Article pubs.acs.org/Macromolecules
Crystalline Polythiocarbonate from Stereoregular Copolymerization of Carbonyl Sulfide and Epichlorohydrin Tian-Jun Yue, Wei-Min Ren,* Ye Liu, Zhao-Qian Wan, and Xiao-Bing Lu State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China S Supporting Information *
ABSTRACT: Carbonyl sulfide (COS) as a carbon source for copolymerization with epoxides has recently received some attention. The introduction of sulfur atom can provide enhancement of important polymer properties compared to the corresponding copolymer from CO2. However, the synthesized copolymers are all amorphous, therefore hindering them to be used as structural materials. Herein, we report the synthesis and characterization of semicrystalline poly(thiocarbonate)s derived from enantiopure epichlorohydrin and COS employing the single-site bifunctional catalyst. The catalyst shows excellent regioselectivity for epichlorohydrin ring-opening at methylene carbon. The copolymerization mechanism has been studied by means of NMR and ESI-MS methods. It is found the reaction temperature plays an important role in the crystallization behavior of the resultant copolymers. That is, at ambient temperature the propagating monothiocarboxylate species favors the nucleophilic attack at the chloromethylene of epichlorohydrin to form an epoxy ring end group, along with the release of chloride ion as a new initiator. This chain termination results in low molecular weight and board distributed copolymers, in accordance with the amorphousness. Alternatively, at reduced temperature such as −25 °C, the monothiocarboxylate species prefers consecutive alternating enchainment of COS and epichlorohydrin to give copolymers with enhanced molecular weights. Of importance, the formed polymer is a typical semicrystalline thermoplastic, possessing a Tg of 15.6 °C and a Tm of 96.7 °C.
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INTRODUCTION Recent attention has focused on the synthesis of poly(thiocarbonate)s as analogues of polycarbonates only with the substitution of sulfur atom(s) for oxygen atom(s) in the main chains. The introduction of sulfur atoms confers attractive features, such as high optical property, remarkable chemical resistance, and excellent heavy-metal capture ability.1,2 For example, polytrithiocarbonate from CS2 with episulfide has an enhanced refractive index compared with the corresponding polycarbonate.3 However, the copolymerization of CS2 with epoxides or oxetane generally suffered from multiple products due to the oxygen−sulfur exchange reaction as well as the concomitant cyclic products.4 Alternatively, the copolymerization of epoxides with carbonyl sulfide (COS, a greenhouse gas, produced from the burning of fossil fuels, volcanic eruption, and animal decay) in the presence of binary or bifunctional catalyst system-based salenCr(III)X complexes was shown to selectively afford the completely alternating copolymers with high molecular weight and narrow monodispersity.5,6 The Cr(III)-based catalyst system in some cases allows for the regioselective polymerization, resulting in the well-defined poly(monothiocarbonate)s with 98 to >99% head-to-tail linkages.7 As a result, the isotactic copolymer could be formed when enantiopure epoxide was employed in the reaction. Unfortunately, no crystallization behavior was observed, therefore hindering them to use as structural materials. On the other hand, epichlorohydrin as an important functionalized material has been widely used in the chemical © XXXX American Chemical Society
industry for preparing epoxy resin, adhesive, and pharmaceutical intermediates.8 It has been also used in the copolymerization with cyclic anhydrides to give the corresponding polyesters.9 More often, epicholorihydrin is an ideal epoxide in the coupling reaction with CO2 as a result of its high reactivity, whereas the cyclic carbonate rather than the polycarbonate usually is the main product.10 In 2011, we succeeded in selectively obtaining the completely alternating polycarbonates from the CO2/epichlorohydrin coupling with the use of a bifunctional Co(III)-based catalyst system.11 Subsequently, a slight adjustment for the binary catalyst by enhancing the steric hindrance around central metal and the cation of the appended ammonium salts resulted in a highly regioregular ring-opening of enantiopure epichlorohydrin, where a typical semicrystalline polymer with a Tg of 42 °C and a T m of 108 °C was produced. 12 As for the copolymerization of COS and epichlorohydrin, however, the regio- and stereoselectivity may be more complicated due to the asymmetric characteristic of COS (Scheme 1). In addition, the sulfur atom, with enhanced nucleophilicity compared with oxygen atom, may cause multiple reactions through attack at the different carbon atoms of epichlorohydrin. Therefore, the synthesis of isotactic polythiocarbonate from epichlorohydrin still remains a challenge. The further exploration on the Received: February 4, 2016 Revised: March 31, 2016
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DOI: 10.1021/acs.macromol.6b00272 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Stereochemsitry Involved in the Enantiopure Epichlorohydrin Ring-Opening during the Copolymerization with COS
entry 1). The resultant poly(chloropropylene monothiocarbonate) has a completely alternating structure, and no oxygen− sulfur exchange reaction occurred. Unfortunately, the enantiomeric excess of the unconverted epichlorohydrin was very low (
