HEXA-N-ALKYLCYCLOTRISILOXANES - SYNTHESIS, MELTING BEHAVIOR AND POLYMERIZATION

A homologous series of hexa-n-alkylcyclotrisiloxanes and poly(di-n-alk ylsiloxane)s has been prepared with alkyl side groups varying in lengt h between 4 and 10 carbon atoms. Melting transition enthalpies of the hexa-n-alkylcyclotrisiloxanes indicated a reversed odd-even effect, sh owing higher melting enthalpies for cyclotrisiloxanes with n-alkyl sid e groups with an odd number of carbon atoms. The melting behaviour of the heptyl- and octyl-substituted cyclotrisiloxanes was rather complex . Multiple melting transitions could be observed, which are presumably due to formation of a kinetically favoured crystalline phase next to the thermodynamically stable crystal modification. All hexa-n-alkylcyc lotrisiloxanes could be polymerized by anionic or cationic ring-openin g polymerization. The observation of longer polymerization times and f ormation of relatively larger amounts of cyclic byproducts in the case of hexadecylcyclotrisiloxane compared to hexaethylcyclotrisiloxane in dicated that with increasing side group length, the anionic polymeriza tion propagation rate constants k(p) decreased, whereas the rate const ants of depolymerization k(dx) increased. Cationic polymerization with trifluoromethanesulfonic acid as the initiator, offered a way to enha nce k(p) relative to k(dx), which allowed the preparation of high-mole cular-weight polymeric products in good yields even in case of cyclotr isiloxanes substituted with long n-alkyl groups. Apart from poly(dimet hylsiloxane), poly(diheptylsiloxane) and poly(dinonylsiloxane), all po ly(di-n-alkylsiloxane)s showed two crystalline phases which were separ ated by a crystal-crystal transition signifying melting of the side gr oups. Starting from poly(diethylsiloxane), poly(di-n-alkylsiloxane)s u p to poly(dihexylsiloxane) displayed a hexagonal columnar mesophase as well, the stability of which increased with longer alkyl side groups. However, further elongation of the n-alkyl side groups to decyl appea red to destabilize the columnar mesophase. Poly(didecylsiloxane) only exhibited a metastable columnar mesophase in combination with the high -temperature crystalline phase.