Heterogeneous Inhibition of Homogeneous Reactions:  Karstedt Catalyzed Hydrosilylation

Abstract

The Karstedt-catalyzed hydrosilylation reaction used in curing of silicone release coatings was investigated using first-principle quantum mechanical techniques (density functional theory) as well as semiempirical methods to estimate solubility parameters. The results we obtain for the catalytic cycle indicate, in agreement with experimental results, that hydrosilylation occurs easily at room temperature. The detailed mechanism we suggest contains the key features of the models previously proposed in the literature by Lewis and ChalkHarrod and adds quantitative estimates of reaction energies and barriers. On the basis of the energy profile for the catalytic cycle in the presence of inhibitor molecules and on solubility parameters for the species involved in the reaction, we conclude that the role of the inhibitors we considered is to phase-separate the catalyst from the substrate. The reaction is thus quenched by introducing a microscopic second phase that interferes with the homogeneous reaction.

Recent advances and actual challenges in late transition metal catalyzed hydrosilylation of olefins from an industrial point of view

“…As a result, these inhibitors are insoluble in polysiloxane substrates. Therefore, these authors conclude that the examined inhibitors do not interfere with the hydrosilylation by binding to the platinum center [138]. Their inhibiting effect is rather based on the formation of a second micro phase containing the catalyst being physically separated from the substrate, i.e.…”

“…The most important and most frequently applied inhibitors are maleates, fumarates, andˇ-alkynoles (Scheme 19) [17,133,137,138] which are typically used in levels of 0.01-10% (w/w) (mostly preferred: 1-5% (w/w)). Their mechanism of action was traced to their ability to act as ligands which block the hydrosilylation reaction by coordination to the metal center, but release the active catalyst under suitable curing conditions [17,137].…”

“…the role of an inhibitor is to act as a phase-separator. The temperature-induced deactivation of the inhibitor is supposed to be caused by the vaporization of the inhibitor, so curing rate and activation temperature are expected to depend strongly on the inhibitor's vapor pressure [138].…”

Catalytic Systems for the Cross‐Linking of Organosilicon Polymers

“…[62] The insoluble nature of the inhibitors in the substrates makes them form liquid globules, which physically isolate the catalyst from the substrate. [42] In other words, the functiono ft he inhibitors is to phase-separate the catalyst.…”

Impact of End-Tethered Polyhedral Nanoparticles on the Mobility of Poly(dimethylsiloxane)

“…Hydrosilylation 15 is an addition polymerisation of a silicon hydride moiety across an unsaturated linkage 16 that has been widely used to prepare POSS-based polymer nanocomposites. 1,2,17,18 Al-ternatively, nanocomposites prepared from polysilicate-based particles studied previously 19,20 also have an inner inorganic framework of silicon and oxygen and particle sizes ranging from 1-3 nm.…”