Letter pubs.acs.org/macroletters
Silicon Acetal Metathesis Polymerization Ertugrul Sahmetlioglu,† Ha Thi Hoang Nguyen, Olivier Nsengiyumva, Ersen Göktürk,‡ and Stephen A. Miller* The George and Josephine Butler Laboratory for Polymer Research, Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States S Supporting Information *
ABSTRACT: A kinetic study revealed that the acid-catalyzed (p-TSA) equilibration of Me2Si(OMe)2 and Me2Si(OEt)2, forming Me2Si(OEt)OMe, is established in 300 min in benzene at room temperature. This silicon acetal metathesis reaction is exploited for the step-growth polymerization of bissilicon acetals (MeOSiMe2OROSiMe2OMe) with metathetical loss of Me2Si(OMe)2. Thus, a convenient and generalized silicon acetal metathesis polymerization (SAMP) method is introduced as the acid-catalyzed copolymerization of a diol (HOROH) and Me2Si(OMe)2, driven by elimination of methanol and/or Me2Si(OMe)2. SAMP constitutes an effective and powerful strategy for manipulating the most common bond in the Earth’s crust, the silicon−oxygen bond.
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eliminating HCl (Figure 1, top). Thus, polysilicon acetals with relatively low molecular weight (Mn ∼ 2000) have been
ilicon is the second most abundant element in the Earth’s crust (28% by mass) after oxygen (46%).1 The silicone polydimethylsiloxane (PDMS) is 38% silicon and 22% oxygen and is the most commonly used silicon-containing polymer, having gained considerable attention since the 1940s. PDMS has repeat units of [−OSiMe2−] with only Si−O bonds along the backbone.2 PDMS can be used for many applications, including adhesives, coatings, contact lenses, biomedical devices, lubricating oils, and elastomers.2−5 Important and specific attributes include low glass transition temperature, high conformational flexibility, biocompatibility, high gas permeability, hydrophobicity, and good oxidative, thermal, and UV light stability.6,7 Although PDMS has such valuable characteristics, its relatively poor mechanical properties prevent it from replacing high-volume commodity plastics (e.g., polyethylene, polypropylene, polyvinyl chloride) in a wider variety of consumer applications. Therefore, the inexhaustible supplies of silicon and oxygen should be transformed into new polymers with improved and tunable thermomechanical properties.8,9 Moreover, with the increasing availability of plant-based polymer building blocks, there exists an opportunity to fashion these new polymers, stated tersely, from plants and sand. It is clear that adjusting the basic structure of siliconcontaining polymers has a profound influence on their thermomechanical properties.10 Distinct from silicones such as PDMS, polysilicon acetals are polymers containing silicon acetal units (−OSiR2O−) connected by hydrocarbon segments (−R′−), having the generalized structure [−OSiR2OR′−].10 While several polysilicon acetals have appeared in the patent literature,11 their thermal properties and other characterizations are rarely delineated. The conventional condensation polymerization protocol combines dichlorodimethylsilane with a diol, © XXXX American Chemical Society
Figure 1. Conventional synthesis of polysilicon acetals from diols via HCl elimination (top). Silicon acetal metathesis polymerization (SAMP) is an alternative strategy that eliminates volatile silicon acetals from bis-silicon acetals (bottom).
reported.12 While this methodology is somewhat effective, it yields stoichiometric amounts of a corrosive acid (HCl) typically passivated by inclusion of a base, which is not atom economical. Hence, we aspired to develop families of sustainable polysilicon acetals via a novel methodology that did not Received: February 2, 2016 Accepted: February 19, 2016
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DOI: 10.1021/acsmacrolett.6b00095 ACS Macro Lett. 2016, 5, 466−470
Letter
ACS Macro Letters directly expel deleterious byproducts. Interchange reactions of the related siloxane functional group (−SiR2OSiR2−) are known to equilibrate linear and cyclic polysiloxanes of various sizes, especially in the presence of a catalyst.13 Thus, in analogy to olefin metathesis,14 acetal metathesis,15 carbonate metathesis,16 or oxalate metathesis17 strategies for polymerization, it seemed that silicon acetal metathesis polymerization (SAMP)18 would be a viable approach for condensing bifunctional monomers to high molecular weight polymers (Figure 1, bottom). In this case, bis-silicon acetals are interchanged, and the step-growth condensation polymerization occurs with elimination of a volatile silicon acetal. The kinetics of silicon acetal metathesis were investigated by reacting symmetrical silicon acetals Me2Si(OMe)2 and Me2Si(OEt)2 with and without added catalyst. In benzene-d6 without added catalyst, a 1:1 ratio of these symmetrical silicon acetals exchanged slowly at ambient temperature, yielding only 16% of the mixed silicon acetal Me2Si(OEt)OMe after 4 days. However, with added p-toluenesulfonic acid (p-TSA) catalyst (
