High-Throughput Synthesis and Characterization of Aryl Silicones by Using the Piers-Rubinsztajn Reaction.

Arylsilicones are widely exploited for their thermal and optical properties. The creation of phenylsilicone elastomers with specific physical properties is typically done by a "one-off" formulation and test process. Herein, it is demonstrated that high-throughput synthesis methods can be used to rapidly prepare a series of arylsilicone elastomers and then the relative impact of different aryl groups on their physical properties is assessed. Aromatic groups were incorporated into polydimethylsiloxane (PDMS) elastomers by exploiting the relative reactivity of different functional groups in the Piers-Rubinsztajn reaction. To analyze trends in the silicone mechanical properties as a function of increasing aryl concentration-structure/property relationships-libraries of elastomers were both quickly synthesized and characterized by using high-throughput suites starting from low viscosity silicone oils/monomers in 96-well plates. Liquid handling parameters were optimized to effectively work with the silicones. Incorporating aryl instead of alkyl crosslinkers into the PDMS backbone increased the silicone elastomer modulus by approximately 50 % (at a crosslink density of 6 %); elastomers prepared with an aromatic crosslinker with three contact points led to much higher moduli compared with those with one contact point at the same crosslink density. When located at precise rather than random points on the silicone chains, diphenylsilicones had lower moduli than analogous monophenylsilicones.

Trace water affects tris(pentafluorophenyl)borane catalytic activity in the Piers-Rubinsztajn reaction.

Improved methods to control silicone synthesis are required due to the sensitivity of siloxane bonds to acid/base-mediated chain redistribution/depolymerization. The Piers-Rubinsztajn reaction employs tris(pentafluorophenyl)borane as an efficient catalyst (<0.1 mol%) for siloxane bond formation from hydro- and alkoxysilanes - typical reactions proceed in open flasks at room temperature within minutes. While advantageous under ideal conditions, the boron catalyst activity may be affected by age, storage conditions and various environmental factors, particularly humidity. Under conditions of high humidity it may be necessary to apply heat and/or use increased catalyst loading in the reactions; there is often an induction time. We examine induction times in the Piers-Rubinsztajn reaction as a function of water concentrations in the reagent or catalyst solution and show that water in the reagent solution or atmosphere is less problematic than water found in the catalyst stock solution. A relatively linear increase in induction time accompanied higher water concentrations in the catalyst solution - no such effect was observed when the water was in the reagent solution. Reaction rates in both scenarios were similar, i.e., not affected by the induction time. Improvements in the stability of catalyst solutions were observed when B(C6F5)3 was stored in low molecular weight silicone oils, and pre-complexed with HSi(OSiMe3)3. These outcomes are ascribed to the ability of HSi groups to outcompete water in binding with B(C6F5)3 to initiate reaction, unless the boron is pre-complexed with water.

Sequential Functionalization of a Natural Crosslinker Leads to Designer Silicone Networks.

Precise silicone networks are difficult to prepare from multiple starting materials because of poor spatial control over crosslink location, competing side reactions, and incompatible catalysts among other reasons. We demonstrate that cure processes catalyzed by B(C6 F5 )3 (the Piers-Rubinsztajn reaction) and platinum-catalyzed hydrosilylation are perfectly compatible, and can be used in either order. It is possible to perform three different, selective, sequential reactions in the same pot using H-terminated silicones as chain extenders in all cases to give explicit networks. Eugenol, a readily available aromatic compound, acts as a trifunctional crosslinker (HO, MeO, HC=CH2 ), each functional group of which can be induced to undergo selective reaction. With platinum catalysis, the reaction of SiH groups with alkenes is fastest, while B(C6 F5 )3 catalyzes reaction at phenols much faster than methoxybenzene. Thus, a variety of H-terminated telechelic siloxanes can be used to form chain extended polymers or elastomers or foams in which the morphology of the material and its constituent parts can be manipulated at will.