Facile Manufacture and Storage of Poly(methylhydrogenosiloxane)s

Abstract

This paper describes the ring-opening polymerization of 2,4,6,8-tetramethylcyclotetrasiloxane (D4 H) in miniemulsion, using mono- and diphosphonic acid surfactants. In the simplest formulation conditions, polymerization proceeds in less than 15 min to generate linear poly(methylhydrogenosiloxane) (PMHS) of large molar masses (typical number-average of 25 kg/mol), of low polydispersities (around 2), and in good yields (up to 94%). Molar masses can be tuned at will since for similar conversions but increasing times, varies between 10 and 30 kg/mol. After 4 h reaction, some redistribution reactions start to proceed, responsible for PMHS chains switching into macrocycles. It was shown that rising the pH to stop the polymerization and breaking the emulsion with excess of alun salt induced hydrolysis/condensation reactions, which rapidly convert the polymer chains into an unwished macrogel. Rather, extracting the polymer by simple centrifugation and using the diacid surfactant induce a physical gelling that very efficiently suppresses side reactions, as shown by minor evolution of the molar mass distribution for 10 months.

Cationic polymerization of 2,4,6,8-tetramethylcyclotetrasiloxane processed by tuning the pH of the miniemulsion

“…The procedures of emulsification, polymerization and emulsion breaking are similar to those previously applied [6,7,10,11]. Typically, 200 mL of an 2.6!10 K3 M aqueous solution of surfactant (NaDBSA or various mixtures of lauric acid/sodium laurate) were emulsified in a beaker by the sonicator (power 7-55 W) during 2 min 20 mL of monomer were added drop by drop at the beginning of the emulsification.…”

“…Ninety percent of linear chains (the rest of low molecular weight cycles) were obtained with molar masses going from 5000 to 70,000 g mol K1 , a priori without formation of networks. Yactine et al [7] recently proposed new conditions to produce high molar masses with high conversions and no formation of gel, using a surfactant bearing mono-and diphosphonic acid groups. The obtained chains present molar masses close to 23,000 g mol K1 and a polydispersity index lower than 2, but with a large amount of macrocycles generated after 5 h reaction.…”

“…Some authors recently suggested [7] that the intraparticular condensation reactions occurring with PDMS cannot exist in the particular case of PMHS synthesis. Indeed, transfer reaction by water molecule produces a silanol chain-end that rapidly rearrange in an irreversible one [6,7] [either (CH 3 ) 3 Si or (CH 3 ) 2 SiH] unable to undergo condensation.…”

“…Indeed, transfer reaction by water molecule produces a silanol chain-end that rapidly rearrange in an irreversible one [6,7] [either (CH 3 ) 3 Si or (CH 3 ) 2 SiH] unable to undergo condensation. Another drawback in PMHS synthesis is the hydrolysis reaction of the silane groups, catalyzed by protons, that is likely to convert the polymer chains into an irreversible network (Scheme 1).…”

“…Surfactants with extreme pKa values (sulfonate and carboxylate fatty acids) were tested, mainly by tuning the pH value of the continuous phase. These surfactants are commercially available (unlike fatty phosphonic acids [7]) and sufficiently hydrophilic to recover PMHS with no surfactant residues (contrary to experiments done with Brij [6]). , from ABCR, was used without further purification.…”