Poly(tetramethyl glycolide) from Renewable Carbon, a Racemization-Free and Controlled Depolymerizable Polyester

Abstract

The development of biomass-based polymers is one strategic step toward achieving a sustainable social system. A typical biomass-based polymer is poly(L-lactic acid) (PLLA), which through a combination of biological fermentation and chemical reaction can be synthesized from naturally abundant biomasses suchas starchor cellulose. PLLAshows goodphysical properties such as crystallinity, thermoplasticity, transparency, and a high melting point (Tm) of around 170 C. It also has the excellent quality of being easily reproduced from the depolymerization product: L,L-lactide. Hence, the likelihood that PLLA will become the plastic material of choice for sustainable systems has been attracting much interest from researchers. However, in practical applications, PLLA does have some drawbacks, such as slow crystallization, low impact resistance, hydrolyzability, and racemization. PLLA readily causes racemization from an L-unit to a D-unit in a chain under heating. Such racemization proceeds by the mechanism of ester-semiacetal tautomerization, causing a decrease in optical purity and crystallinity. This is a serious problem in the reproduction of practical materials via thermal depolymerization and repolymerization. A fundamental and complete solution to this problem requires a modification of the chemical structure of lactic acid. In this study, in order to overcome the problems associated with PLLA while preserving its superior properties, a biomass-based and racemization-free polymer: poly(tetramethyl glycolide) (PTMG) possessing superior depolymerizability for the reproduction is developed. Previously, PTMG has been synthesized from petroleum by wholly chemical processes involving the ring-opening polymerization of tetramethyl glycolide (TMG), which is a cyclic dimer of R-hydroxyisobutyric acid (HIBA). HIBA itself has also required preparation over many steps from petroleum using the cyanhydrin method for methyl methacrylate production. The methyl methacrylate production has been improved by some novel production processes such as the AVENEER method. Recently, a biosynthesis method of HIBA from renewable carbons has been achieved. PTMG shows a highTm at 185-190 C and a characteristic thermal degradability into methacrylic acid, TMG, acetone, etc. However, the derivation of PTMGfrombiomass and its controlled depolymerization into monomers, which will become required for many commonly used polymers in a future, are newly proposed in this study. Renewable resources: D-/L-lactic acids and pyruvic acid derived from biomasses are employed as starting materials for the synthesis of HIBA in this study, which is an acyclic monomer of PTMG. The biomass-based HIBA is prepared bymethylation of the acids and then converted into the cyclic dimer: TMGby a cyclic esterification. The following synthesis of polymer PTMG is carried out by a ring-opening polymerization of TMG. Moreover, the controlled depolymerization of PTMG is performed either to return toTMG or to convert to methacrylic acid depending on the use of a specific catalyst for each monomeric product. Two synthetic routes of HIBA from the renewable resources were performed. One was the direct methylation of D-/ L-lactic acid derivatives after the abstraction ofR-hydrogen on a chiral carbon: the other was themethylation of an R-keto group of apyruvic acid derivative by theGrignard reaction,which is an oxidized form of corresponding D-/L-lactic acid derivatives. Although the direct methylation has been reported in our recent research, themethylationof thepyruvic acidderivative is anew finding introduced in this study. Results of the methylation are listed in Table 1. The methylation of methyl pyruvate by the Grignard reaction showeda50%yield at room temperature.On the other hand, the direct methylation of the hydroxyl-group protected ethyl D-/L-lactate (HPEL), with protection provided by a methoxymethyl group, gave relatively high yields of 5475%at-84 C.Thesemethylation reactionshave the advantage of using multiple renewable resources: pyruvic acid and L-, D-, and D-/L-lactic acids without the need of high optical purity. HIBA was obtained by the hydrolytic deprotection of methylated products with high yields of >74%. The cyclic dimerization ofHIBAproceeded smoothly in the presence of the dehydration catalyst methane: sulfonic acid to isolate the cyclic dimer: TMG in a 67% yield. The ring-opening anionic polymerization of TMG that followedwas achieved by using three initiators: EtOLi, n-BuLi, and t-BuLi, resulting in the preparation of a high molecular weight PTMG (Mn 90 000) as shown in Table S1. Previously, Deibig et al. showed Tm of PTMG in a range of 180-190 C, but no glass transition (Tg) temperature was reported. The isolated PTMG showed Tm and Tg at 191 and 70 C (Figure 1), respectively, about 15 C higher than those of PLLA. TheTg transition signal was very weak, and theTm peak shifted into higher temperatures of up to 206 C with a corresponding increase in the heat treatment temperature. The weak Tg signal and high Tm value of PTMG suggest superior crystallization and heat resistance, respectively. Scheme 1. Total Synthetic Processes of PTMG and Its Depolymerization