A crosslinked polymer having perfluorocyclobutane rings is prepared in a process comprising the steps of: (a) contacting monomers having two dimerizable perfluorovinyl groups; (b) exposing the monomers to sufficient heat and for a sufficient time that a (linear) polymer containing perfluorocyclobutane rings is formed; and (c) exposing the polymer to sufficient crosslinking initiating means and for a sufficient time such that crosslinking occurs. Crosslinked polymers, so prepared are novel and have tensile strength and other physical proeprties enhanced over the properties of the corresponding linear polymer.

This invention relates to crosslinked polymers having perfluorocyclobutane 
rings, and to thermal processes to prepare such polymers. 
Dimerization of certain perfluorovinyl compounds has been reported and is 
discussed, for instance, in Chambers, Fluorine in Organic Chemistry, 
John Wiley, New York, 1973, pp. 173-191; S. Patai, The Chemistry of 
Alkenes, Wiley Interscience Publishers, 1964, p. 779; M. Hudlicky, 
Chemistry of Organic Fluorine Compounds, 2nd ed., Halsted Press (John 
Wiley and Sons), 1972, p. 450; and Tarrant ed., Fluorine Chemistry 
Reviews, Vol. 2, Marcel Dekker, 1968 pp. 1-52. In general, the 
dimerizations are easily sterically hindered and have not been used to 
prepare long chain molecules. A report of dimerization linking two 
molecules of such compounds as perfluoropropylene and perfluoropentene-1, 
included speculation that the reaction could be used for perfluoroalkyl 
perfluorovinyl compounds wherein the alkyl radical has 1 to 20, "or even a 
higher number" of carbon atoms. See, McCone et al., U.S. Pat. No. 
3,316,312. 
Certain polymers having perfluorocyclobutane groups in their backbone and 
monomers suitable for preparing such polymers are disclosed in copending 
U.S. Application Ser. Nos. 364,667 filed June 9, 1989; U.S. Application 
Ser. No. 364,666 filed June 9, 1989: U.S. Application Ser. No. 364,686 
filed June 9, 1989: U.S. Application Ser. No. 364,665 filed June 9, 1989. 
Polymers are prepared from monomers containing more than one 
perfluorovinyl group. Such polymers are generally linear and thermoplastic 
except when the polymers are prepared from monomers having more than two 
perfluorovinyl groups per molecule. In most instances, use of even small 
amounts of monomers containing more than two of the perfluorovinyl groups 
results in polymers having crosslinked or branched molecular structures. 
These crosslinked or branched polymers generally have enhanced mechanical 
properties as compared to their thermoplastic counterparts, such as 
tensile strength and modulus and flexural strength and modulus. Such 
crosslinked or branched polymers are not, however, generally thermally 
processable, such as by injection molding or extruding, as are the 
thermoplastics. 
It would be desirable to produce a polymer containing perfluorocyclobutane 
rings which polymer could be thermally processed as a thermoplastic then 
cured into a crosslinked polymer. 
SUMMARY OF THE INVENTION 
In one aspect, the invention is a process for preparing a crosslinked 
polymer having perfluorocyclobutane rings comprising the steps of: 
(a) contacting monomers having two dimerizable perfluorovinyl groups: 
(b) exposing the monomers to sufficient heat and for a sufficient time that 
a polymer containing perfluorooyclobutane rings is formed: and 
(c) exposing the polymer to sufficient crosslinking initiating means and 
for a sufficient time such that crosslinking occurs. 
In another aspect, the invention is a crosslinked polymer produced by that 
process. 
The crosslinked polymers are advantageously elastomeric in the range from 
their glass transition temperatures to the temperatures at which 
degradation is observed, which is generally on the order of 
400-450.degree. C. Compared to their thermoplastic counterparts, the 
crosslinked polymers exhibit enhanced solvent resistance and increased 
mechanical strength, without loss of advantageous electrical properties, 
such as low dielectric constant and dissipation factor.

DETAILED DESCRIPTION OF THE INVENTION 
Polymers are formed by thermal reaction of monomers having two dimerizable 
perfluorovinyl groups such that perfluorocyclobutane groups are formed. A 
dimerizable perfluorovinyl group is a perfluorovinyl group which reacts 
with another such group to form a perfluorocyclobutane ring. Thus, 
resulting polymers have at least two perfluorocyclobutane groups. The term 
polymer is used herein to refer to any compound having at least two 
perfluorocyclobutane groups formed from perfluorovinyl groups, and 
includes oligomers which have from about 2 to about 100 repeating units 
and preferably have a molecular weight of from about 300 to about 30,000. 
Depending on the molecular structure connecting the perfluorocyclobutyl 
groups, the number of perfluorocyclobutane groups can vary from as few as 
two up to thousands. 
Any monomer having two dimerizable perfluorovinyl groups and which monomer 
is thermally polymerizable into crosslinkable polymers is suitably used in 
the practice of the invention. Whereas polyaddition of perfluorovinyl 
groups to form perfluoroaliphatic polymers (like polytetrafluoroethylene), 
not generally having perfluorocyclobutane groups, takes place in the 
presence of free radicals or free radical generating catalysts, 
dimerization to form perfluorocyclobutane groups takes place thermally. In 
the thermal polymerization of diperfluorovinyl compounds, substantially 
linear polymers having little branching are believed to be formed. In the 
practice of this invention, certain of these substantially linear polymers 
are crosslinked. Crosslinking involves chemical reactions that 
interconnect polymer molecules. As these reactions proceed, a polymer 
network is formed. Early in a crosslinking process, there are molecules 
having a wide variety of molecular weights: molecular weight increases 
with increasing extent of crosslinking. At a point in the progress of 
crosslinking, the gel point is reached. This point is defined as the point 
when there is sufficient crosslinking that the polymer is no longer 
soluble in a solvent for the corresponding uncrosslinked linear polymer. 
