An extraordinarily rapid polymerization of vinylpentafluorocyclopropane: Highly stereo- and regioselective synthesis of unsaturated fluoropolymers

Article abstract of DOI:10.1021/ja021289i

Vinylpentafluorocyclopropane 1 was prepared from the reaction of 1,1,2-trifluoro-4-bromobutene and hexafluoropropylene oxide at 190 °C, following by treatment with KOH. 1 is stable at low temperature (-40 °C) for 7 years, but it rearranged readily to 2,3,3,4,4-pentafluorocyclopentene-1, 2, at above 80 °C (E_a = 28.7 kcal/mol). Under radical conditions, 1 extraordinarily rapidly polymerizes to give highly crystalline Z-fluoropolyolefin (CF₂CF₂CF=CHCH₂)_n, 3, which is very useful for cross-linking and grafting but difficult to obtain by other means. The stereochemistry of 3 was further confirmed by radical addition of iodine to 1 to form Z-ICF₂CF₂CF=CHCH₂I, 4, exclusively. The rapid polymerization with high stereoselectivity and regioselectivity could be rationalized by effects of a favorable polar transition state of a high ring strain and electron-deficient pentafluorocyclopropyl and a relative electron-rich double bond of 1. Copyright

Full text of DOI:10.1021/ja021289i

Published on Web 01/01/2003
An Extraordinarily Rapid Polymerization of Vinylpentafluorocyclopropane:
Highly Stereo- and Regioselective Synthesis of Unsaturated Fluoropolymers†
Zhen-Yu Yang*
DuPont Central Research and DeVelopment, Experimental Station, P.O. Box 80328,
Wilmington, Delaware 19880-0328
Received October 21, 2002 ; E-mail: Zhenyu.yang-1@usa.dupont.com
Fluoropolymers, particularly saturated fluoropolymers, have
excellent thermal and chemical stability, low surface energy, high
insulating ability, and low refractive index and dielectric constant,
but their limited solubility and high crystallinity necessitate extreme
processing conditions.1 Nonfluorinated polymers containing double
bonds are useful materials which can be further modified by cross-
linking or grafting to improve properties. These materials can be
readily prepared by radical polymerization of 1,3-dienes. However,
perfluorobutadiene does not polymerize to give a high-molecular
weight polymer with radical initiators, although anion-catalyzed
polymerization was reported recently.2,3 Radical polymerization of
solid while standing at room temperature for a few hours. When
fluoromonomers to make fluoropolymers containing double bonds
is very important since the fluorinated double bonds are highly
polarized and can be used as electrophilic centers for cross-linking
or grafting.4 The polarized double bonds of fluoropolymers are
usually generated by base elimination of hydrofluoride from
vinylidene fluoride-based polymers; however, this process is
constrained by the limited diffusion of dehydrofluorination agents
through these polymers and the generation of harmful volatiles.5
Other fluoropolymers are extremely resistant to double bond
generation due to their high stability.6 I have designed and
synthesized the monomer vinylpentafluorocyclopropane 1 to pro-
duce polymers with polarized double bonds that are not accessible
through radical polymerization of fluorinated dienes or dehydrof-
luorination of other fluoropolymers.
The polymerization of nonfluorinated vinylcyclopropanes is well
documented, and the polymer structures obtained are dependent
on the polymerization conditions. With cationic initiators, vinyl-
cyclopropane gave a polymer with predominantly 1,2-structure
units, resulting from polymerization of the double bond.7 However,
under radical conditions vinylcyclopropanes mainly gave ring-
opened polymers with double bonds in backbones.8 That is due to
the fact that the rate of ring-opening of a cyclopropylcarbinyl radical
is very fast (108 s-1).9 Introduction of two fluorines to a cyclopropyl
carbinyl radical further facilitates the ring-opening reaction in 3
orders of magnitude, because the fluorine substituents affect both
the structure and reactivity of cyclopropane.10 One would expect
vinylpentafluorocylopropane 1 to be even more reactive to polym-
erization under radical conditions due to the high strain energy in
perfluorinated cyclopropane systems. Synthesis of 1 is straightfor-
ward from the low cost and commercially available starting material
CF2dCFCH2CH2Br. Reaction of CF2dCFCH2CH2Br with hexaflu-
oropropylene oxide at 190 °C gave the fluorinated cyclopropane
derivative in 82-90% yield.11 Elimination of HBr was ac-
complished by treatment with KOH in ethanol and water to form
the desired monomer 1 in 92% yield.
