Spirocycles as stable alternative of networks - polycondensations of oligoether diols with metal tetraalkoxides

Germanium, tin, titanium and zirconium tetraalkoxides were polycondensed with poly(ethylene glycol)s (PEGs) and poly(tetrahydrofuran) (PTHF) diols at temperatures between 100 and 200 degreesC. The lengths of the diols were varied from PEG-300 over PEG-400, PEG-600 and PEG-1000 to PEG-2000. In the case of PTHF diols number average molecular weights (M(n)s) of 650, 1000 and 2000 were used. When Ge(OEt)(4) was polycondensed, all reaction products were soluble. In the case of Sn(OtBu)(4), all products obtained from diols with M(n)s greater than or equal to 600 Da were soluble, whereas a stiff gel was obtained from PEG-400. From Ti(OnBu)(4) and Zr(OEt)(4) soluble spirocycles were only obtained for diols with M(n)s greater than or equal to 1000, whereas the shorter diols yielded stiff networks. In the case of Ti(OnBu)(4) and PEG-1000, it was observed that at temperatures less than or equal to 120 degreesC initially a stiff gel was formed obviously by a kinetically controlled polycondensation. Upon heating to 140 degreesC this gel underwent an irreversible transformation into syrupy, soluble spirocycles due to equilibration of Ti-O-R bonds. The thermodynamic stability of the spirocycles is explained by strain-free rings and a gain in entropy. Preliminary experiments with the spirocycles of zirconium proved their usefulness as initiators for the ring-expansion polymerization of epsilon-caprolactone. (C) 2004 Elsevier Ltd. All rights reserved.