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

Synthesis of Branched Polymers by Cationic Polymerization 23
15,000 g mol –1 due to polymer precipitation, exhibited close to theoretical M n s
and M w /M n s in the range 1.3–2.0. The structure of the products has been analyzed
by 1 H NMR spectroscopy and found to be essentially identical to those obtained
by tricumyl chloride. The reaction between tricumyl methyl ether (15)
and BCl 3 was investigated by Zsuga et al. using 13 C and 11 B NMR spectroscopy in
CH 2 Cl 2 at –30 °C [29]. According to the results, tricumyl methyl ether and BCl 3
yield tricumyl chloride and BCl 2 OCH 3 in a fast reaction, thus the true initiator
may be the chloro derivative. Interestingly the corresponding exchange reaction
did not take place with tricumyl acetate (16)/BCl 3 system which also efficiently
initiates the polymerization of IB [30]. The product upon quenching the polymerization
however was the chloro functional three arm star PIB. Similarly to tricumyl
methyl ether, tricumyl alcohol (17), only partially soluble in CH 3Cl at –
50 °C, was found to be rapidly converted to the soluble choride derivative in a reaction
with BCl 3. Thus three arm star PIBs have also been obtained by premixing
tricumyl alcohol with BCl 3 for 10 min followed by the addition of IB [31, 32].
Polar solvents such as CH 2Cl 2 or CH 3Cl are poor solvents for PIB and therefore
the MW that can be obtained with BCl 3 is limited. In contrast to BCl 3, TiCl 4
coinitiates the polymerization of IB even in moderately polar solvent mixtures,
which dissolve high MW PIB at low temperatures. Organic esters, halides, and
ethers can all be used to initiate living polymerization of IB. Ethers are converted
to the corresponding chlorides almost instantaneously, while the conversion of
esters is somewhat slow. Alcohols are inactive with TiCl 4 alone but have been
used in conjunction with a mixture of BCl 3 and TiCl 4; BCl 3 converts the alcohols
to the active chloride which is activated by TiCl 4. Well defined three arm star PIB
of controlled MW have been obtained by many groups [32–34] with the 14 or
16/TiCl 4 initiating systems or by using 17 with the combination of BCl 3 and TiCl 4
under similar conditions, i.e., in CH 3 Cl or CH 2 Cl 2 /hexane (40/60 v/v) solvent
mixture at –80 °C in the presence of a Lewis base.
Four arm star PIB has been prepared by living polymerization with the
3,3',5,5'-tetra(2-acetoxy-isopropyl)biphenyl (TCumOAc, 18)/BCl 3 initiating system
in dilute solutions in the –35 to –80 °C range [35].
(18)
In CH 3 Cl/n-hexane (40/60 v/v) mixtures, very low conversion and ill-defined
products were obtained, presumably due to the very low solubility of the 18/BCl 3
complex. Precipitation was also observed in pure CH 3 Cl when [IB]>0.129 mol l 1 .