heated to 80 °C or above, compound 1 was readily isomerized to
give 2 in high yield (Ea ) 28.7 kcal/mol).12 In contrast, rearrange-
ment of nonfluorinated vinylcyclopropanes to cyclopentenes re-
quired much higher temperatures (Ea ) 49.7-51.7 kcal/mol),13
while introduction of gem-difluorines to cyclopropane facilitated
the rearrangement reaction to give difluorocyclopentene (Ea ) 41.5
kcal/mol).14
When monomer 1 was treated with bis(perfluoropropionyl)
peroxide (3P) in 1,1,2-trichlorotrifluoroethane at 40 °C, rapid
polymerization occurred to form a white solid in greater than 90%
yield. The polymer is insoluble in common organic solvents such
as acetone, ethyl acetate, acetonitrile, THF, DMF, hexafluoroben-
zene, and other fluorocarbon solvents at room temperature, so that
its molecular weight could not be measured by GPC.
Polymerization of nonfluorinated vinylcyclopropanes gave amor-
phous polymers with both cis and trans double bonds as well as
cyclobutyl rings.8 In contrast, monomer 1 rapidly polymerizes with
surprisingly high stereoselectivity and regioselectivity. Only 1,5-
ring polymerization and Z-double bond conformations were ob-
served. The polymer microstructure was characterized by FTIR and
multinuclear NMR. The absorption of the double bond (1719 cm-1
)
1
was observed by IR. H and 19F NMR analyses were obtained on
Monomer 1 is a low-boiling (bp 30 °C) liquid. It is stable at low
temperature, having been stored at -40 °C for more than seven
years without decomposition, but it polymerized to give a white
1
the molten polymer at 135 °C. The H NMR spectrum revealed a
doublet of triplets for the vinyl hydrogen at 5.84 ppm, with a
coupling constant for the doublet of 31.2 Hz, indicating a trans
arrangement for the vinyl hydrogen and fluorine atoms. A multiplet
† Publication no.: 8346.
9
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J. AM. CHEM. SOC. 2003, 125, 870-871
10.1021/ja021289i CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
at 3.22 ppm corresponds to the methylene hydrogens.15 19F NMR
revealed the vinyl fluorine at -123.0 ppm and two sets of methylene
fluorines -111.8 and -117.0 ppm, respectively. No fluorocyclo-
butyl structure was observed. The integration ratio was precisely
consistent with the polymer structure formed via a clean ring-
opening polymerization. The molecular weight (Mn) of 3 made by
3P initiator and bis(4-tert-butylcyclohexylperoxy)dicarbonate was
6.5 × 104 and 7.5 × 104, respectively, based on NMR end group
analysis.
The Z-double bond configuration in 3 was further confirmed by
chemical conversion. Under radical conditions, 1 reacted with iodine
in CH2Cl2 at 0 °C to room temperature to give the diiodide 4 in
high yield. GC showed a single peak, and 1H NMR analysis
exhibited a doublet of triplets at 5.97 ppm with coupling constants
of 30.1 and 9.0 Hz, respectively. The larger coupling is due to the
trans F-H coupling, indicative of the Z-olefin.
stability and exhibited 10% weight losses above 400 °C in N2. DSC
showed that the copolymers were highly crystalline, but their
melting points were only a few degrees different from that of the
homopolymer, indicating incorporation of only small amounts of
these comonomers.18
Acknowledgment. I thank Mr. R. E. Smith, Jr. for technical
assistance and Dr. M. Hofmann for his comments.
