Chemistry Reference
In-Depth Information
16.4 Linear Monomers
The first report of CCPs of linear monomers concerned dehydrohalogenation of
para-halophenols and para-halothiophenols; Lenz et al. [
89
,
90
] studied poly-
merizations of alkali metal salts of 4-halothiophenols as part of a broader study on
In agreement with a cationic radical mechanism, the reactivity of the haloatoms
decreased in the order J [ Br [ Cl [ F. Furthermore, it was found that chain
growth and monomer consumption did not agree with the Carothers equation and
high oligomers were formed at low conversion. These polymerizations were
performed in solution, but the poly(phenylene sulfide) precipitated from the
reaction mixture and the chain growth continued in the solid state, albeit a 50-100
times lower rate [
90
].
Despite the complex mechanism, these polymerizations showed the charac-
teristic features of CCP of aromatic monomers. The delocalization of electron
density from the nucleophilic (anionic) ''a''-function to the electrophilic ''b''-
function across the aromatic ring (mesomeric effect) considerably reduces the
electrophilicityof the monomer relative to the ''b''-end group of oligomers and
polymers. This means, that the first step, namely the reaction between two
monomers, is the slowest step of the entire polymerization process.
The oxidative polymerization of 2,6-dimethylphenol (invented by Hay et al.
[
91
], see also
Chap. 8
) and the polymerization of potassium 4-bromo-2,6-dim-
ethylphenoxide (see Formula
16.6
) have both the character of CCPs. The former
CCP needs oxygen as reaction partner and a Cu
1+
amine complex as catalyst. Heitz
et al. [
92
] observed that contrary to a normal polycondensation high oligomers
were formed at low conversions, and he formulated a speculative radical-cation
mechanism. The CCP of 4-bromophenoxide salts needs Cu
2+
or other oxidizing
metal ions as reaction partners and also involves a radical-cation mechanism [
93
].
When 4-methyl- or 4-tert.butylphenol are added as initiators, linear chains having
one OH end group are formed, whereas addition of tetramethyl bisphenol yields
telechelic polyethers (see Formula
15.6
).
Control of Mn was feasible below 10 kDa and DPs \ 1.5 were found. Higher
molar masses with little control via the M/I ratio were obtained from other initi-
ators [
94
-
97
]. Numerous polymerizations of metal salts of various halophenols
were reported by several research groups beginning with the work Torey and
Hunter in 1911 [
98
]. In many cases, more than one haloatom was attached to the
phenol and branching or other side reactions occurred. The hypothesis of CCP
versus polycondensation was not discussed. A detailed review of all these poly-
merizations has been published [
99
].
Whereas all aforementioned CCPs are characterized by a radical mechanism,
syntheses of aromatic polyethers via CCPs involving normal nucleophilic substitu-
tion at activated C-Cl or (better) C-F bonds were also reported. The reactivities of 4-
oxo-4
0
-fluorobenzophenone and 4-oxo-4
0
-chlorodiphenylsulfone (see Formula
16.6
,
bottom) were examined by a research group of ICI [
100
-
103
]. Although oxide anion
