142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

Synthesis of Branched Polymers by Cationic Polymerization 35
(27) R=H
(28) R=O-Si(CH 3) 2-H
The above method appears to have serious limitations. First, the availability
of common methylcyclosiloxanes is limited, and second, steric compression prevents
quantitative hydrosilylation of neighboring SiH groups.
To eliminate steric congestion, Majoros et al. [51] prepared a new octafunctional
siloxane linking agent by moving the SiH group away from the rigid cyclic
core skeleton (28). Using H 2PtCl 6.H 2O as catalyst, although star formation was
apparent, 60–70% of PIB allyl remained unreacted even after 144 h. Karstedt's
catalyst {bis(divinyltetramethyl disiloxane) platinum(0)} was more efficient, although
the majority of PIB allyl still remained unreacted. The reaction was further
complicated by the formation of higher order stars by star-star coupling,
which could be suppressed by increasing the [C=C]/[SiH] ratio from 1.0 to 1.66.
While star-star coupling was considered as a side reaction in the above reports,
Omura and Kennedy [52] attempted to exploit core-core coupling of small
methylcyclosiloxanes in the presence of moisture under hydrosilylation conditions
using PIB allyl to build star PIBs with many arms. The synthetic strategy is
shown in Scheme 6.
Kinetic studies of primary and higher order star formation concluded that
well-defined first order stars with narrow molecular weigth distribution could
be prepared with [SiH]/[C=C]=1.25 at room temperature whereas higher order
stars were obtained with [SiH]/[C=C]=4.0 at 120 °C. While primary star formation
was very slow and could require up to a week to complete at room temperature,
higher order star formation was essentially complete in 24 h. Higher order
stars with up to 28 arms have been prepared by this method. Intrinsic viscosities
and branching index g' were also studied. The intrinsic viscosities of stars were
much lower than those of linear PIBs of the same MW. As expected, it was found
that g' values of stars depend on the number of arms and not on the MW of the
arms. The stars were found to be resistant to acids and bases suggesting that the
PIB corona protects the vulnerable core.