Rather, the polymer swells in the solvent. Theoretically, either the 
weight average molecular weight diverges to infinity in an infinite 
sample, or a first macromolecular cluster grows to be on the order of the 
sample size when the sample is finite. At the gel point, the polymer 
system loses its solubility and a steady-shear viscosity approaches 
infinity. A decrease in percent elongation as measured by the procedure of 
ASTM D882-83 is also observed. Preferably, the decrease in percent 
elongation is at least about 10 percent, more preferably at least about 20 
percent of the percent elongation. At the gel point, there are still 
unattached polymer molecules within a polymer network system. As these 
molecules are crosslinked into the network, stiffness increases and the 
mechanical strength of the polymer (e.g. as measured by the procedures of 
ASTM D882-83 and ASTM D790-81) is enhanced. The viscosity also continues 
to increase. The gel point at a temperature can be determined 
rheologically by the process of H. H. Winter et al. in J. Rheology, 30 
(2), 367-382 (1986) and 31(8), 683-697, (1987); and Macromolecules, 22, 
411-414, (1989). As measured by the procedure taught by Winters, 
crosslinked polymers of this invention preferably have gel points within 
about two hours at about 360.degree. C., more preferably in less than 
about two hours at 320.degree. C., most preferably in less than about two 
hours at 280.degree. C. Measurements at temperatures below about 
320.degree. C. are more indicative of a preferred crosslinking because 
crosslinking at such temperatures is accompanied by thermal decomposition. 
Before crosslinking, solid polymers of the invention are generally 
thermoplastic. Viscosity of either a melt or solution of the polymer 
increases as crosslinking occurs until the gel point and resulting 
unsolubility is reached. The crosslinked polymers are preferably 
elastomeric, that is, the polymer can generally regain its shape after 
deformation. That deformation is indicated by elongation measurements 
greater than about 100% at temperatures above the glass transition 
temperature (Tg) of the polymer. The crosslinked polymers preferably 
retain their elastomeric properties at temperatures of from their glass 
transition temperatures to the temperatures at which they are observed to 
degrade, preferably about 400.degree. C. The glass transition temperature 
varies with the composition of the polymer. 
Crosslinking also increases a polymer's tensile strength as measured by the 
procedures of ASTM D882-83. The increase is preferably up to about 1000%, 
more preferably from about 10% to about 500%, most preferably of from 
about 10% to about 100% increase. Also the polymer's tensile and flexural 
modulus as measured by the procedures of ASTM D882-83 and ASTM D790-81, 
respectively, also increases, preferably up to about 1,000%, more 
preferably of from about 10% to about 500%, most preferably of from about 
10% to about 100%. Additionally, the fluorine-containing structures of 
such crosslinked polymers preferably retain relatively low dielectric 
constants. 
Although any monomer having two dimerizable perfluorovinyl groups and which 
is crosslinkable is suitably used, crosslinked polymers of the invention 
are preferably prepared from monomers having two perfluorovinyl groups 
separated by at least one hydrocarbyl group having at least one carbon 
atom between the perfluorovinyl groups. 
Furthermore, when the perfluorovinyl groups are attached to aliphatic 
carbons or separated from aliphatic carbons by single atoms such as 
oxygen, the perfluorovinyl groups are preferably primary or secondary 
because tertiary perfluorovinyl groups are generally sterically hindered 
with respect to formation of perfluorocyclobutane rings, more preferably 
the perfluorovinyl groups are primary because secondary perfluorovinyl 
groups tend to rearrange. Preferably, to avoid rearrangement and 
facilitate polymer formation and crosslinking the monomers have structures 
such that resulting polymers have hydrocarbyl groups (preferably aromatic 
rings), perfluorocyclobutane rings and at least one non-carbon atom such 
as oxygen, silicon, boron, phosphorus, nitrogen, selenium, tellurium 
and/or sulfur atom (each optionally substituted) in the backbones. 
The monomers preferably have a structure represented by Formula I: 
EQU CF.sub.2 .dbd.CF--X--R--X--CF.dbd.CF.sub.2 
wherein R represents an, optionally inertly substituted group; and each X 
is independently a bond or any group which links R and a perfluorovinyl 
group (hereinafter linking structures), said structures being inert. By 
"inert" it is meant that the structures or substituents do not react 
undesirably with perfluorovinyl groups or interfere undesirably with 
polymerization (perfluorocyclobutane formation) of the monomers. 
Linking structures X are each independently a linking structure such as a 
bond, an oxygen atom, carboxylic and thiocarboxylic ester groups, other 
sulfur containing structures, perfluoroalkylene, perfluoroalkylene ether, 
alkylene, acetylene, phosphorus containing groups such as phosphines, 
carbonyl and thio carbonyl groups: seleno: telluro: nitrido: 
silicon-containing groups such as silanediyl, trisilanediyl 
tetrasilanetetrayl, siloxanediyl, disiloxanediyl, trisiloxyl, 
trisilazanyl, or silylthio groups; boron-containing groups such as 
boranediyl or methylboranediyl groups; a combination thereof, or any other 
group which is inert, which molecularly links R to a perfluorovinyl group, 
and which provides a molecular structure in which the perfluorovinyl group 
is sufficiently reactive to form a perfluorocyclobutane ring. For 
instance, X is preferably other than a perfluoroalkylene group because 
perfluorovinyl groups attached to perfluoroalkylene groups generally 
require temperatures greater than about 300.degree. C. to dimerize and are 
subject to isomerization. 
It is preferred that at least one of X is not a bond. More preferably, X is 
independently selected from the group consisting of groups having at least 
one non-carbon atom between the perfluorovinyl groups and R, such as 
groups containing oxygen, sulfur, selenium atoms, tellurium atoms, 
silicon, boron, phosphorus or nitrogen between R and the perfluorovinyl 
group, e.g. oxygen atoms, sulfur atoms, (thio) carboxylic ester groups, 
phosphines, (thio) carbonyl groups, seleno, telluro, silanediyl, 
trisilanediyl, trisilazanyl or silylthio, boranediyl groups. Preferred 
groups have S, O, Si, N or P, more preferably S, 0, or Si between R and 
the perfluorovinyl group, such as carbonyl, thiocarbonyl, sulfone, 
sulfoxy, silanediyl, amines (optionally inertly substituted), oxygen or 
sulfur atoms. Most preferably there is a single atom other than carbon: 
even more preferably it is oxygen or sulfur, among those groups preferably 
an ether or sulfide linkage, because monomers having such linking 
structures advantageously form perfluorocyclobutane groups at lower 
temperatures than are needed with such groups as perfluoroalkyl groups and 
are more stable than monomers where the perfluorovinyl group is attached 
directly to R, particularly when R is aromatic. Monomers having such 
linking structures are also relatively easily prepared. 
