Direct Synthesis, Microstructure Characterization And Thermal Stability Study Of Cross-linked Polysiloxanes Via Anionic Ring-opening Copolymerization Of Polyhedral Oligomeric Silsesquioxanes (POSS) And Cyclosiloxanes

This dissertation successfully fabricated cross-linked polysiloxanes under base catalysts such as potassium hydroxide (KOH) siloxanolate or tetramethylammonium hydroxide (Me₄NOH) siloxanolate by anionic ring-opening copolymerization of cyclosiloxanes and Polyhedral Oligomeric Silsesquioxanes (POSS) as multifunctional monomers. A series of cross-linked polysiloxanes of kinds and different cross-linking density were synthesized by altering the kind and stoichiometry of POSS and cyclosiloxanes. And the mechanisms of anionic ring-opening copolymerization of POSS and cyclosiloxanes as well as the influences of the polar additive N, N-dimethylacetamide (DMAc) on gelation time were studied in detail. The characterization techniques including gel content and swelling ratio, GPC, FT-IR. solid-state ¹³ C and ²⁹ Si NMR,XRD,DSC and TG were used to discuss detailedly on the microstructure and thermal stability of the obtained cross-linked polysiloxanes.Studies on monomers, catalysts and polymerization temperature were our main focuses. One or Both of octamethylcyclotetrasiloxane (D₄) and octaphenylcyclotetrasiloxane (Ph₈D₄) , as well as one of three POSS including Octaisobutyl-POSS, Dodecaphenyl-POSS, Octaphenyl-POSS were used as comonomers to anionic ring-opening copolymerize under base catalysts such as KOH siloxanolate (at 100～120℃) or Me₄NOH (at 80～100℃) siloxanolate in this experiment. The following four anionic ring-opening copolymerization systems were studied: Octaisobutyl-POSS /D₄ anionic ring-opening copolymerization system (A); Dodecaphenyl-POSS/D₄/Ph₈D₄ anionic ring-opening copolymerization system (B); Octaisobutyl-POSS/D₄/Ph₈D₄ anionic ring-opening copolymerization system (C); Octaphenyl-POSS /D₄/Ph₈D₄ anionic ring-opening copolymerization system (D).No matter Octaisobutyl-POSS which has good solubility in D₄ or Dodecaphenyl-POSS and Octaphenyl-POSS which have poor solubility in D₄, they cannot homopolymerize even for more than 24 hrs under basic catalysts such as KOH, siloxanolate (at 100～120℃) or Me₄NOH (at 80～100℃) siloxanolate. However, they can quickly copolymerize with cyclosiloxanes at the same condition. Thereby, it can be concluded that POSS and cyclosiloxanes were inclined to form random copolymer, rather than block copolymer. The mechanism of random copolymerization commonly accords with the homopolymerization mechanism of monomers; therefore, the copolymerization mechanism of POSS and cyclosiloxanes under basic catalysts is the same as the homopolymerization of cyclosiloxanes. so they all belong to anionic ring-opening polymerization.The anionic ring-opening copolymerization mechanism was illuminated with the example of copolymerization system (A). The base such as KOH or Me₄NOH reacts with D₄ to generate the corresponding siloxanolate catalyst. which initiates D₄ to yield the base catalyzed (or anionic) active species. The chain propagation is believed to proceed first with the nucleophilic attach of the anionic active species on the silicon atom of siloxane linkage and then with the subsequent redistribution of electron cloud density. Then the siloxane linkage is dissociated by heating to yield new anionic active centers, which continue to react and then propagate gradually. Finally the random cross-linked polysiloxane is formed because the octaisobutyl-POSS contains the O{[C(CH₃) ₃] (O-)SiO}-units (i.e. T bonding).The effect of the polar additive DMAc on gelation time was discussed. The results indicated that they were difficult to react and didn't even gel 24 hrs by anionic ring-opening copolymerization of POSS and cyclosiloxanes without any polar additives in the four copolymerization systems of POSS and cyclosiloxanes. But the polar additives DMAc, can dramatically affect the anionic ring-opening copolymerization. It not only made the copolymerization of POSS and cyclosiloxanes possible,but also decreased the gelation time distinctly. And the rate of polymerization increased greatly with the increase of DMAc.In copolymerization system (A), when the content of Octaisobutyl-POSS monomer was O, the obtained polymer can be thoroughly removed through the extraction procedure. The gel content was infinitesimal and the swelling ratio was infinite, so it was linear polysiloxane. When a trace amount of Octaisobutyl-POSS was added, the obtained polymer can not be thoroughly removed through the extraction procedure, the remainder was about 80% and the swelling ratio also decreased dramatically. So it can be concluded that Octaisobutyl-POSS participated the anionic ring-opening copolymerization reaction and the obtained polymer was cross-linked polysiloxanes. In addition, as the octaisobutyl-POSS monomer increased, the swelling ratio decreased and the gel content was unchanged. The same conclusions can be drawn from the other copolymerization systems.The GPC analyses of the soluble part of the obtained polymers in copolymerization system (B) through the extraction procedure showed that the soluble polysiloxanes were mainly composed of the oligomers with low molecular weight of several hundreds to thousands, and few polymers with molecular weight higher more than ten thousands. Therefore, it can be believed that the polysiloxanes with higher molecular weight were almost cross-linked.The results of solid-state FT-IR. ¹³ C and ²⁹ Si NMR, XRD showed again that POSS was reacted with cyclosiloxanes via anionic ring-opening copolymerization and the obtained polymers were cross-linked polysiloxanes with very low crystalline degree. In the former three polymerization systems. POSS monomers were copolymerized nearly in accordance with the designed value and the reactions were more complete more than polymerization system (D). which was much less than its designed value.The DSC and TG results indicated that the obtained cross-linked polysiloxancs exhibited distinct glass transition temperatures (T_g) and excellent thermal stability. Compared to that with KOH siloxanolate, the cross-linked polysiloxane synthesized with Me₄NOH siloxanolate had more preferable thermal stability.