Chemistry Reference
In-Depth Information
Further interesting findings resulted from syntheses of aromatic poly(ether
sulfone)s [
64
,
65
]. The fraction of cycles increased at the expense of linear chains
with increasing conversions. Due to the lower reactivity, 4,4
0
-dichlorodiphenyl
sulfone yielded lower molar masses than 4,4
0
-difluorodiphenyl sulfone, with the
consequence that the fraction of cycles was lower. Poly(ether sulfone)s derived
from tert-butyl catechol ((d) in Formula
7.3
) proved to be particularly well suited
to MALDI-TOF mass spectrometry. The polyether with the highest molar mass
gave a spectrum showing mass peaks of cycles up to 20 kDa. After fractionation
mass peaks of cyclic polyethers up to 27 kDa were achieved and no signals of
linear chains were detectable. However, the fraction above 27 kDa certainly
contained linear chains, because in a real experiment 100 % conversion without
any side reaction cannot be achieved. The formation of cyclic polyethers in syn-
theses of poly(benzonitrile ether)s (e.g. (a) in Formula
7.4
), of poly(pyridine
ether)s ((b) in Formula
7.4
) and of poly(ether ketone)s was also screened by
MALDI-TOF mass spectrometry [
66
-
68
].
Due to the aforementioned work of Horbach et al. [
50
,
51
], Kricheldorf et al.
[
69
-
72
] reinvestigated the hydrolytic interfacial polycondensation of bisphenol-A
bischloroformiate and also the interfacial phosgenation of bisphenol-A and bi-
sphenol-M. Tertiary amines and quarternary ammonium or phosphonium salts
were used as catalysts, and the reaction conditions were optimized for high
molecular weights (indicating high conversions). For reasons discussed in
Chap. 8
,
extremely high conversions and molar masses were achieved (Mn's up to
400 kDa). As overriding trend, it was found that the fraction of cyclic polycar-
bonates increased with the molar mass of the entire reaction products. For the best
samples the MALDI-TOF mass spectra exclusively displayed peaks of cycles
observable up to 20 kDa. After fractionation mass spectra were obtained exhib-
iting peaks of cycles up to 50 kDa [
69
] or 55 kDa [
71
] (Fig.
7.7
). This mass
corresponds to a DP around 220 or to 2,600 ring forming atoms. This result allows
three important conclusions:
(A) Flory's statement [
1
] that the end groups of long chains will not meet in the
course of a normal KC polycondensation is definitely wrong. Cyclization can
compete with chain growth at any chain length.
(B) The conclusions of Horbach et al. [
50
,
51
] were justified.
(C) Since cyclic DNA in protozoa and other microorganisms frequently consist of
approx. 400 base pairs corresponding to 2500 ring atoms, the gap between
large cyclic biopolymers, and cyclic polymers formed in a normal KC poly-
condensation is closed.
Concerning point (C) Stasiak [
73
], who synthesized cyclic DNA in test tubes by
means of the enzyme ligase stated: ''Ligase which is frequently used for DNA
circularization experiments waits until thermal motion brings together DNA ends
in the correct polarity (3
0
with 5
0
) and the connects these ends covalently''. Cer-
tainly long DNA chains may form sequence-dependent loops and other confor-
mations favoring ring closure more than the conformations of a freely rotating
chain. Nonetheless, the formation of large DNA rings in living organism is not a