R is suitably any inert molecular structure, preferably a molecular 
structure which facilitates formation of perfluorocyclobutane rings or 
crosslinking and/or imparts desirable physical properties to polymers or 
oligomers prepared from the monomers. For the purpose of imparting 
desirable physical properties to polymers, R preferably contains at least 
one carbon atom in the molecular chain between X's because monomers having 
at least one carbon atom between X's when X is other than a bond, tend to 
have desirable stability and to produce polymers having desirable physical 
properties. Alternatively, the carbon atom is in a side chain: for 
instance, --R-- can be --N(CH.sub.3)--, --N(CH.sub.2 
CH.sub.3)--P(CH.sub.3)--, --P(CH.sub.2 CH.sub.3)-- and the like. Carbon 
atoms(s) in R are suitably in aliphatic, cycloaliphatic, aromatic, 
heterocyclic groups and the like and combinations thereof. Additionally, R 
optionally contains groups or has substituents which are inert, that is 
which do not undesirably interfere with the formation of crosslinks or 
perfluorocyclobutane rings from perfluorovinyl groups. Inert substituents 
include ether, carbonyl, ester, tertiary amide, carbonate, sulfide, 
sulfoxide, sulfone, nitrile, alkyl phosphonate, tertiary amine, alkyl 
phosphate, alkyl silyl, chlorine, bromine, fluorine, alkyl, arylalkyl, 
alkylaryl, cycloalkyl, aromatic, heterocyclic, alkoxyl, aryloxy groups and 
the like, which inert substituents are suitably in any position, for 
instance, in a polymer backbone between X's and/or appended to such a 
backbone. Carbon-containing inert substituents on R preferably contain 
from about 1 to about 50, more preferably from about 1 to about 12 carbon 
atoms because of the stability and ease of working with monomers of lower 
molecular weight. R, including inert substituents preferably has a 
molecular weight (MW) of from about 14 to about 20,000, more preferably 
from about 75 to about 15,000 and most preferably from about 75 to about 
5,000. These ranges include monomeric and oligomeric R groups. In the case 
of monomers which are other than oligomeric, R preferably has from about 1 
to about 50, more preferably from about 6 to about 25, carbon atoms 
because molecular weights above this reduce the contribution to properties 
made by the fluorine-containing substituents when R is alkyl or aromatic 
hydrocarbon. As previously discussed, the nature of R as well as the 
perfluorocyclobutane content of the polymers can vary broadly according to 
the type of products desired. 
Preferably, for polymers having good plastic properties such as tensile 
strength and flexibility, at least one carbon atom of R is in the 
molecular chain between X's and is part of an aromatic nucleus. Aromatic 
groups are desirable because of improved physical properties of the 
polymers and ease of manufacture of the monomers. For both ease of 
manufacture of the monomer and monomer stability, when R is aromatic, each 
X is preferably independently sulfur or oxygen. The aromatic group can be 
any molecular structure having aromatic character, advantageously having 
at least one six membered aromatic ring, suitably having any number of 
such six-membered rings fused together or connected by bonds or linking 
structures. R preferably has from about 1 to about 50 such rings, more 
preferably from about 1 to about 10 rings, more preferably containing from 
about 6 to about 25 carbon atoms, most preferably R has at least 2 to 
about 4 aromatic rings to impart properties such as hardness and/or 
stiffness to a polymer. The aromatic fragment is suitably unsubstituted or 
inertly substituted. Inert substituents on an aromatic R include, for 
instance, the inert substituents listed for R generally. Exemplary 
aromatic molecular fragments include, for instance, perchlorophenylene, 
phenylene, biphenylene, naphthylene, dichlorophenylene, nitrophenylene, 
p,p'(2,2-diphenylene propane) [--C.sub.6 H.sub.4 --C(CH.sub.3).sub.2 
--C.sub.6 H.sub.4 ]; p,p'-(2,2-diphenylene -1,1,1,3,3,3 hexafluoropropane) 
[--C.sub.6 H.sub.4 --C(CF.sub.3).sub.2 --C.sub.6 H.sub.4 --], preferably 
biphenylene: phenylene; 9,9'-diphenylfluorene, oxydiphenylene: 
thiodiphenylene; 2,2-diphenylene propane: 2,2'-diphenylene, 
1,1,1,3,3,3-hexafluoropropane; 1,1-diphenylene-1-phenylethane; 
naphthalene; and anthracene. 
For the purpose of facilitating crosslinking, more preferably, R is a group 
which reacts with perfluorovinyl groups residual in a substantially linear 
polymer to form a crosslinked or branched molecular structure. The 
reaction of R with the perfluorovinyl groups is suitably initiated by 
heat, free radicals, wave energy, or any other crosslinking initiating 
means, but preferably by heat. Most preferably R includes a structure 
having two double, triple or aromatic bonds (hereafter multiple bonds) 
separated by a single bond. Such structures are recognized in the art as 
latent dienes. Preferably the latent dienes are suitable for reactions of 
the Diels- Alder type, more preferably suitable for such reactions with 
perfluorovinyl groups in the monomers, most preferably suitable for such 
reactions with perfluorovinyl ether groups under conditions used for 
crosslinking. The single bond is preferably a carbon to carbon single 
bond. Each of the multiple bonds is independently suitably a multiple bond 
between any two atoms, preferably between a carbon atom and any other atom 
(e.g. --C.dbd.O, --C.dbd.C--, --C.tbd.N) more preferably a carbon to 
carbon bond. Exemplary of preferred R groups include, for instance, 
biphenylene, 9.9'-diphenylfluorene, flourene, cyclopentadienylene, furan 
and anthracene. 
Even though Diels-Alder reactions of perfluorovinyl groups are rare (See D. 
D. Coffman, et al., J. Am.Chem. Soc.. 71, 490-496 (1949); E. T. McBee, et 
al., J. Am. Chem. Soc., 77, 915-917 (1955); J. J. Drysdale, et al., J. Am. 
Chem. Soc., 80, 3672-3675 (1958)), monomers capable of such reactions are 
observed to give crosslinked polymers having gel points at temperatures 
generally lower than similar polymers formed from monomers wherein double 
bonds are separated by more than one single bond. 
Most preferably, R has aromatic carbon atoms at least one of which is 
bonded directly to X, most preferably aromatic carbon atoms of R are 
bonded directly to each X because perfluorovinyl groups bonded to X, said 
X being bonded to aromatic groups are generally more reactive in forming 
perfluorocyclobutane rings. 
