Research On The Controllable Synthesis Of Silicone Oligomer

Polydimethylsiloxane (PDMS), due to its excellent resistance to high/low temperature, anti-oxidation, anti-shear viscosity-temperature coefficient, low surface tension, water-repellentance, anti-foaming, stripping, electrical insulation, physical inertia and other properties, is widely used in machinery, electric, textile, paint, medicine and other sectors of the national economy.At present, most commercialized PDMS are dimethyl siloxane high polymers, with the molecular weight in the range of 1000 to 10,000 and the degree of polymerization 10 to 126. Because of their hydrophobicity and hydrophilicity, it is hard for them to dissolve or disperse inwater system. On the other hand,the linear PDMS oligomers, with their low degree of polymerizationand the short molecular chains, such as di-, tri- and tetramers, are able to dissolve and dispersebothinoilsystemand in methanol, acetone and polyethers, which brings new specific applications for a linear dimethylsiloxane oligomer. However, with the most target products focused on dimethyl siloxanepolymer,the study on synthesis and characterization of linear PDMS oligomers is rare.Here we use TiO2/SO42-solid superacid as catalyst, octamethylsiloxanes and hexamethyldisiloxane as raw materials, optimize the reaction conditions and develop an efficient and environmental friendly process to product linear dimethyl siloxanealkoxy oligomer (MD4M). We also explore the kineticsof the heterogeneous catalysisof D4 ring-opening reactionwith solid superacid, build a macrokinetics model and establish research foundation and basisfor controllable preparationof PDMS oligomers withsolidsuperacid catalysts.Polydimethylsiloxane oligomer (MD4M) was synthesized with octamethylcyclotetrasiloxane (D4), hexamethyldisiloxane (MM) as the raw material andsolidsuperacidas catalyst, characterizedby means of GC, FT-IR and 29Si-NMR. The optimumreactiontemperaturewas chosen based on the D4 conversion rate under different reaction temperatures by using homogeneous design experimental project. At the same time, GC internal standard method was applied to track the real-time reaction process. The optimal reaction time was thusdetermined. On these basis, the influences ofdosagesof MM and catalysts, as well as the catalyst type on the yield of MD4M were studiedwith single factor experiments. The most optimum synthetic conditions is:the ratioof raw D4 and MM is 0.75:1, theamount ofcatalyst, TiO2/SO42- solid superacid, is 2% of the total mass, the reaction temperature is 100℃ and the reaction time is 7h. In order to verify the reliability and repeatability, the experiments were repeated and the catalyst was recycled.The results show that the conversion rate of D4 reached 87% and there was a raise in the yield of the target product compared to using sulfuric acid as a catalyst, which indicate that the optimum conditions are reliable. Moreover, the catalytic activity remained the same after more than 3 times circulation and thus can be used to replace sulfuric acid as an acid catalyst in the ring-opening polymerization of D4 to synthesize the dimethyl siloxane oligomers.Withthe optimized reaction temperature and reaction time, which are stated in the second chapter, thedegree of polymerization of the redesigned polydimethylsiloxanewas 160. The most suitable catalystamountwas determined as 8% by the pre-reaction. TiO2/SO2- solid superacid was used as heterogeneous catalyst to explore the kinetics of cationic ring-opening polymerization of D4 andGC internal standard method was applied to track and monitor the polymerization product in real-time. The effects of reaction temperature, stirring intensity, viscosity, molecular weight regulator, the particle size of catalyst and other factors were studied in theD4 bulk polymerization reaction. The results showed that the relative optimum reaction temperature is 100℃ and the stirring intensity should bemore than500 rmp. Also, the effect of outer diffusion to the polymerization reaction is almost negligible and the automatic acceleration affects different ring polymerization rates in a similar way. Thus the process of derivationis simplified. The particle size of TiO2/SO42- solid superacid used was 200 mesh and there was few influence on the internal diffusion within this range. Considering the above factors, we establish a macro-dynamics model on D4 ring-opening polymerization, as the derivation of kinetic equations was greatly simplified, which is shown as follows:[Dsum] t=[Dsum]eq+([D4]0-[D4] eq) exp (kA,4[A*]t)-[Dx] eqexp (-kA,x[A*]t)The further analysison the results shows that the cationic ring-opening polymerization of D4 meets the Arrhenius law and is a first order reaction at the macro level. By fitting the regressed experimental data, the curve of the dynamic model and correlation equations of parametersis achieved, as shown above, where [D4]0 is a known quantity and [D4]cq, [Dx] eq and [Dsum]eq can be determined by GPC and GC. However, kA,4, kA,x and [A*] are hard to determinatealone in selected conditions. Hence, combinations are applied as k4=kA,4[A*], kx=kA,x[A*], andthe related kinetic parametersthus can be obtained with the experimental data:k40=2.466×103s-1, kx0=3.154×103s-1, Ea,4=32.6kJ·mol-1, Ea,x=34.1kJ·mol-1.