The first polycondensation through a free radical chain process

Full text of DOI:10.1021/ja970165g

8718
J. Am. Chem. Soc. 1997, 119, 8718-8719
Scheme 1
The First Polycondensation through a Free Radical
Chain Process
Takeshi Endo,* Nobuo Torii, Toshikazu Takata,†
Tsutomu Yokozawa,‡ and Toshio Koizumi§
Research Laboratory of Resources Utilization
Tokyo Institute of Technology, Nagatsuta-cho
Midori-ku, Yokohama 226, Japan
Scheme 2
Department of Applied Chemistry, College of Engineering
Osaka Prefecture UniVersity, Sakai, Osaka 593, Japan
Department of Applied Chemistry
Faculty of Engineering, Kanagawa UniVersity
Rokkakubashi, Kanagawa-ku, Yokohama 221, Japan
Department of Chemistry, National Defense Academy
Hashirimizu, Yokosuka 239, Japan
addition of thiol to allene as a model reaction similarly proceeds
to give corresponding 1:1 adduct in a certain selectivity.6,8 When
the radical addition of thiophenol to chloroallene (1) (1:1 ratio)
was carried out, we obtained a few products including 1,2-bis-
(phenylthio)propene (2, 15%) and 1-chloro-2-phenylthiopropene
(11%, a formal 1:1 adduct) produced by central carbon attack
of sulfur.9 We noticed the formation of 2, because it could be
emerged presumably through an elimination of hydrogen
chloride from the corresponding 1:2 adduct. To scavenge the
hydrogen chloride evolved during the reaction, propylene oxide
(PO) was added to the reaction system to increase the yield of
2 to 44%. Further, use of 2 mol of thiophenol enhanced the
yield up to 81%, but another 1:1 adduct, 1-chloro-3-phenylthio-
propene (3), was still produced in 9% yield in addition to 10%
of unidentified product (UIP) (Scheme 1). Therefore photo-
initiated radical reaction10 was examined to elevate the product
selectivity by lowering the temperature from 60 to 20 °C. As
a result, the yields of 2 and 3 were 91 and 7%, respectively,
whereas the formation of UIP was almost suppressed.
The polymerization of 1 and aromatic dithiols 4-6 was
conducted at 20 °C for 72 h in toluene under photoirradiation
(Scheme 2, Table 1). The usual workup gave a polymeric
product of which structure was determined mainly by the NMR
spectra as the polymer formed by polycondensation. Typical
1H NMR spectrum of the polymer PS is shown in Figure 1
(top). The simple signal pattern of the spectrum indicated that
the polymer presumably consists only of 1,2-dithiopropene unit,
while the regioisomeric ratio of cis and trans is nearly 1:1. The
13C NMR spectrum supported the proposed structure (Figure
1). The selected results of the polymerizations under varying
conditions are summarized in Table 1.
The polymerization of 1 and 4 was carried out under the
conditions same as those of Scheme 1a (Table 1, run 1) and
gave only 12% yield of polymer with Mn 1300 (peak top value)
which was roughly equal to Mn calculated from the product ratio
of Scheme 1a. In the polymerization under the photoirradiation
at 60 °C (run 2), the yield increased to 24%, although Mn was
fairly low. However, at 20 °C the yield increased to 37% (run
3). With prolonged reaction (run 4), the polymer with Mn 2700
was obtained in 71% yield. It was found that use of dithiol in
slight excess of up to 1.2 equiv enhances both the yield and
molecular weight (runs 5-8). Further loweing of the reaction
temperature (7 °C) affected the results (run 10) little. Decrease
in concentration gave a little better yield and higher Mn
presumably because of the reduced viscosity of the polymeri-
zation system (run 11).
ReceiVed January 21, 1997
The free radical chain reaction is one of the most important
polymerization processes due to its practical advantages, e.g.,
its susceptibility to water much lower than that of ionic
polymerization. The free radical process has been applied only
to addition polymerization and polyaddition. Recent develop-
ment of free radical ring-opening polymerization1 has largely
expanded the possibility of ring-opening polymerizations which
have been limited to ionic process.2 Meanwhile, polyconden-
sations usually proceed with an ionic mechanism via an acid-
or base-catalyzed reaction,3 in addition to those involving
intervention of organometallic intermediates.4 However, to the
best of our knowledge, there has been no example of polycon-
densation through a free radical chain process. Provided
polycondensation through a free radical chain process could
occur, a new entry to polymer synthesis would be opened and
the availability of polycondensation as useful synthetic method
would be enlarged. During our extensive study on polymeri-
zation of allenes and their homologues as vinyl-functionalized
vinyl monomers,5-7 we have recently found the first example
of polycondensation through free radical chain process, which
is disclosed in this paper.