Some specific combinations of X and R are especially preferred: when R is 
aromatic, at least one X is preferably other than a bond, more preferably 
neither X is a bond, because attachment of perfluorovinyl groups directly 
to aromatic R renders the perfluorovinyl groups more thermally and 
oxidatively unstable than when said groups are attached, for instance to 
oxygen or sulfur. When R is a perfluoroalkyl group or a 
perfluoroalkylether group, at least one X is preferably other than a bond, 
most preferably no X is a bond or a perfluoroalkyl group, because 
perfluorovinyl groups linked directly to perfluoroalkyl groups require 
temperature in excess of about 300.degree. C. to dimerize and are subject 
to isomerization. 
Monomers useful in the practice of the invention are suitably prepared by 
any method which links molecular structures having perfluorovinyl groups 
to other molecular structures or which forms perfluorovinyl groups. 
Monomers are preferably prepared by the process taught in copending 
application U.S. Application Ser. No. 364,665 filed June 9, 1989 and 
incorporated herein in its entirety. 
Before crosslinking, substantially linear polymers or oligomers thermally 
produced from the preferred monomers preferably have a repeating unit 
formula represented by Formula II wherein R and X are 
EQU --[X--R--X--Q].sub.n -- 
defined above; Q is a perfluorocyclobutane group and n is an integer 
representing the number of repeating units, which is preferably from about 
2 to about 100,000, more preferably from about 2 to about 10,000, most 
preferably from about 3 to about 5,000. Polymer chains of these lengths 
are terminated by residual perfluorovinyl groups unless the vinyl groups 
have reacted with other materials. 
To form such polymers, the monomers are heated to a temperature and for a 
time sufficient to form perfluorocyclobutane rings. Temperatures suitable 
for forming perfluorocyclobutane rings differ with the structure of the 
monomer. In general, temperatures above about 40.degree. C. are suitable 
for formation of perfluorocyclobutane rings, preferably the temperature is 
above about 50.degree. C., more preferably above about 100.degree. C., 
because these temperatures result in formation of the rings at 
successively faster rates. Temperatures above about 450.degree. C. are 
preferably avoided because perfluorocyclobutane groups are generally 
thermally unstable above such temperatures. More preferably a temperature 
of from about 105.degree. C. to about 350.degree. C., most preferably from 
about 105.degree. C. to about 250.degree. C., is used to produce the 
perfluorocyclobutane rings at a convenient rate. Within that range, a 
temperature of from about 100.degree. to about 230.degree. is generally 
most preferred for cyclization of perfluorovinyl aromatic or aliphatic 
ethers or sulfides, while a temperature of from about 50.degree. C. to 
80.degree. C. is needed to form perfluorocyclobutane groups when the 
perfluorovinyl group is attached directly to an aromatic ring. In the case 
of perfluoroalkylperfluorovinyl groups, however, temperature of at least 
about 300.degree. C., preferably at least about 350.degree. C., is 
generally required. Details of the polymerization process are given in 
U.S. application Ser. No. 364,667 filed June 9, 1989 (Attorney Docket No. 
C-37,374), which is incorporated herein in its entirety. 
Polymers prepared by such a process are suitably solids, fluids or gels, 
preferably solids or fluids, most preferably solids. The solids preferably 
maintain plastic characteristics such as tensile strength well above 
ambient temperatures (e.g. above about 25.degree. C.) and have glass 
transition temperatures from well below ambient to well above ambient 
temperatures. A particularly preferred group of such polymers have glass 
transition temperatures (Tg) above ambient (25.degree. C.), preferably 
above 60.degree. C. and most preferably above 100.degree. C. In general, 
the polymers having Tg above ambient result from monomers of Formula I 
wherein R is aromatic, and the polymers having Tg above 60.degree. C. when 
R contains more than one aromatic ring. A particular desirable property of 
polymers where R is aromatic and not substituted with polar substituents 
(e.g. nitro, sulfonate, carboxy) is the combination of good physical 
properties and good electrical properties. Dielectric constants and static 
dissipation factors (as measured according to the procedures of ASTM 
D150-87) preferably range from about 2.2 to about 3.0 and from about 
0.0002 to about 0.005 respectively. Glass transition temperatures increase 
from about ambient when R is phenyl to about 170.degree. C. when R is 
biphenyl to 230.degree. C. when R is 9,9-diphenylfluorene. 
Such polymers can be crosslinked by any crosslinking initiating means such 
as by heat, by free radicals, or by wave energy. This crosslinking 
preferably occurs substantially without crosslinking agents such as 
monomers having three or more functional groups reactive to form the 
polymers (trifluorovinyl groups). By substantially without, it is meant 
that while materials which act as crosslinking agents or catalysts for 
crosslinking may incidentally be present in very small quantities, 
preferably less than about 0.01 weight percent of the polymer, more 
preferably less than about 0.01 weight percent crosslinking agent and less 
than about 0.001 weight percent catalyst for crosslinking, such materials 
are not deliberately added. Free radicals are suitably provided by use of 
any means of supplying free radicals such as by use of compounds known in 
the art for producing free radicals, e.g. organic azo compounds. Wave 
energy is suitably supplied by means such as ultraviolet light, radiant 
heat (e.g. infrared light), microwaves, X-rays, or particle beam 
radiation. Such means are within the skill in the art. Neither thermal 
crosslinking nor crosslinking using wave energy require addition of a 
catalyst such as a free radical initiator and such methods are, therefore, 
preferred. Thermal crosslinking is, however, more preferred. 
Thermally crosslinked polymers are prepared from such thermally formed 
polymers containing perfluorocyclobutane rings by heating the polymers to 
a temperature sufficient to result in crosslinking, that is for chemical 
bonds to form between at least some of the polymer molecules. The 
temperature for such crosslinking is higher than that required for thermal 
(linear) polymerization, preferably it is at least about 50.degree. C. 
degrees higher than the temperature required for thermal (linear) 
polymerization, more preferably from about 250.degree. C. to about 
400.degree. C., most preferably from about 280.degree. C. to about 
380.degree. C., even more preferably from about 280.degree. C. to about 
340.degree. C. These temperatures are suitably maintained for a time 
sufficient to achieve a preselected degree of crosslinking. Such times are 
preferably from about 1 minute to about 10 days, more preferably from 
about 15 minutes to about 1 day (24 hours), most preferably from about 15 
minutes to about 8 hours. 
