The first synthesis of well-defined star polymers consisting of
eight polyisobutylene (PIB)
arms emanating from a calixarene core is described. The synthesis
was accomplished with the use of
octafunctional calixarene derivative initiators which, in conjunction
with BCl3/TiCl4 coinitiators,
induce
the living polymerization of isobutylene (IB). Specifically, the
initiators were tert-hydroxy and
tert-methoxy
derivatives (5 and 6, respectively) of
5,11,17,23,29,35,41,47-octaacetyl-49,50,51,52,53,54,55,56-octamethoxycalix[8]arene. The tert-methoxy
derivative (6) is soluble in polymerization charges
(CH3Cl/hexanes) at −80 °C and was used preferentially. To gain insight
into the mechanism of polymerization,
model experiments using a monofunctional analogue,
2-(p-methoxyphenyl)-2-methoxypropane (8), of
the
octafunctional initiators were also conducted. According to these
model studies, 8 is an efficient initiator
for the living polymerization of IB, and the initiator residue can be
spectroscopically quantitated as the
head group of PIB. Well-defined stars with close to theoretical
composition and molecular weights were
obtained in the presence of BCl3−TiCl4
mixtures; however, polymerizations were unsatisfactory with
the
use of only BCl3 or only TiCl4.
Polymerizations were carried out in two stages in one reactor.
In the first
stage, the reaction was initiated with the
6/BCl3 combination using ∼25% of the required
amount of IB
in CH3Cl at −80 °C, followed by the second stage,
in which hexanes, TiCl4, and the balance of the
monomer
were added. Gel permeation chromatography, by the use of on-line
RI, UV, and laser light scattering
detectors, was used to determine the molecular weight and composition
of the linear and octa-arm star
polymers. The number-average number of arms,
N̄
n,arm, and the weight-average number of
arms, N̄
w,arm,
were determined directly by core destruction of the calixarene moiety,
followed by molecular weight
determination of the surviving arms; according to these studies,
N̄
n,arm and
N̄
w,arm were 7.6 and 7.7,
respectively.
