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

Synthesis of Branched Polymers by Cationic Polymerization 9
were: the length of the arms (DP n ), the initial concentration of the linear precursor
[poly(IBVE)], and the value of the molar ratio r=[divinyl compound]/[poly(IBVE)].
The major conclusions are the following:
– when r is increased, the yield of star polymer increases together with its MW
and its average arm number; these last two points being correlated with an increase
of the weight fraction of the core
– when [poly(IBVE)] is increased, the MW of the final product as well as the average
number of arms increases (in the studied series, the star polymer yield
was high and independent of [poly(IBVE)] because very favorable conditions
were used, i.e., high value of r and short arm)
– when the length of the arms is short, the overall MW is lower but the star polymer
yield as well as the number of arms is higher; this indicates that the
intermolecular linking reaction of the intermediate block copolymer of IBVE
and 1 is sterically less hindered for shorter chains.
In addition to the effect of the experimental conditions, the influence of the
nature of the arms and of the divinyl compound was also studied. It was shown
that bulkiness of the arms strongly influences the yield of star polymer; for instance,
arms of poly(cetyl vinyl ether) were linked in very low yield as compared
with poly(IBVE). The influence of the structure of the divinyl ether was investigated
and appears to be of great importance. Coupling with 3 and 4 led to low
yield of star polymer, while the efficiency of 1 and 2 was much higher. The explanation
provided by the authors was that compact and flexible spacers between
the two vinyl groups of 3 and 4 could lead to smaller cores where further
reaction of incoming chains would be sterically hindered.
2.1.1.2
Poly(alkoxystyrenes) n
Preparation of star polymers of p-methoxystyrene (p-MOS) and p-tert-butoxystyrene
(tBOS) using two different bifunctional vinyl compounds 1 and 5 was reported
by Deng et al. [5].
Living cationic polymerization of both styrenic monomers was carried out
with the use of the HI/ZnI 2 initiating system in CH 2Cl 2 at –15 °C in the presence
of tetra-n-butylammonium iodide. The obtained living polymers of p-MOS of
various lengths were allowed to react with both divinyl monomers 1 and 5 with
a ratio r=3 to 7. With 1 the yield of star polymer was very low and a large amount
of poly(p-MOS) remained unreacted. This was ascribed to the much higher reactivity
of the divinyl ether compared with the styrenic monomer. This led to a
very fast second stage polymerization and the major part of the linear precursor
(5)