A particularly useful aspect of the present invention includes preparing 
the substantially linear polymers according to processes previously taught 
and exposing the linear polymer to conditions suitable for crosslinking in 
a shaping apparatus, e.g. an extruder, a mold, or other apparatus suitable 
for heating or other crosslinking initiation, e.g. for a time and at a 
temperature sufficient for crosslinking. This allows handling of a polymer 
which melts at a relatively lower temperature and producing an elastomer 
or thermosetting polymer having stability at temperatures sufficient to 
melt the substantially linear polymer. Crosslinking advantageously occurs 
without the need for a solvent or a high pressure apparatus, and proceeds 
without the evolution of volatile compounds or by-products. 
Crosslinked polymers formed by the process of the invention differ from 
crosslinked polymers prepared from monomer mixtures containing monomers 
having more than two perfluorovinyl groups in that they are prepared 
directly from bifunctional monomers or thermoplastic polymers thereof 
without addition of a catalyst or a multifunctional crosslinking agent. 
Also, polymers prepared from a single monomer having more than one 
crosslinking agent in general have a very high crosslink density and are 
more rigid and inflexible solids than polymers crosslinked by the process 
of the invention. 
The following examples are offered to illustrate but not to limit the 
invention. In each case, percentages are weight percent unless otherwise 
indicated. Examples (Ex.) of the invention are indicated numerically, 
while comparative samples (C.S.) are not examples of the invention and are 
indicated with letters. 
All gas chromatography/mass spectrometry (GC/MS) analyses of monomers and 
intermediates are performed on a Finnigan 1020 GC/MS using a 30 meter 
RSL-150 fused silica capillary column. All gas chromatography/mass 
spectrometry (GC/MS) analyses of fluid polymer samples are performed on a 
Finnigan 4500 GC/MS using a 60 meter DB-1 fused silica capillary column, 
with the GC program run at 290.degree. C. isothermal. Liquid 
chromatography/mass spectrometry (LC/MS) is performed on a Finnigan 4500 
mass spectrometer using acetonitrile - water eluent and a moving belt 
LC/MS interface. 
Dynamic Mechanical Spectroscopy (DMS) measurements are performed on a 
Rheometrics RDS-7700 rheometer in parallel plate mode using 25 mm plates 
at 10% strain and a series of frequencies. Differential scanning 
calorimetry (DSC), thermomechanical analysis (TMA) and thermogravimetric 
analysis (TGA) is performed on a Perkin Elmer 7000 thermal analysis 
system. 
Dielectric constant and dissipation factor measurements are conducted 
according to the procedures of ASTM D150-87. Tensile strength and modulus 
and percent elongation were measured on an Instron model 1125 according to 
the procedures of ASTM D-882-83. 
Gel Permeation Chromatography (GPC) is performed on a Waters 720 GPC 
instrument using a methylene chloride eluent and a series of 
Micro-styragel.RTM. columns of 10,000, 1,000, 500 and 100 angstrom pore 
sizes. Reported values are standardized against polystyrene. 
Granular zinc is activated by washing in 0.1 N hydrochloric acid (HCl) 
followed by drying in a vacuum oven at 0.5 torr and 140.degree. C. for 10 
hours. 
Infrared (IR) spectra are measured on a Beckmann Microlab 600 model 
spectrophotometer. Nuclear Magnetic Resonance (NMR) spectra are measured 
on a Varian EM360 spectrometer using 19F (fluorine 19) or 1H (hydrogen) 
mode. 
The gel point determination involves a dynamic mechanical spectroscopy 
technique performed at 10% strain in a parallel plate mode using a 1 mm 
gap. Storage and loss moduli are measured at frequencies from 0.1 to 100 
radians per second, measuring 10 different frequencies per decade of 
frequency, every 15 minutes. Measurements are carried out isothermally. 
The ratio of the loss modulus (G") to the storage modulus (G'), known as 
the loss tangent, or tan delta (G"/G'), is plotted against time for a 
number of frequencies. The point at which the value of tan delta becomes 
independent of frequency (as is indicated by a convergence of plots of the 
log tan delta vs. time at various frequencies to a point) is known as the 
gel point, and represents the time at which the size of the polymer chains 
in the sample increase to a size which is on the order of the size of the 
sample. At this point the sample is considered crosslinked on a 
macromolecular scale, and exhibits properties such as enhanced mechanical 
strength and insolubility in solvents for the thermoplastic precursor. 
EXAMPLE 1 
PREATION, POLYMERIZATION AND CROSSLINKING OF 
4,4'-BIS(TRIFLUOROVINYLOXY)BIPHENYL 
Dimethyl sulfoxide (DMSO) (1800 ml) is placed in a 5-liter 5-necked flask 
fitted with a mechanical stirrer, a Dean-Stark phase separating trap 
topped with a nitrogen padded reflux condenser, and a thermocouple 
attached to a temperature controller. The solvent is stirred and purged of 
oxygen by blowing in nitrogen through a dip-tube placed below the surface 
of the liquid while 4,4'-dihydroxybiphenyl (454g, 2.44 mole) is added to 
the flask. 
The system is stirred and purged for 20 minutes, then potassium hydroxide 
(85% pellets) (322g, 4.88 mole) is added slowly. The stirred mixture is 
then heated to 120.degree. C. The temperature is held at 120.degree. C. 
for 1.5 hours, then the heat is turned off and the mixture is allowed to 
cool to room temperature. Toluene (600 ml) which has been thoroughly 
purged with nitrogen is added to the solution and the resulting mixture is 
heated to reflux (135.degree. C.). Water is azeotropically removed from 
the reactor through the Dean-Stark trap for a total of 4 days, cooling the 
reactor once after 24 hours to allow for salt formation to be broken up by 
opening the flask under a nitrogen sweep and scraping the sides with a 
spatula. After 4 days the Dean-Stark trap is removed and replaced with a 
Soxhlet extractor containing anhydrous sodium sulfate. The toluene is then 
refluxed through the Soxhlet extractor for 7 hours to dry the toluene. 
