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

Synthesis of Branched Polymers by Cationic Polymerization 31
2.3.1
A n -Type Star Homopolymers
2.3.1.1
Poly(vinyl ethers) n
Monofunctional malonate ions were shown to terminate quantitatively living
cationic chain ends of poly(vinyl ether)s to give stable carbon-carbon bond [45]
even when they were used in stoichiometric concentration [46]. The poor solubility
of their multifunctional counterparts in organic solvents could be overcome
by the use of 18-crown-6 to dissolve them in THF. Coupling reactions of
living poly(IBVE), formed by initiation with the HI/ZnI 2 system, were performed
using the trifunctional coupling agent 22 and the tetrafunctional 23.
With 22, a three arm polymer was recovered in 56% yield and with 23, only three
out of the four anions reacted to give three arm polymer in 85% yield. Such incomplete
reactions were explained by poor solubility as well as steric hindrance
at the coupling sites.
(22)
(23)
The same authors chose another very reactive nucleophilic function, the silyl
enol ether group, which upon reaction with living cationic chain ends of poly(vinyl
ether)s, also leads to a carbon-carbon bond with formation of a ketone
(Scheme 4). Model reactions of living poly(IBVE) with various monofunctional
silyl enol ethers [47] showed that the a-substituent R should have electron-donating
properties in order to increase the electron density on the double bond.
Scheme 4