Synthesis And Characterization Of Polysilazanes And Hyperbranched Polycarbosilazanes

This thesis mainly deals with the synthesis and characterization of polysilazanes and hyperbranched polycarbosilazanes with desired functional groups.Firstly, three polysilazanes were synthesized via ammonolysis of mixture of MeHSiCl₂ and MeViSiCl₂ with different feeding ratios. Then, the forth polysilazane was prepared by further polymerization of the third polysilazane with NH₄Cl as the catalyst. The UV-curing and pyrolysis behaviors for the polysilazanes are also studied. Spectral analyses of the polysilazanes show that functional groups have been successfully incorporated in the polysilazanes. Research on the UV-curing behaviors indicates that all the four polymers synthesized can be cured under UV-irradiation, and free radical polymerization among vinyl groups mainly occurrs during the curing process, meanwhile, along with the condensation reaction between =N-H and vinyl groups. However, the UV-curing rate is relatively slow, which may be attributed to the effect of silicon atom's d-ovbit on the reactivity of vinyl groups. The relatively fast curing rate in nitrogen atmosphere indicates that the oxygen, may play a role of an inhibitor in the free radical polymerization, and can indeed reduce the curing rate at a certain degree. But it is not the main reason for the slow curing rate. Study on their pyrolysis behavior indicates that further reactions among =N-H, vinyl and≡Si-H groups occur during the heating process. Besides, introduction of≡Si-H groups to the polysilzanes can greatly improve the resulting ceramic yield, and the ceramic yield increases with the increase in the content of ≡Si-H groups.In addition, a novel dichlorosilane carrying acryloyl group was also synthesized via modification of methyltrichlorosilane with 2-hydroxyethyl arcylate. Then, polysilazane with acryloyl groups was prepared via ammonolysis of the dichlorosilane synthesized, and it was well characterized by IR and ¹H-NMR spectroscopy. Study on its UV-curing behavior shows that it can be cured within 40 s in air atmosphere and 10 s in nitrogen atmosphere, respectively. And, the mechanism of its UV-curing and pyrolysis are similar as mentioned above.Due to their low viscosity and multifunctionalilty, the hyperbranched polycarborsilzane are novel polymers with many potential applications. So we also designed and synthesized four of these hyperbranched polycarborsilzanes via hydrosilylation reaction. Firstly, four monomers, bis(N,N-diallylamino)methylsilane (M₁), bis(N,N-ethylallylamino)methylsilane (M₂), (N,N-diallylamino)dimethylsilane(M3) and bis(dimethylsilyl)allylamine (M4), were synthesized via aminolysis of MeHSiCl? or MeiHSiCl with allylamine. diallylamine or ethylallylamine. Spectral and element analyses show that all the monomers are consistent with their desired structures.Hyperbranched polycarbosilazanes were prepared from the four monomers via hydrosilylation addition reaction with HiPtClg or Pt/C as the catalyst. Also, the polymers are well consistent with their desired structures, characterized by FTIR, 'H-NMR, 29Si-NMR spectroscopy and Laser Light Scattering (LLS) instrument. Spectral results show that all the polymers consist of =Si-N-Cs linkages with hyperbranched architecture carrying many functional =Si-H or vinyl end groups. Besides, a-hydrosilylation mainly occurs during the polymerization.With M3 as an example, the effect of outside conditions, such as catalysts, monomer feeding manners and solvents, on polymerizations and the molecular weights are studied to further understand the rule of this kind of polymerization. It is found that polymerizations can be conducted fast with FbPtC^ as catalyst, but the corresponding molecular weights are lower than that with Pt/C as catalyst. Toluene, used as solvent, can slow down the polymerization;nevertheless, it has little effect on the molecular weights. Slow addition polymerization process can greatly improve the molecular weights and the size of.polymers comparing with the "one-pot" process, if using H2PtCl(, as the catalyst.