High molecular weight samples of poly(di-n-pentylsiloxane)(PDPeS), pol
y(di-n-hexylsiloxane) (PDHS), and poly(di-n-decylsiloxane) (PDDS) have
been prepared by means of anionic (PDPeS, PDHS) and cationic (PDDS) r
ing-opening polymerization of the corresponding hexa-n-alkylcyclotrisi
loxanes. The phase behavior of the polymers has been studied by polari
zing microscopy, differential scanning calorimetry, wide-angle X-ray s
cattering, and Si-29 solid-state NMR spectroscopy. Like the homologues
with shorter allyl substituents, all three new polysiloxanes showed t
wo disordering transitions and formed a conformationally disordered me
sophase before isotropization. Elongation of the alkyl side groups fro
m ethyl to n-hexyl improved the stability of the hexagonal columnar me
sophase, which is reflected in an increase in the mesophase temperatur
e interval from DETLAT = 43-degrees-C for poly(diethylsiloxane) (PDES)
to more than 310-degrees-C in the case of PDHS. Optical polarizing mi
croscopy indicated the formation of extended chain lamellae within thi
s state. When the alkyl side groups were elongated to n-decyl, however
, the conformationally disordered state was only observed by temperatu
re-dependent Si-29 chemical shift anisotropy measurements as an underc
ooled phase in coexistence with the high-temperature crystalline phase
.