Supporting Information Available: Experimental procedures for
1, 2, 3, 4 and a copolymer and characterization data for all compounds
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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The extraordinarily rapid polymerization of 1 may be attributed
to the high strain of the pentafluorocyclopropyl ring and the
favorable polar transition state between an electron-deficient
pentafluorocyclopropyl ring and a relatively electron-rich double
bond. The high Z-stereoselectivity of the double bonds in polymer
3 and the diiodide 4 could be rationalized by the favored transition
state A for the ring-opening of a pentafluorocyclopropylcarbinyl
radical to a Z-allyl radical rather than the disfavored transition state
B, which experiences a steric repulsion between the bulky R group
with the pentafluorocyclopyl group.16
(10) Tian, F.; Bartberger, M. D.; Dolbier, W. R., Jr. J. Org. Chem. 1999, 64,
540.
(11) Sargent, P. B.; Krespan, C. G. J. Am. Chem. Soc. 1969, 91, 415. Sargeant,
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SSSR, Ser. Khim. 1990, 8, 1844.
(12) Kinetic data for rearrangement of 1 to 2 are: ∆Gq ) 28.7 kcal/mol, ∆Hq
) 26.7 kcal/mol, and ∆Sq ) -5.5 eu (log A ) 12.7, Ea ) 28.4 kcal/
mol). see Smart, B. E.; Kusic, P. J.; Roe, d. C.; Yang, Z. Y. J. Fluorine
Chem. 2002, 117, 199.
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Am. Chem. Soc. 1982, 104, 2494.
(15) A small multiplet was observed at 6.05 ppm. This probably corresponds
to the cis-vinyl hydrogen. The ratio of the peak at 5.84 ppm to the peak
at 6.05 ppm is 14 to 1.
(16) The ring-opening of a nonfluorinated cyclopropyl carbinyl radical does
not have high stereo- and regioselectivity. See: Marino, P. S.; Bay, E. J.
Org. Chem. 1980, 45, 1763. Beckwith, A. L. J.; Moad, G. J. Chem. Soc.,
Perkin Trans. 2 1980, 1473. Ratier, M.; Pereyre, M.; Davies, A. G.;
Sutcliffe, R. J. Chem. Soc., Perkin Trans. 2 1984, 1907. Fluorine
substituents have remarkable effects on stereo- and regioselectivity. Ring-
opening of gem-difluorocyclopropylcarbinyl radical gave E-olefins. See:
Morikawa, T.; Uejima, M.; Kobayashi, Y. Chem. Lett. 1988, 1410.
Morikawa, T.; Uejima, M.; Yoda, K.; Taguchi, T. Chem. Lett. 1990, 467.
The high strain energy of perfluorocyclopropyl further enhances the
stereoselectivity of ring-opening due to much better interaction between
the SOMO of the radical and the σ* orbital of the perfluorocyclopropyl
illustrated in the most favored transition state.
The regiospecific polymerization is due, at least in part, to the
rigidness of the fluorocarbon chain and the lack of cyclization to
four-membered rings which are common in nonfluorinated systems.8
In addition, the propagation rate is faster than that of the
hydrocarbon counterpart due to the favorable polarity match-ups
in the transition state for addition of a fluorinated radical to a
hydrocarbon double bond.17
TGA and DSC were used to evaluate the thermal characteristics
of the homopolymer 3. TGA (20 °C/min) revealed that, despite its
unsaturation, polymer 3 has good thermal stability with a 10%
weight loss temperature of about 400 °C in N2 and 375 °C in air.
DSC analysis of 3 displayed a melting transition at 130 °C with no
detectable glass transition above room temperature. In contrast to
amorphous polymers from nonfluorinated vinylcyclopropanes, the
high crystallinity of 3 is probably a result of segments packing
regularly due to the fact that 3 has highly uniformed steric structures.
3 can readily be pressed into a tough and clear thin film at 150 to
180 °C.
(17) Dolbier, W. R., Jr. Chem. ReV. 1996, 96, 1557. Dolbier, W. R., Jr.; Li,
A.; Smart, B. E.; Yang, Z. Y. J. Org. Chem. 1998, 63, 5687.
(18) Yang, Z. Y. U.S. Patent 5,420,367, 1995.
The monomer 1 also copolymerizes with other fluorinated
monomers, such as tetrafluoroethylene, chlorotrifluoroethylene, and
perfluoropropyl vinyl ether. All of the copolymers had good thermal
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