Bisallenes react with dithiols to afford corresponding poly-
addition products in high yields under typical radical conditions
via selective formation of central carbon-sulfur bond.6 Radical
* Author to whom all correspondence should be addressed.
† Osaka Prefecture University.
‡ Kanagawa University.
§ National Defense Academy.
(1) (a) Endo, T. Kobunshi 1986, 35, 272. (b) Endo, T.; Yokozawa, T.
In New Method for Polymer Synthesis; Mijs, W. J., Ed.; Plenum Press:
New York, 1992; p 155. (c) Bailey, W. J.; Chen, P. Y.; Chen, S. C.; Chaio,
W.-B.; Endo, T.; Gapud, B.; Kuruganti, V.; Lin, Y.-N.; Ni, Z.; Pan, C.-Y.;
Shaffer, S. E.; Sidney, L.; Wu, S.-R.; Yamamoto, N.; Yamazaki, N.;
Yonezawa, K.; Zhou, L. L. Makromol. Chem., Macromol. Symp. 1986, 6,
81.
(2) (a) Saegusa, T., Goethals, E., Eds.; Ring-Opening Polymerization;
ACS Symposium Series 59; American Chemical Society: Washington, DC,
1997. (b) Ivin, K. J., Saegusa, T., Eds.; Ring-Opening Polymerization;
Elsevier: New York, 1984.
(3) (a) Marvel, C. S.; Markhart, A. H., Jr. J. Am. Chem. Soc. 1948, 70,
993. (b) Marvel, C. S.; Aldrich, P. H. J. Am. Chem. Soc. 1959, 81, 1978.
(c) Marvel, C. S.; Markhart, A. H., Jr. J. Am. Chem. Soc. 1951, 73, 1064.
(d) Marvel, C. S.; Roberts, W. J. Polym. Sci. 1951, 6, 711. (e) Marvel, C.
S.; Cripps, H. N. J. Polym. Sci. 1952, 8, 313. (f) Marvel, C. S.; Olson, L.
E. J. Polym. Sci. 1957, 26, 23.
(4) For example, see: Ueda, M.; Ichikawa, F. Macromolecules 1988,
21, 1908.
(5) Yokozawa, T.; Tanaka, T.; Endo, T. Chem. Lett. 1987, 1831. (b)
Yokozawa, T.; Ito, N.; Endo, T. Chem. Lett. 1988, 1955. (c) Mizuya, J.;
Yokozawa, T.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 1990, 28,
2765. (d) Idem. J. Am. Chem. Soc. 1989, 111, 743.
(6) (a) Sato, E.; Yokozawa, T.; Endo, T. Macromolecules 1993, 26, 5185,
5187. (b) Idem. Makromol. Chem., Rapid Commun. 1994, 15, 607. (c)
Idem. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 669.
(7) (a) Endo, T.; Tomita, I. Prog. Polym. Sci. 1997, 22, 565. (b) Idem.
Trends Macromol. Res. 1994, 1, 151.
(8) (a) Pasto, D. J.; Hermine, G. L. J. Org. Chem. 1990, 55, 685. (b)
Ogawa, A.; Kawakami, J.; Sonoda, N.; Hirao, T. J. Org. Chem. 1996, 61,
4161.
(9) This result was consistent with that of Pasto et al.,8a although they
reported no details such as yield of the products. Other products: abnormal
1:1 adduct, 1-chloro-3-phenylthiopropene (6%), and 1:2 adduct, 1,2-bis-
(phenylthio)-2-chloropropane (19%).
(10) Reaction conditions: Pyrex filter, toluene, 20 °C, 72 h, PO (2 equiv),
and irradiation with a 400 W high-pressure mercury lamp.
S0002-7863(97)00165-0 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 37, 1997 8719
Scheme 3
photochemically dissociates to give thiyl radical. The poly-
condensation mainly proceeds via initial attack of the thiyl
radical at central carbon of 1 and subsequent attack of the
resulting thiyl radical at the halogen-bonded carbon via anti-
Markovnikov mode leading to R-thioradical11 from which
halogen radical is eliminated to afford 1,2-dithiopropene unit.
