Hexa-n-alkylcyclotrisolixanes. Synthesis, melting behaviour and polymerization

A homologous series of hexa-n-alkylcyclotrisiloxanes and poly(di-n-alkylsiloxane)s has been prepared with alkyl side groups varying in length between 4 and 10 carbon atoms. Melting transition enthalpies of the hexa-n-alkylcyclotrisiloxanes indicated a reversed odd-even effect, showing higher melting enthalpies for cyclotrisiloxanes with n-alkyl side 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-alkylcyclotrisiloxanes could be polymerized by anionic or cationic ringopening polymerization. The observation of longer polymerization times and formation of relatively larger amounts of cyclic byproducts in the case of hexadecylcyclotrisiloxane compared to hexaethylcyclotrisiloxane indicated that with increasing side group length, the anionic polymerization propagation rate constants kp decreased, whereas the rate constants of depolymerization kdx increased. Cationic polymerization with trifluoromethanesulfonic acid as the initiator, offered a way to enhance kp relative to kdx, which allowed the preparation of high molecular-weight polymeric products in good yields even in case of cyclotrisiloxanes substituted with long n-alkyl groups. Apart from poly(dimethylsiloxane), poly(diheptylsiloxane) and poly(dinonylsiloxane), all poly(di-n-alkylsiloxane)s showed two crystalline phases which were separated by a crystal-crystal transition signifying melting of the side groups. Starting from poly(diethylsiloxane), poly(di-n-alkylsiloxane)s up 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 appeared to destabilize the columnar mesophase. Poly(didecylsiloxane) only exhibited a metastable columnar mesophase in combination with the high-temperature crystalline phase