Review pubs.acs.org/IECR
New Chemistry in Functional Aliphatic Polyesters Rong Tong* Department of Chemical Engineering, Virginia Polytechnic Institute and State University, 635 Prices Fork Road, Blacksburg, Virginia 24061, United States ABSTRACT: Aliphatic polyesters are important biodegradable polymers with wide applications. Polyesters prepared by ring-opening polymerization often have a limited range of properties, because of the minimal functional diversity of available lactone monomers. In this review, synthetic strategies in preparing functional lactone monomers are highlighted, as well as recent controlled polymerization strategies to synthesize functional aliphatic polyesters, which include proton-transfer polymerization, ring opening polymerization of O-carboxyanhydrides, radical ring-opening polymerization of cyclic ketene acetals, and copolymerization of epoxide/anhydride.
1. INTRODUCTION Polyesters can generally be divided into semiaromatic and aliphatic polyesters. Semiaromatic polyesters such as poly(ethylene terephthalate) are used in a variety of bulk applications, because of their good mechanical strength and other characteristics, which will not be discussed here. Aliphatic polyesters are widely used in everyday applications ranging from clothing and packaging to agriculture and biomedicine.1−7 They are regarded as potentially sustainable alternatives to petroleum-based polymers, because of their numerous renewable sources, hydrolytic degradation to benign products, and high biocompatibility.8 One of the most common routes to polyesters in industry is the step-growth polymerization of diacids or diesters with diols. However, such method is energy-intensive because a smallmolecule byproduct, water or an alcohol, is removed at high temperature. In addition, this polymerization usually have a broad molecular weight distribution (dispersity (Đ) or Mw/Mn, where Mw is the weight-average molecular weight and Mn the number-average molecular weight), ∼2.9,10 Alternatively, aliphatic polyesters are preferred to be prepared through chain-growth ring-opening polymerization (ROP) of lactones.11 Such polymerization has been studied extensively, and a wide variety of initiators, including organocatalysts, metal alkoxides, and various metal complexes, has been explored and reviewed elsewhere (Figure 1).12−21 However, polyesters prepared by this method often have a limited range of properties,
because of the minimal functional diversity of available lactones, which further prevents the post-polymerization functionalization. Besides the ROP of lactones, other polymerization pathways such as proton-transfer polymerization, ROP of O-carboxyanhydrides, radical ROP of cyclic ketene acetals, and copolymerization of epoxide/anhydride have been recently developed to prepare functional polyesters. In this review, I will not only discuss the preparation of functional lactone monomers but also highlight recent strategies to synthesize functional aliphatic polyesters via controlled polymerization.
2. FUNCTIONAL LACTONES FOR RING-OPENING POLYMERIZATION One general strategy to prepare functional polyesters is to polymerize functional lactone monomers. Prior to the discussion of synthetic strategies, it is worth mentioning that the thermodynamic polymerizability of lactones is strongly related to ring size; and computational studies show that ring strain is highest for β-propiolactone (4-membered ring) and much lower for γ-butyrolactone (5-membered ring) and δ-valerolactone (6-membered ring).22 This explains that only a limited number of 5-membered lactone monomers were reportedly used for ROP.23 Notably, the modification of polyester by transforming functional groups post-polymerization has been extensively reviewed24−27 and will not be discussed here. 2.1. β-Lactone. β-lactone can usually be prepared via the catalytic asymmetric [2 + 2] cycloadditions of ketenes and aldehydes,28−32 which, in most cases, require the isolation of enolate equivalents such as silyl ketene acetals.33 Carbonylation of epoxides using catalysts of the form [metal-based Lewis acid]+[Co(CO)4]− has recently emerged as a reliable direct Received: Revised: Accepted: Published:
Figure 1. Ring-opening polymerization of lactone and lactide. © XXXX American Chemical Society
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February 6, 2017 March 22, 2017 March 23, 2017 March 23, 2017 DOI: 10.1021/acs.iecr.7b00524 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Review
Industrial & Engineering Chemistry Research
Figure 2. Regioselective synthesis of β-lactone via the carbonylation of epoxide.
approach to β-lactones when using terminal or symmetrically 2,3-disubstituted epoxides as substrates (Figure 2).34,35 Regioselective carbonylation of asymmetric disubstituted epoxides can also be achieved to prepare either cis- or trans-β-lactones using the same catalyst system.36,37 2.2. Lactide. Homobifunctional lactide can be prepared via the dimerization of functional α-hydroxyl acids catalyzed by p-toluenesulfonic acid under reflux (Figure 3a).38−41 The yields of these lactides were relatively low (
