Synthesis and characterization of new, liquid, branched poly(methylvinylborosiloxanes)

Abstract

New liquid branched poly(methylvinylborosiloxanes) (br-PMVBS) of random structure were synthesized in three steps. By reacting boric acid with an excess of dimethyldichlorosilane (Me₂SiCl₂) in dry ether a “borosiloxane precursor”: tris(chlorodimethylsilyl) borate B(OSiMe₂Cl)₃ was prepared. In the second step of synthesis ether solution of B(OSiMe₂Cl)₃ was added to a mixture of appropriate organic chlorosilanes (Me₂SiCl₂, MeViSiCl₂, MeSiCl₃, and Me₃SiCl) and all reagents were reacted with stoichiometric amounts of water, in the presence of pyridine (as an acceptor of HCl), in dry ether, at low temperature (usually at -10 to 0 C). In order to fully react (“to block”) trace silanol groups, reactions of intermediate PMVBS with additional batches of Me₃SiCl were carried out in the third step, C₅H₅N·HCl was filtered off and washed with a dry ether. The solvent was distilled off from filtrates and low molecular weight siloxane oligomers were removed by a vacuum distillation at 130-150 C. Chemical structures of br-PMVBS were confirmed by elemental analysis and spectroscopic methods (FTIR, emission atomic spectroscopy ICP-AES, and NMR: ¹H, ²⁹Si and ¹¹B). On the basis of analysis of their ²⁹Si-NMR spectra the microstructure of polysiloxane chains was proposed. The prepared br-PMVBS had in their structures: triple branching borosiloxane units: BO_1.5 and in some cases methylsiloxane moiety CH₃SiO_1.5 (T). They contained linkages: Si-O-Si, Si-O-B, vinyl(methyl)siloxane functional groups (CH₂=CH)MeSiO (Dvi), dimethylsiloxane mers (CH₃)₂SiO (D), and non-reactive trimethylsiloxy terminal groups (CH₃)₃SiO_0.5 (M), but they did not have: hydroxyl functional groups: Si-OH and B-OH, and sensitive to water B-O-B linkages. Molecular weights of br-PMVBS (M_n = 1500-3300 g/mol; M_w = 3800-7400 g/mol) and their polydispersity (M_w/M_n = 2.0-2.5) were determined by a size exclusion chromatography (SEC).