Dow Silicones Corp.

Volatile methylsiloxanes (VMS) are man-made, nonbiodegradable chemicals produced at a megaton-per-year scale, which leads to concern over their potential for environmental persistence, long-range transport, and bioaccumulation. We used directed evolution to engineer a variant of bacterial cytochrome P450 BM3 to break silicon-carbon bonds in linear and cyclic VMS. To accomplish silicon-carbon bond cleavage, the enzyme catalyzes two tandem oxidations of a siloxane methyl group, which is followed by putative [1,2]-Brook rearrangement and hydrolysis. Discovery of this so-called siloxane oxidase opens possibilities for the eventual biodegradation of VMS.

Silicones have a plethora of uses but have limitations in placement errors and continuous crosslinking.To address these challenges, the authors sought to develop a catalyst that would not only contain a latency period, thereby limiting placement errors but could be immobilized in an elastomer, thus thwarting continued crosslinking.Five new N-heterocyclic carbene ligands were synthesized to feature alkene pendant arms of various lengths, (1,3-di(pent-4-en-1-yl)-1H-imidazol-3-ium bromide) ([^C5Im]Br), 1,3-di(but-3-en-1-yl)-1H-imidazol-3-ium bromide ([^C4Im]Br), and sterics (1-(2,6-diisopropylphenyl)-3-(pent-4-en-1-yl)-1H-imidazol-3-ium bromide ([^C5Im^Dipp]Br), and 1-methyl-3-pent-4-en-1-yl)-1H-imidazol-3-ium bromide ([^C5Im^Me]Br) were targeted to understand how alkene length influences latency.An alkane version of the ligand (1,3-dipentyl-1H-imidazol-3-ium bromide) ([^C5_satIm]Br) was also developed for comparison.Upon deprotonation and subsequent reaction of the ligand with Karstedt's catalyst, a new family of Markó's catalysts were developed.Stoichiometric reactions with silanes demonstrated that the pendant ligand arms can be hydrosilylated.Immobilization of the catalyst via intramol. hydrosilylation effectively prevented post-cure crosslinking.

The Harkins relationship, that the spreading coefficient of an adhesive on a release coating is the difference between the work of adhesion of the materials and the work of cohesion of the adhesive, has been found to apply to a variety of silicone release coatings.The works of adhesion and cohesion were estimated from contact angle data using the Owens and Wendt approach.The prediction of the Harkins relationship is obeyed by almost all the combinations of pressure-sensitive adhesive and release coatings we have examinedRelease occurs when the spreading coefficient is neg. and does not when it is pos.The main exception to this general spreading coefficient rule is the failure of polytetrafluoroethylene to release polydimethylsiloxane-based pressure-sensitive adhesives.The cause is believed to be roughness of the polytetrafluoroethylene surface.