After 7 hours, the Soxhlet is replaced with a Dean-Stark trap, and toluene 
(300 ml) is removed from the reactor by simple distillation. The reaction 
mixture is then cooled to 30.degree. C. in an ice water bath and 
1,2-dibromotetrafluoroethane (1300g, 5.00 mole) is added slowly dropwise 
over three hours at a rate that maintains a reactor temperature of 
35.degree..+-.2.degree. C. When the addition is complete the reaction 
temperature is allowed to stabilize (not increasing in temperature when 
the ice bath is removed) and then a heating mantle is applied to the 
flask. The reactor is heated to 50.degree. C. for 8 hours, then allowed to 
cool to room temperature with constant stirring. The crude reaction 
mixture is filtered to remove the potassium bromide salts, and the 
precipitate is washed with acetone. The filtrates are combined and 
thoroughly evaporated to remove acetone, DMSO and residual toluene. The 
solid residue is subjected to a 2 liter Kugelrohr bulb-to-bulb 
distillation to provide the crude product. This material is dissolved in 
750 ml of methylene chloride and is washed first with mild aqueous 
potassium bicarbonate (500 ml, approximately. 0.2 M), then with mild 
aqueous hydrochloric acid (HCl) (500 ml, approximately 0.05 M), then twice 
with distilled water (500 ml each). After complete phase separation the 
product layer is removed and evaporated, and the residue is fractionally 
distilled (138.degree.-148.degree. C., 0.35 torr) to provide 1031.1g (1.90 
mole, 77.9% yield) of 4,4'-bis(2-bromotetrafluoroethoxy)biphenyl, melting 
point 71.degree.-73.degree. C. The Infrared (IR) spectra of the product 
has the following peaks (cm.sup.-1): 1601,1492 (indicating an aromatic 
double bond); 1199-1107 (indicating carbon-oxygen and carbon fluorine 
bonds): 842, 788 (indicating aromatic character). The gas 
chromatograph/mass spectrometer (GC/MS) indicates peaks at the following 
mass to charge ratios: (m/e) =545 (29.8%): 543 (48.9%); 541 (23.8%): 365 
(48.7%): 363 (50.9%); 337 (30.3%); 335 (34.7%); 168 (33.7%): 156 (78.3%): 
140 (36.7%): 139 (90.1%): 129 (37.4 %): 128 (100.0%): 127 (33.2%): 102 
(32.9%); 76 (41.1%): 63 (34.3%), consistent with a product of 
4,4'-bis(2-bromotetrafluoroethoxy)biphenyl. 
Bromine is eliminated from this product by the following procedure: 
Into a 1-liter 5-necked flask equipped with a mechanical stirrer, a 
thermocouple attached to a temperature controller, a powder addition 
funnel and a reflux condenser, is placed freshly distilled diglyme (200 
ml) and fresh zinc powder (36.Og, 0.55 mole). 
The mixture is stirred and heated to 130.degree. C. Powdered 
4,4'-bis(2-bromotetrafluoroethoxy)biphenyl (100g, 0.184 mole) is added 
very slowly via the powder addition funnel over 3.5 hours. The mixture is 
then stirred mechanically at 115.degree. C. for 1 hour, after which, 
heating is turned off and the mixture is allowed to cool to room 
temperature. The solution is centrifuged to remove the zinc salts. Then 
the liquid is decanted, and the zinc salts are washed with acetone and 
centrifuged again. The liquid portions are combined and evaporated 
thoroughly, and the residue is dissolved in methylene chloride and washed 
with 0.05 M hydrochloric acid. The methylene chloride solution is 
evaporated to provide 62.45g (0.180 mole) of 
4,4'-bis(trifluorovinyloxy)biphenyl of 94.5% purity in 98% yield. 
The product is then recrystallized in ethanol to give product of 99.99% 
purity in greater than 70% recovery, melting point 44.degree.-46.degree. 
C. 
The IR spectrum shows peaks at (cm.sup.-1): 1833 (indicative of a 
perfluorovinyl group): 1601,1491 (indicative of an aromatic double bond): 
1231, 1196-1132 (indicative of carbon-oxygen and carbon-fluorine bonds 
respectively); 818 (indicative of aromaticity). 
The GC/MS spectrum has the following peaks: m/e: 346 (31.3%): 153 (13.8%); 
152 (100.0%): 151 (27.0%) 150 (11.7%); 76 (14.9%): 63 (14.9%). 
Differential scanning calorimetry (DSC) analysis of the 
4,4'-bis(trifluorovinyloxy)biphenyl monomer (20.degree. C. to 360.degree. 
C. at 20.degree. C./minute) indicates a sharp endotherm of melting 
beginning at 45.degree. C., followed by a broad exotherm beginning at 
about 170.degree. C., interpreted as corresponding to the heat of 
cyclization of the trifluorovinyl groups to form hexafluorocyclobutane 
rings. 
Monomer, 4,4'-bis(trifluorovinyloxy)biphenyl, (60.Og, 0.173 mole) is placed 
in a 1 liter 3-necked round bottom flask with 75 ml of 
perfluorotetradecahydrophenanthrene (Multifluor.RTM. APF 215 commercially 
available from Air Products). The flask is fitted with a mechanical 
stirrer and a nitrogen padded reflux condenser. After purging the flask 
thoroughly with nitrogen, the mixture is stirred and heated to reflux. 
Initially, upon heating the melted monomer is not miscible with the 
solvent, but as the temperature rises the two phases become homogeneous. 
After stirring at reflux for approximately 45 minutes, a polymer phase 
separates: and, after stirring at reflux for a total of 3 hours, the phase 
separated polymer becomes viscous enough to seize the stirring shaft. The 
cooled polymer is removed from the flask and evaporated under high vacuum 
(approximately 0.50 torr) at about 220.degree. C. for 3 hours to remove 
residual solvent. A portion of this polymer is compression molded at 
250.degree. C. to provide a light yellow, transparent flexible plastic 
film. Another portion is dissolved in tetrahydrofuran (THF) and placed in 
an evaporating dish to make a solvent-cast film. After the solvent is 
evaporated overnight, a light yellow thin film is peeled from the dish. 
This sample exhibits excellent flexibility and transparency. 
An IR spectrograph of the film has the following peaks (cm.sup.-1): 1601, 
1490 (indicating aromatic double bonds): 1302, 1194-1115 (indicating 
carbon-oxygen and carbon-fluorine bonds), 818 (indicating aromaticity). 
DSC analysis of this polymer indicates a Tg transition at 148.degree. C. 
Dynamic mechanical analysis (DMS) gives a Tg value of 170.degree., and gel 
permeation chromatography (GPC) indicates a weight average molecular 
weight of 85,000 as standardized against polystyrene. 
