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

Synthesis of Branched Polymers by Cationic Polymerization 11
with increasing r. Between 48 and 96 h the MW increased dramatically due to
intermolecular star-star coupling, the extent of which was proportional to r. Due
to star-star linking, the MWD of the final product after 96 h was broad (weight
average number of arms=110, M w /M n =2.9). For star PIBs with longer arms
(M w =18,700–116,100 g mol –1 ) the polymerization of IB and linking was performed
at –80 °C. It was found that with increasing arm length the rate of star
formation rapidly diminished, presumably due to the lower rate of star-star coupling.
Based on the observation that star-star coupling is absent when the molecular
weight of the arm is higher than M w =18,700 g mol –1 , it was postulated
that relatively large arms sterically hinder star-star coupling. This however may
not be the only factor determining the presence or absence of star-star coupling
since linking of the longer arms was carried out at lower temperature (–80 °C).
The effect of the nature of the divinyl monomer was also studied; in contrast to
6, diisopropenylbenzene (DIPB, 7) was found to be inefficient.
The synthesis of multiarm star PIB has also been attempted by the TMP-
Cl/TiCl 4 initiating system in CH 3Cl/hexane (40/60 v/v) at –80 °C in the presence
of pyridine using 6 and 7 as the core forming monomers [8]. Similar to findings
by Marsalko et al. [7], DIPB was found to be inferior in comparison with DVB,
due to slow and incomplete star formation. With DVB the star polymer formed
more rapidly and using a [DVB]/[TMPCl]=10 ratio to effect linking, star with
DP n(arm) =250 and the average number of arms=23 was obtained in 18.5 h with
only 4% unlinked PIB arm. Importantly, star-star linking was found to be absent
at –80 °C, and thus the product exhibited a relatively narrow MWD. The effect of
the PIB arm length on the synthesis of multiarm star PIB was also investigated
[9]. Similarly to results reported by Marsalko et al. [7, 10], it was found that with
increasing arm MW (from 10,000 to 56,000 g mol –1 ), dramatically increased
linking times (from 24 to 568 h) were necessary to ensure high incorporation of
the PIB arms into the star molecule. Simultaneously, the weight average number
of arms decreased from 54 to 5 respectively. It was also found that the intrinsic
viscosity of the star PIB was much lower than that of a linear analog of an equivalent
MW.
Structure-property relationship of multiarm star PIBs has been investigated
by a variety of techniques including viscometry, pour points, electron microscopy,
and ultrasonic degradation [11]. The intrinsic viscosity of star PIBs changes
very little in the 30–100 °C range in contrast to that of linear PIBs of the same
MW which increases strongly with temperature. The viscosity of star PIBs was
mainly determined by the MW of the arm and was relatively independent of the
number of arms. Transmission electron micrograph of a star PIB showed a
spherical shape with 55±4 nm in diameter which was in reasonable agreement
with the radius of gyration R g=27 nm determined from light scattering. The
(7)