Effect of 15-crown-5 ether on the secondary processes of chain redistribution and cyclization of anionic copolymerization of dimethyl- and methylphenylcyclosiloxanes

Achieving a Rapid and Selective Ring Opening Polymerization of Dimethylcyclosiloxanes via Potassium Silanolate Catalysis: Understanding the Role of Potassium Chelation on Polymerization Rate and Backbiting

“…The first is the rate enhancement observed with larger cyclicsas D 6 and D 7 have respective stepwise enhancements in the rate over D 4 and D 5 with potassium silanolate. While part of this may arise from reported differences in activation enthalpies, this behavior also likely arises, in part, from charge separation of the silanolate catalyst, which can also be seen when using crown ethers or other Lewis bases to facilitate charge separation. , Considering the potassium silanolate catalyst, as the potassium cation becomes more fully chelated by the free cyclosiloxane monomer, the silanolate anion will become more reactive. This is exemplified by the trend of improving chelation of the potassium moving from D 4 and D 5 to D 6 and further improving chelation with D 7 .…”

“…8,9 Other reports show that chelants like crown ethers can provide a rate enhancement in D 4 AROP, although these approaches can have toxicity and cost challenges. 10,11 There have also been a handful of catalystbased strategies reported in recent years to avoid the formation of cyclosiloxane coproducts. Regarding PDMS made via polycondensation, judicious choice of catalysts has been shown to reduce cyclic levels, with both Brønsted and Lewis acids showing good efficiency to generate high-molecularweight polymers with low cyclics.…”

“…Anionic ring opening polymerization (AROP) of ring-strained hexamethyl cyclotrisiloxane (D 3 ) has long been reported to generate little-to-no cyclosiloxane coproducts post polymerization. However, the costs associated with D 3 ’s isolation and the subsequent polymerization process preclude it from replacing PDMS feedstocks like D 4 in the production of commodity silicones. , Other reports show that chelants like crown ethers can provide a rate enhancement in D 4 AROP, although these approaches can have toxicity and cost challenges. , There have also been a handful of catalyst-based strategies reported in recent years to avoid the formation of cyclosiloxane coproducts. Regarding PDMS made via polycondensation, judicious choice of catalysts has been shown to reduce cyclic levels, with both Brønsted and Lewis acids showing good efficiency to generate high-molecular-weight polymers with low cyclics. − Catalyst-based strategies can be used in the AROP of cyclosiloxanes to make PDMS with a nonequilibrium level of cyclosiloxane byproducts, with both superbases or specialized polymerization initiators showing improvements .…”