Dielectric constant and dissipation factor measurements performed on this 
polymer give the following results: 
______________________________________ 
Frequency Dielectric 
Dissipation 
(kHz) Constant Factor 
______________________________________ 
1.01 2.58 0.0007 
10.0 2.57 0.0004 
1000.0 2.55 0.0004 
______________________________________ 
A sample plaque of the biphenyl perfluorocyclobutyl ether polymer (12 
cm.times.8 cm.times.1 cm) is placed inside a baking dish in a vacuum 
drying oven and is cured under vacuum at 310.degree. C. for 20 hours. The 
sample is removed and a coupon is cut (6.0 cm.times.1.2 cm.times.0.33 cm) 
for dynamic mechanical analysis. The analysis shows a Tg of 175.degree. C. 
with no complete melt occurring, as is evidenced by maintenance of a 
storage modulus up to and including 344.degree. C. This cured 
(crosslinked) polymer also does not dissolve in THF but swells into a gel. 
A sample of the thermoplastic biphenyl perfluorocyclobutyl ether polymer is 
placed in a Rheometrics RMS-605 Mechanical Spectrometer using 25 mm 
parallel plates with a 1 mm gap. Using a 10% strain, storage and loss 
moduli are measured from 0.1 to 100 radians per second, measuring ten 
different frequencies per decade of frequency, every 15 minutes. 
Isothermal measurements are carried out according to the technique of H. 
H. Winter and F. Chambon, J. Rheology, 30(2), 367-382 (1986) except that 
the gel points are measured at 320.degree. C. and 360.degree. C. The log 
of tan delta (G"/G'), that is the logarithm of the ratio of the loss 
modulus (G") to the storage modulus (G'), is plotted against time for 
various frequencies. A gel point is indicated by convergence of the 
various frequency dependent tan deltas in the plot at 15 minutes. A second 
experiment is run under similar conditions, but at 320.degree. C. 
isothermal; a plot of tan delta against time indicates a gel point at 80 
minutes. Furthermore, samples after being heated beyond the gel points are 
not soluble in tetrahydrofuran. Thus, the system appears to crosslink in 
15 minutes at 360.degree. C. and in 80 minutes at 320.degree. C. 
Crosslinked samples exhibit some reddish brown or yellow color. 
Physical property determinations on samples of the thermoplastic 
(uncrosslinked) polymer and on the crosslinked polymer show significant 
differences as indicated in the table below: 
______________________________________ 
Physical Property Comparison 
Thermoplastic 
Polymer Crosslinked Polymer 
______________________________________ 
Tensile Strength.sup.1 
5,500 psi 7,200 psi 
Tensile Modulus.sup.1 
200,000 psi 255,000 psi 
Percent Elongation.sup.1 
12% 4% 
Flexural Strength.sup.2 
10,800 psi 8,700 psi 
Flexural Modulus.sup.2 
234,000 psi 315,000 psi 
Dielectric Constant.sup.3 
2.57 (10 kHz) 
2.59 (10 kHz) 
Dissipation Factor.sup.3 
0.0004 (10 kHz) 
0.0006 (10 kHz) 
______________________________________ 
.sup.1 As determined by the procedures of ASTM D88283. 
.sup.2 As determined by the procedures of ASTM D79081. 
.sup.3 As determined by the procedures of ASTM D15087. 
This data shows that crosslinking increases tensile strength and modulus as 
well as flexural modulus while reducing elongation without substantial 
change in electrical properties. 
EXAMPLE 2 
PREATION, SOLUTION POLYMERIZATION AND CROSSLINKING OF 
9,9-BIS(4'-TRIFLUOROVINYLOXY]PHENYL)FLUORENE 
Into a 2 liter 5-necked round bottom flask fitted with a mechanical 
stirrer, Dean-Stark trap topped with a nitrogen padded reflux condenser 
and a thermocouple attached to a temperature controller, are placed DMSO 
(650 ml) and toluene (200 ml). While the stirred solution is purged with 
nitrogen, 9,9-bis(4'-hydroxyphenyl)fluorene (200.0g, 0.57 mole) is added 
to the flask. While purging with nitrogen continues, potassium hydroxide 
(85% pellets, 77.5g, 1.17 mole) is added all at once, and the mixture is 
heated to 100.degree. C. with constant stirring. After two hours, the 
temperature is increased until the solution begins to reflux (130.degree. 
C.). Water is removed by azeotropic distillation for 24 hours. The 
Dean-Stark trap is replaced by a Soxhlet extractor containing anhydrous 
sodium sulfate, and the toluene is refluxed through the Soxhlet for 5 
hours. A small amount of toluene (60 ml) is then removed by simple 
distillation. Then the reactor is cooled to 35.degree. C. Addition of 
1,2-dibromotetrafluoroethane (315g, 1.21 mole) via dropping addition 
funnel is then maintained at a rate that keeps the reaction temperature at 
35.degree.-38.degree. C. When the addition is complete, the mixture is 
heated at 50.degree. C. for 8 hours, then cooled to room temperature with 
constant stirring. The mixture is filtered, and the precipitate is washed 
twice with acetone. The filtrates are combined and evaporated thoroughly. 
The residue from the evaporation is washed with water to remove residual 
potassium bromide (KBr). After the residue is air dried for 24 hours, it 
is purified by column chromatography (on neutral alumina, using hexane 
eluent) to provide as product, 
9,9-bis(4'-[2"-bromotetrafluoroethoxy]phenyl)fluorene (331.4g, 0.468 mole, 
82% yield), melting point 157.degree.-158.degree. C. 
The LC/MS spectrum has peaks at: m/e: 710 (53.0%): 709 (34.0%): 708 
(100.0%): 707 (23.3%): 706 (49.8%); 513 (28.4%) 511 (28.5%): 438 (12.8%): 
437 (52.4%): 436 (14.7%): 435 (55.8%); 355 (15.7%): 290 (33.9)%: 289 
(19.5%): 239 (35.9%): 228 (36.2%): 227 (38.9%): 226 (47.3%): 202 (27.7%): 
157 (47.2%); 131 (27.6%): 129 (23.1%). 
The product from the above reaction (18.85 g, 0.027 mole) is combined with 
freshly activated granular zinc (5.00g, 0.076 mole) in glyme and heated at 
reflux overnight. After cooling, the reaction mixture is decanted and 
centrifuged to remove suspended zinc salts. The solvent is removed by 
vacuum evaporation, and the residue is purified by column chromatography 
on neutral alumina using hexane as an eluent to provide as product 
9,9-bis(4'-trifluorovinyloxyphenyl)fluorene (5.55 g, 011 mole, 40% yield), 
melting point 115.degree.-116.degree. C. 
