tailieunhanh - ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 5

Phản ứng của axit cacboxylic với anhydrit cũng thường được sử dụng cho liên kết ngang của prepolymers diepoxide. Các phản ứng đã được thực hiện ở nhiệt độ cao hơn so với trước đây, trong sự hiện diện của các amin đại học, xảy ra thông qua việc mở vòng của tạo ra từ các anhydrit: | MAIN REACTIONS USED IN STEP-GROWTH POLYMERIZATION 245 The reaction of carboxylic acids with anhydrides is also commonly used for the cross-linking of diepoxide prepolymers. The reaction has to be carried out at higher temperature than previously and in the presence of tertiary amines occurs through ring-opening of the oxiranes by the carboxylate generated from anhydrides R-CO Q V- XO R R-COO-CH2-CH-R At high temperature other reactions occur which make the structure of the resulting networks extremely complex. Indeed secondary hydroxyls formed upon ringopening of epoxides can in turn react with the oxiranes of the precursor ROH HO Such reactions increase the density of cross-linking of the network formed. . Substitution Reactions on Silicon Atoms Only a minor portion of industrially produced polysiloxanes is obtained by chain polymerization of cyclosiloxanes octamethylcyclotetrasiloxane . Most are synthesized by water-induced hydrolysis of dialkyldichlorosilanes followed by selfcondensation of the disilanol formed. The starting monomer is dimethyldichlorosilane which is prepared by copper-catalyzed reaction of methyl chloride on metal silicon. The hydrolysis of the chlorinated derivative CH3 Cl Si Cl 2H2O Ch3 cH3 2HCl HO Si OH T CH3 corresponds to a nucleophilic substitution. In the presence of bases the condensation occurs by nucleophilic substitution and the result of the self-condensation of silanol groups is poly dimethylsiloxane CH3 nHO-Si OH T CH3 CH3 3 H-TO-Si l-OH n - 1 H2O n CH3 Depending on whether the silanol function is carried by a mono- di- or trivalent monomer one may have termination polymerization or cross-linking. The reactivity of silanols is closely related to the nature of the alkyl groups the number 246 STEP-GROWTH POLYMERIZATIONS of hydroxyls carried by the silicon atom and the size of the polysiloxane carrying them. Thus CH3 2Si OH 2 is the most reactive among dialkylsilanediols. The mechanism of the acid-catalyzed condensation of .