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

34 B. Charleux, R. Faust
for the coupled products with acetoxy and methacryloyloxy functionality
(yield>95%). For the coupled product with the styryl terminal group, the yield
was lower (~90%). Structural analysis using 1 H NMR spectroscopy was performed
after separation of the main product by preparative gel permeation
chromatography. F n was close to the theoretical value 4, indicating that the final
product had the expected four arm structure with one functional group at the
end of each arm.
2.3.1.2
Poly(isobutylene) n
In view of the excellent shear stability of silicone oils, it was theorized that shear
stable multiarm star PIBs could be prepared using cyclosiloxane cores [50]. The
synthesis was accomplished in two steps. First, allyl terminated PIB of desired
MW was prepared by reacting living PIB with trimethylallylsilane. Linking was
effected by hydrosilylation of the allyl-functional PIB with cyclosiloxanes carrying
six or eight SiH groups (respectively 26 and 27) in the presence of H 2PtCl 6
catalyst at 180 °C for an extended period of time. With relatively low MW allyl
functional PIB (M n=5200 g mol –1 ), after 3 days of linking using 26 at a [C=
C]/[Si-H]=1 ratio, six arm star PIB was obtained in ~80% yield. With an arm
MW of M n=12,600 g mol –1 however, in addition to the expected star PIB and unreacted
PIB arm, the product also contained a much higher MW component. It
was theorized that this arose by star-star coupling in the presence of adventitious
water. In contrast to allyl-functional PIB, linking of isopropenyl functional
PIBs was less successful, as the amount of unreacted PIB arm was ~50%, even
with short arms. Experiments with the octafunctional hydrogenoctasilsesquioxane
27 yielded stars with significantly lower than eight arms even with low arm
MW. With arm MWs of M n =12,600–19,200 g mol –1 , the number of arms of the
primary stars was only ~5. In addition, higher than expected MW stars were also
obtained probably by star-star coupling. 13 C relaxation NMR studies indicated
that the mobility of the arms is not limited by steric compression between them.
Apparently, there is enough room around 27 to place eight arms, although access
to the unreacted Si-H sites may become limited after five to six arms have been
placed.
(26)