Use of excess amount of 1 (1.1 equiv) results in enhanced
yield and molecular weight and use of a larger excess of 1 (∼1.5
equiv) does not affect them as seriously, unlike usual polycon-
densation (Table 1), clearly suggesting that the second addition
of thiyl racial is somewhat faster than the first one. This would
be confirmed from the fact that only 65% of 1 was exhausted
while 100% of thiophenol was consumed, when the addition
reaction was carried out in 1:1 ratio (AIBN, 60 °C). Attack of
thiyl radical at terminal carbon of 1 should result in a termination
reaction from the results of the model reaction (Scheme 1).
Meanwhile, due to the greater leaving ability of the Br radical,
bromoallene (7) was employed instead of 1.12 Table 1 (runs
13 and 14) indicates that use of 7 causes certain increase in
yield and molecular weight,13,14 indicating some favorable effect
of the replacement of Cl with Br.
Figure 1. 1H (CDCl3) and 13C NMR (benzene-d6) spectra of polymer
(PS1) prepared by free radical polycondensation of 1 and 4 at 20 °C
under photoirradiation.
Thus, the above-mentioned results unambiguously suggest
that this type of novel polycondensation through free radical
chain process can be categorized as an addition-condensation.
It should be of great significance to establish the first “radical
polycondensation”, and some excellent molecular designs based
on the results of this study will provide further useful examples
to develop the new polymerization “radical polycondensation”.
Table 1. Free Radical Polycondensation of Haloallenes and
Dithiolsa
molar
polymer (PS)
ratio of
allene/ temp time
yield
run allene dithiolb dithiol (°C) (h)
(%)c Mn Mw/Mn
d
d
1e
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
1
2
2
4
4
4
4
5
5
5
5
5
5
5
6
4
5
1.1
1.1
1.1
1.1
1.0
1.1
1.2
1.3
1.5
1.1
1.1
1.1
1.1
1.2
60
60
20
20
20
20
20
20
20
7
48 PS1 12 1300 NDf
58 PS1 24
450
5.78
2.72
2.52
1.81
1.94
1.64
1.83
1.42
1.79
2.15
2.01
3.84
2.63
48 PS1 37 1500
72 PS1 71 2700
72 PS2 40 5000
72 PS2 45 5900
72 PS2 57 5900
72 PS2 42 6000
72 PS2 34 4700
72 PS2 44 5600
72 PS2 63 8300
72 PS3 85 4600
72 PS1 94 6400
72 PS2 58 5800
JA970165G
(11) A broad signal appearing around 3.5 ppm in the 1H NMR spectrum
may be due to the methylene signal of the polymer unit formed by the
terminal addition of thiol to 1.
(12) The structure of the polymer from 7 is not so simple in comparison
with that from 1: the 1H NMR indicates contamination of a few units
different from the 1,2-diarylthiopropene unit. This is in good accordance
with the model reaction using thiophenol. Rate of terminal carbon attack
of thiyl radical (>30%, 8 + 9) is much higher than that of 1 (7%): the
product formed by the terminal carbon attack can further react with thiyl
radical to yield 8. Therefore, the terminal carbon attack does not result in
termination unlike the case of 1: the two main chain structures including
a small amount of graft structure can be observed.
10
11g
12
13
14
20
20
20
20
a Polymerization conditions: concentration, 0.6 M in toluene;
photoirradiation by a 400 W high-pressure mercury lamp in a pyrex
tube. b Abbreviations of the dithiols are shown below. c Yield of
cyclohexane-insoluble polymer for 4 and yield of ether-insoluble
polymer for 5 and 6. d Estimated by GPC (eluent, tetrahydrofuran,
polystyrene standards). e The polymerization was carried out under the
conditions shown in Scheme 1. f Not determined. g [1] ) 0.15 M.
(13) To suppress the terminal carbon attack, 1-bromo-3-methyl-1,2-
butadiene (10) was used under the same conditions as Scheme 2. However,
low molecular wight polymer was obtained (74% yield, Mn 820, Mw/Mn
2.09). Unlike the radical addition of thiol to 3-methyl-1,2-butadiene yielding
1,2-adduct via the central carbon attack,14 the polymerization would proceed
through initial attack of thiyl radical at the bromo-substituted terminal
carbon, because the polymer is suggested to have isopropylidene moiety
by the model reaction, in which a different type of 1:2 adduct 11 is
selectively formed.
Without photoirradiation or 2,2′-azobisisobutyronitrile (AIBN),
no reaction took place at all; whereas, equimolar TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl) added to the reaction
system also suppressed the reaction completely. These results
evidently suggest the free radical chain reaction. In the plausible
mechanism (Scheme 3), either a radical species formed from
radical initiator abstracts hydrogen of the thiol group or the thiol
(14) Jacobs, T. L.; Illingworth, G. E. J. Org. Chem. 1963, 28, 2692.