polyesters remains a challenge. [27,28,35] Under these conditions, no product was detected by using 1 HNMR spectroscopy ( Table 1, entries 1a nd 2). Indeed,t he biodistribution of cyclic aliphatic polyesters (cAPEs)i sd ifferent from that of linear APEs, [10] which offers an opportunity to develop new drug deliverys ystems.…”

“…[27,28,35] Under these conditions, no product was detected by using 1 HNMR spectroscopy ( Table 1, entries 1a nd 2). At the beginning, we explored rac-LA polymerization in THF and toluene solutions following the standard procedures reported previously.…”

“…[5] Amongo thers, polymers with ac yclic topologya re an intriguing synthetic target because the lack of chain-ends and the high conformational constraintl ead to materialp roperties that are different from their linear analogues. [28] In recent years,wehave developed anumber of Fe catalystsf or the activation of CO 2 and oxiranes for the production of carbonates [29][30][31] anda chieved the production of aliphatic polycarbonates (APCs) by using an [OSSO]-typel igand system. [8] However,t he formation of cyclic polyesters remains a challenge.…”

“…[9] Such materials are highly attractive foru se in spe-cialized fields such as biomedicine. [28] In recent years,wehave developed anumber of Fe catalystsf or the activation of CO 2 and oxiranes for the production of carbonates [29][30][31] anda chieved the production of aliphatic polycarbonates (APCs) by using an [OSSO]-typel igand system. In this regard, the use of ac atalytic system based on ab iocompatiblem etal such as Fe would be of great interest because it would reduce hazards derived from metal residues in the final product.…”

“…In this regard, the use of ac atalytic system based on ab iocompatiblem etal such as Fe would be of great interest because it would reduce hazards derived from metal residues in the final product. [28] In recent years,wehave developed anumber of Fe catalystsf or the activation of CO 2 and oxiranes for the production of carbonates [29][30][31] anda chieved the production of aliphatic polycarbonates (APCs) by using an [OSSO]-typel igand system. Actually, the number of reports in which the production of cAPEs by means of aw ell-defined metal complex is described is limited, [24] and mosti nvolve the use of Al- [25][26][27] and Zn-based catalysts.…”