Free-radical chain generation of ketene during the synthesis of liquid crystalline aromatic polyesters

Article abstract of DOI:10.1016/j.tetlet.2009.08.099

At the high temperatures used for the preparation of liquid crystalline aromatic polyesters (LCPs), ketene cleavage occurred from acetoxyarenes. Ketene is known to oligomerize to colored oligomers which may be responsible for an undesirable yellow color i

Full text of DOI:10.1016/j.tetlet.2009.08.099

Tetrahedron Letters 50 (2009) 6743–6744
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Free-radical chain generation of ketene during the synthesis of liquid
crystalline aromatic polyesters
*
Mihaiela C. Stuparu, Juhua Xu, H. K. Hall Jr.
C. S. Marvel Laboratories, Department of Chemistry, The University of Arizona, Tucson, AZ 85721, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2009
Revised 26 August 2009
Accepted 27 August 2009
Available online 2 September 2009
At the high temperatures used for the preparation of liquid crystalline aromatic polyesters (LCPs), ketene
cleavage occurred from acetoxyarenes. Ketene is known to oligomerize to colored oligomers which may
be responsible for an undesirable yellow color in LCPs. Thermolysis of the model compound p-acet-
oxybenzoate gave ketene oligomers and methyl p-hydroxybenzoate, which oligomerized. A free-radical
chain reaction mechanism for ketene formation was demonstrated by the reaction of tributyltin hydride
with haloacetoxyarenes.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
perature some of the mixed anhydride intermediates thermolyze
to ketene and aromatic carboxylic acid; a nitrogen sweep carried
This study explored the possibility of ketene formation and
oligomerization contributing to the yellowing observed during
aromatic main chain liquid crystal polymer (LCP) synthesis.1,2
Such yellowing limits potential LCP uses as optical materials. In
earlier LCP work, it was shown that at high polymerization tem-
ketene out of the reaction to a trap where its occurrence was
demonstrated by the reaction with aniline to form acetanilide
(3).3 If the ketene is left in the reaction, it can oligomerize; and
polyketene is variously described as ranging from yellow4 to dark
brown-black.5
(Mechanism A)
(concerted)
O C
H CH2
O
H2C C O
+
OH
OH
H
C C
n
polyketene
O
(Mechanism B)
H2C C O
+
O
H2C C O
(free radical chain)
O
O
+
+
, etc
O
OH
H3C C O
H2C C O
Scheme 1.
* Corresponding author. Tel.: +1 520 621 6325; fax: +1 520 621 8407.
E-mail address: hkh@u.arizona.edu (H.K. Hall Jr.).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.099

6744
M. C. Stuparu et al. / Tetrahedron Letters 50 (2009) 6743–6744
O
O
O
O
O
H3CO C
O C CH3
n
H3CO C
O H +
n
H3CO C
O C CH3
n
ΔΔ
1
: n = 1
6: n = 2-7
3: n = 1
2: n = 2
4
: n = 2
5: n = 3-7
7: higher oligomers (n > 7)
8: polyketene
Scheme 2.
O
O
Bu3SnH
BuO)
9 gives 3 + 8
H3CO C
O C CH2X
n
10
11
3
4
8
8
gives
gives
+
+
(t
2
9
: n = 1, X = Cl
10: n = 1, X = I
11: n = 2, X = Cl
Scheme 3.
The mechanistic question arises: Is this ketene formation a con-
certed electrocyclic reaction (Scheme 1, A) or a free-radical chain
reaction (Scheme 1, B). The literature6–11 supports either
mechanism.
4. Conclusions
At high temperature polymerization used in LCP synthesis,
model compounds 1 and 2 cleave to ketene and two sets of p-oxy-
benzoyl oligomers. To decide whether the cleavage mechanism
was either a concerted or a free-radical chain, the feasibility of
the latter mode was demonstrated at lower temperatures. The pos-
tulated chain-carrying acyloxy radical was generated by either
hydrogen or halogen abstraction and shown to readily lose ketene.
2. Reactions in the absence of free-radical generators
(Scheme 2)
Heating either model acetoxyarene 1 or 2 at 280–320 °C for
20 min gave a dark yellow reaction mixture. The mixtures were
analyzed by GC–MS. Isolation of low molecular weight products
was done by flash column chromatography. Higher oligomers were
detected using ESI-MS techniques. An intractable residue, soluble
only in pentafluorophenol, presumably a mixture of higher molec-
ular weight oligoesters, was also formed. The yellow component of
8 was highly polar, requiring methanol for elution but it could not
be more closely characterized. These results are explained by ther-
mal cleavage of 1 or 2 to hydroxyesters 3 and 4 which can undergo
intermolecular methanolysis to form 3–5.
Acknowledgements
We thank Solvay Advanced Polymers for support, Mr. Jeffrey
Robertson for the thermolysis of 6, Dr. Ian W. Jones, and Professor
R. B. Bates for assistance.
References and notes
1. Yoon, H. N.; Charbonnean, L. F.; Calundann, G. W. Synthesis Processing and
Properties of Thermotropic Liquid Crystal Polymers. Advanced Materials,
Weinheim, Wiley-VCH, 1992; Vol. 4, pp 206–213.
2. Negi, Y. S.; Goyal, R. K. Int. J. Plastics Tech. 2003, 7, 99–118 [CAN 142:23533r].
3. Han, X.; Williams, P. A.; Padias, A. B.; Hall, H. K., Jr.; Sung, H. N.; Linstid, H. C.;
Lee, C. Macromolecules 1996, 29, 8313.
3. Reactions in the presence of free-radical generators
(Scheme 3)
4. Yamashita, Takeya, Japan Tokkyo Koho [CAN 78 31587].
5. Khemani, K. C.; Wudl, F. J. Am. Chem. Soc. 1989, 111, 9124–9125.
6. Nishida, S.; Imai, T.; Tsuji, T. Chem. Lett. 1974, 11, 1303–1304.
7. Barefoot, A. C.; Carroll, F. A. J. Chem. Soc., Chem. Commun. 1974, 9, 357.
8. Kaspar, M.; Prochaszka, M. Collect. Czech. Chem. Commun. 1974, 39, 3124–3130.
9. (a) Ghibaudi, E.; Colussi, A. J. J. Chem. Soc., Chem. Commum. 1984, 7, 433–434;
(b) Ghibaudi, E.; Colussi, A. J. Int. J. Chem. Kinet. 1984, 16, 1575–1583.
10. Taylor, R. J. Chem. Soc., Perkin Trans. 2 1988, 2, 1839.
Hydrogen abstraction was achieved by heating 1 with di-t-butyl
peroxide at 150 °C under argon gave a mixture of 3 and polar yel-
low material 8, along with unreacted 1. More decisively, halogen
abstraction was achieved by heating the halo compounds 9–11
with tributyltin hydride in the presence of di-t-butyl peroxide
which converted them to 3 and 4 in virtually quantitative yields.
11. Chihara, T.; Kaniguchi, S. Chem. Lett. 2002, 1, 70–71.