Chemistry Reference
In-Depth Information
revealed an a-transition around 330 C, a b-transition around 455-460 C, and
melting with rapid degradation above 540 C[
83
]. Again the a-transition yielded a
mobile smectic-E phase favoring efficient SSP. The polyester of 4-hydroxybi-
phenyl-4
0
-carboxylic acid (see Formula
14.4
) was first prepared by the authors [
84
,
85
]. Its crystal modifications were also studied by Liu et al. [
86
]. Phase transitions
at 305 and 409 C were difficult to detect in the DSC trace and depended very
much on the reaction conditions. Due to the high thermostability of this polyester
the Tm was as high as 585 C, and successful polycondensations were performed
at temperatures up to 400 C, but unfortunately no information about DPs was
reported. The polyester of 4-mercaptobenzoic acid was also first prepared by the
author [
87
]. The dimensions of the crystal lattice below and above the a-transition
at 365 C were quite similar to those of POB. Successful SSPs yielded DPs up to
200 [
87
] and, surprisingly, whiskers were prepared below the a-transition [
88
].
Due to the low thermostability transformation into a crosslinked resin began
shortly above the a-transition [
88
]. Oligoesters of 4-hydroxycinnamic acid were
prepared by Elias using thionyl chloride [
89
] and whiskers were later prepared by
the Kimura group [
90
,
91
]. The same group also reported on SCCG of an acety-
lated hydroxyl carboxylic acid containing an azomethin group (see Formula
13.4
)
[
92
], but neither information on DPs nor on phase transitions was disclosed
[
90
-
92
].
Further studies of SCCG concerned hydroxyl carboxylic acids containing het-
erocycles (see Formula
14.4
and
14.5
)[
93
-
97
]. The acetylated carboxylic acids
and their trimethylsilyl esters were used as monomers. Due to the high thermo-
stabilities of these polyesters most polycondensations were conducted at 350 or
400 C. Whisker-like crystals were obtained in most cases suggesting that efficient
SCCG took place, but no information on DPs or molar masses was disclosed.
Furthermore, SCCG of poly(ester amide)s from N-(4
0
-hydroxybenzoyl)-4-amino-
benzoic acid or its trimethylsilyl ester (see Formula
14.5
) were reported [
98
].
Various crystalline morphologies were found including ''shavings'', but detailed
information the chain growth in the solid state was not provided. From the anal-
ogous vanillic monomer only copolyesters containing 4-HBA were prepared [
99
].
All these polyesters presented above have in common that the syntheses are
based on hydroxycarboxylic acids. Little attention has been paid to SSP of
diphenols and aromatic dicarboxylic acids [
100
-
105
]. Whiskers of poly(hydro-
quinone terephthalate) were prepared by Yamashita et al. [
100
]., but a phase
transition below Tm was only observed for copolyesters rich in 4-HBA units [
101
].
In one paper of the author [
102
] acetylated hydroquinone or 4,4
0
-dihydroxybi-
phenyl were polycondensed with terephthalic acid or 4,4
0
-biphenyl dicarboxylic
acid and their trimethylsilyl esters. In this way four homopolyesters were prepared
at 350 or 400 C. No phase transitions were detected blow 500 C. No information
on the extent of the chain growth was given. Unfortunately, end group analyses do
not yield reliable information about DPs for a
2
? b
2
polycondensations. Yet, the
relatively high concentrations of CO
2
H end groups suggest that oligomers having
two CO
2
H end groups preferentially precipitated from the reaction mixture, but an
excess of such oligomers in the solid state hinders efficient chain growth.
