Chemistry Reference
In-Depth Information
here particularly effective. By means of a more electrophilic initiator, Ds as low as
1.1 were achieved. Using polystyrene containing an activated C-F end group, two-
block copolymers or miktoarm star copolymers were also synthesized by this
approach [
109
].
Further studies of the Yokozawa group concerned CCPs of aromatic polyesters.
At first, polymerizations of 4-trimethylsiloxybenzoyl chloride and its anion (see
Formula
16.7
), first reported by Yokozawa et al. [
110
] were studied Yet the
insolubility of poly(oxibenzoate) in all inert solvents prevented a satisfactory
characterization of the reaction products [
111
]. Therefore, a new class of mono-
mers (see Formula
16.7
d) was synthesized. The aliphatic side chains rendered the
polyesters soluble in various inert solvents, and the activated heterocyclic amid
group eased purification by recrystallization To prevent transesterification the
polymerization temperature had to be lowered to 30 C[
112
,
113
].
Most of the work of the Yokozawa group was dedicated to CCPs of monomers
yielding soluble N-alkyl polyaramides [
114
-
117
]. At first monomers derived from
4-aminobenzoic acid were studied (see Formula
16.8
, top). Deprotonation by a
metal amide yielded nitrogen anions which deactivated the ester groups of the
monomers, so that with an electrophilic initiator perfect CCPs were obtained
(typically in THF at 20 C). The ethyl esters of N-alkyl-3-amino benzoic acid also
proved to be useful monomers. The transmission of electronic effects via the
p-electrons is relatively weak in this case, but it is supported by a positive
inductive effect.
Due to the shorter distance between amino and carbonyl group. In addition to
numerous homopolesters having low DPs two-block copolymers either containing
3- and 4-aminobenzoyl units or different substituents or a block having a different
backbone were prepared. Star-shaped polyamides were obtained from multifunc-
tional initiators, and hyperbranched polyamides from diethyl 5-(N-methyl-
amino)isophthalate (see Formula
16.8
)[
118
]. Various details of all these CCPs
were presented and discussed in two review articles [
119
,
120
] and in five more
recent publications [
112
-
125
].
16.4.1 Catalyst-Transfer Polymerizations
Syntheses of fully conjugated aromatic polymers by C-C coupling from dihalo-
aromatics frequently proceed via CCP involving migration of the catalyst to the
active chain end. A typical example is syntheses of poly(alkyl thiophene)s as
outlined in Formula
16.9
each growing step involves elimination of MgBrCl, but
the Ni-catalyst migrates to the new chain end forming an activate end group.
Formally the catalyst also plays the role of an initiator, so
that the DPs parallel the M/I ratio. Furthermore, Yokozawa et at. [
126
,
127
]
obtained PDs as low as 1.1 under optimized reaction conditions, whereas high DPs
were reported in the original work of McCullough [
128
,
129
]. Both research
groups explored and improved the polymerization of dihalo thiophenes in various
