Xiyin Gao

Controlled synthesis and characterization of poly[methyl(3,3,3-trifluoropropyl)siloxane] with selective end groups

Anionic ring-opening polymerization of 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane in the bulk was studied using dilithium diphenylsilanediolate as an initiator (I); and N,N-dimethylformamide (DMF), bis(2-methoxyethyl)ether (Diglyme), and 1,2-dimethoxyethane (DME) as promoters (P). A detailed study on the kinetics of polymerization with different molar ratios of promoter to initiator ([P]/[I]) that were equal to 2.0, 4.0, and 6.0 revealed that the yield of linear polymers was the highest when [P]/[I] = 2.0 for all the promoters, among which DME was the most efficient promoter for suppressing the backbiting reactions. The reaction promoted by DME had a very broad “termination window” with the highest yield of linear polymer and very narrow molar mass distribution. The results of the matrix-assisted laser desorption/ionization time of flight mass spectrometry indicated that the intermolecular redistribution occurred during the process of polymerization. PMTFPS with end groups such as vinyl, hydroxyl, hydrogen and chloromethyl were prepared and characterized by 1H NMR, 29Si NMR and FT-IR. Polymers having vinyl end groups displayed higher thermo stability than those having hydroxyl end groups under nitrogen.

Nonequilibrium Anionic Ring-opening Polymerization of Tetraphenyltetramethylcyclotetrasiloxane (D4Me,Ph) Initiated by Sodium Isopropoxide

GRAPHICAL ABSTRACT Abstract The nonequilibrium anionic ring-opening polymerization (NAROP) of tetraphenyltetramethylcyclotetrasiloxane (D4Me,Ph) in the presence of initiator (sodium isopropoxide) and promoters including 12-crown-4, N-methyl-2-pyrrolidinone (NMP), N,N-dimethylformamide (DMF), and diglyme was investigated. It was found that the polymerization could be accelerated sharply by the increase of reaction temperature, as well as the mole ratio of promoter to initiator ([p]/[i] ratio). The NAROP rate of D4Me,Ph decreased in the order of promoters: 12-crown-4>> NMP >DMF >diglyme. In the presence of NMP and sodium isopropoxide ([p]/[i] ratio = 3.0, 10 h at 70°C), the polydispersity index (PDI) of the poly(methylphenylsiloxane) obtained is 1.52, and the conversion rate of the monomer can reach over 90%. The byproduct pentaphenylpentamethylcyclopentasiloxane (D5Me,Ph), which encountered in the 12-crown-4 promoted system upon prolonged reaction time, was not observed according to the 29Si NMR of the reaction mixtures. In conclusion, NMP turned out to be the best choice among those promoters to realize the NAROP of D4Me,Ph.

Mechanism of the antioxidation effect of α-Fe2O3 on silicone rubbers at high temperature

The high temperature resistance of silicone rubber (SR) could be significantly improved by adding hematite (α-Fe2O3). In this study, the variation of α-Fe2O3 and polysiloxanes was investigated by ultraviolet-visible absorption spectroscopy, X-ray diffraction, Mossbauer spectroscopy, and 29Si NMR spectroscopy after SR aging for 12 h in air at 350 °C. The results indicate that Fe3+ is reduced to Fe2+ by organic free radicals (R˙) in the process of aging. Fe3+ captures R˙ to protect the polysiloxanes from being destroyed and is transformed into Fe3O4. Moreover, magnetic Fe3O4 is only found in the inner layer of SR; however, it is not detected in the outer layer. Therefore, a new mechanism of antioxidation is proposed: on the outer part of SR, the reduction–oxidation cycle of Fe3+ is formed because of sufficient oxygen; however, inside the SR, α-Fe2O3 is gradually reduced to Fe3O4.