The LC/MS spectrum has peaks at: m/e: 511 (29.3%): 510 (91.9%): 337 
(37.2%): 316 (16.1%): 315 (19.7%); 313 (12.8%); 241 (15.5%); 240 (52.8%): 
239 (100.0%): 237 (15.6%); 207 (14.1%): 158 (28.7%): 157 (53.1%): 155 
(14.4%): 150 (28.8%): 145 (18.3%): 144 (16.5%): 120 (15.1%). 
Into a 50 ml round bottom flask fitted with a nitrogen padded reflux 
condenser, mechanical stirrer and a thermocouple attached to a temperature 
controller are placed 9,9-bis(4'-trifluorovinyloxyphenyl)fluorene (3.0g, 
0.0059 mole) and diphenyloxide (5.0 ml). The mixture is stirred and heated 
to reflux (255.degree. C.) for 22 hours. The diphenyloxide (DPO) solvent 
is evaporated under high vacuum on a 100 milliliter Kugelrohr bulb to bulb 
apparatus (0.03 mm, 165.degree. C.) to provide the polymer product, which 
is dissolved in methylene chloride and cast into a thin film. 
Gel permeation chromatography analysis of the polymer indicates a weight 
average molecular weight of 135,000 as standardized against polystyrene. 
DSC analysis indicates a Tg transition at 224.degree. C. 
It is notable that this polymer of 9,9-bis 
(4,4'-trifluorovinyloxyphenyl)fluorene, which is polymerized in DPO, 
attains a high molecular weight and forms a solvent cast film with good 
physical properties such as flexibility. The polymer is soluble in 
acetone, dichloromethane and tetrahydrofuran. 
A sample of the 9,9-bis(4'-oxyphenyl)fluorene perfluorocyclobutyl ether 
polymer is placed in the Rheometrics RMS-605 Mechanical Spectrometer using 
the conditions of Example 1. A plot of the tan delta against time for a 
360.degree. C. isothermal experiment indicates a gel point in less than 10 
minutes. A similarly run experiment at 320.degree. C. isothermal indicates 
a gel point in 35 minutes. 
These gel points are indicative of curing of the samples into crosslinked 
polymer systems. Thus, the system appears to crosslink in less than 10 
minutes at 360.degree. C. and in 35 minutes at 320.degree. C. Samples of 
the polymer heated to the gel point are insoluble in acetone, 
dichloromethane and tetrahydrofuran. The Tg is measured by differential 
scanning calorimetry to be 240.degree. C. 
EXAMPLE 3 
PREATION, POLYMERIZATION AND CURING OF 
2,2-BIS((4-PERFLUOROVINYLOXY)PHENYL)PROPANE 
The procedure for preparing the diperfluorovinyl compound of Example 1 is 
followed except that smaller scale equipment is used with about half the 
amounts of solvent for the various steps using 100.0g, 0.44 mole of 
para-bisphenol A in place of the 4,4'-dihydroxybiphenyl; 59.Og, 0.89 mole 
of potassium hydroxide pellets: 240g, 0.92 mole of 
1,2-dibromotetrafluoroethane; and 25.Og, 0.38 mole of granular zinc. The 
toluene mixture is heated to reflux (125.degree. C.). Water is removed by 
azeotropic distillation for a total of 48 hours, without cooling after 24 
hours. Before addition of the 1,2-dibromotetrafluoroethane, the toluene is 
dried by refluxing through the extractor for 20 hours. After cooling to 
room temperature the reaction flask is cooled to 18.degree. C. in an ice 
bath. The 1,2-dibromotetrafluoroethane is added at a rate that maintains a 
reaction temperature of 18.degree.-22.degree. C. Then the mixture is 
allowed to warm to room temperature, and is heated slowly to 50.degree. 
C. and stirred at 50.degree. C. for 6 hours. 
A crude reaction mixture forms and is filtered. The filtrate is evaporated 
thoroughly leaving a residue which is chromatographed on neutral alumina 
(50-200 mesh) using hexane as eluent to provide 113.9g (44% yield) of a 
product having a GC/MS with peaks at the following mass to charge ratios 
(m/e): 587 (4.6%): 585 (9.2%): 583 (5.0%): 572 (44.5%): 570 (100%): 568 
(52.0%): 397 (11.3%); 395 (11.0%): 315 (27.1%): 313 (28.2%): 299 (28.0%): 
297 (30.4%): 181 (31.1%), 179 (37.6%); 167 (29.1%): 165 (37.8%): 131 
(33.6%); 129 (36.9%): 115 (32.4%); 101 (35.7%): 91 (31.3%): 77(33.5%) 
consistent with a bisphenol A bis(2-bromotetrafluoroethyl) ether, 99+% 
pure by GC/MS analysis. 
After reaction with the granular zinc at 105.degree. C., the bisphenol A 
bis(2-bromotetrafluoroethyl) ether (103.4g, 0.176 mole) is placed in a 100 
ml dropping addition funnel and added at a rate that maintains a reaction 
temperature of 105.degree.-108.degree. C. When the addition is complete 
the mixture is stirred at 108.degree. C. for 2.5 hours, then cooled to 
room temperature. The mixture is centrifuged to remove the solids; the 
precipitate is separated and washed with acetone and again separated by 
centrifuging. The liquid portions are combined and evaporated leaving a 
residue which is chromatographed through a neutral alumina column using 
hexane as an eluent to provide 42.8g (63% yield) of a product having a 
GC/MS with peaks at the following mass to charge ratios (m/e): 388 
(17.5%): 374 (20.0%): 373 (100%): 276 (30.4%); 215 (46.7%); 199 (12.8%): 
179 (24.8%): 178 (50.0%): 152 (15.6%): 118 (24.0%): 117 (18.3%): 115 
(17.1%): 102 (19.9%): 89 (23.5%): 77 (22.6%): 76 (29.9%) bisphenol A 
bis(trifluorovinyl) ether. 
The bisphenol A bis(trifluorovinyl) ether monomer (13.6g) is combined with 
14.0 ml of Multifluor APF 215 solvent in a 100 ml 3-necked round bottom 
flask fitted with a mechanical stirrer and a nitrogen padded reflux 
condenser. Stirring is begun as the mixture is heated to reflux. The 
mixture is stirred at reflux for 5 hours, then allowed to cool to room 
temperature. A layer of polymer phase-separates to the top of the solvent. 
This polymer is removed and evaporated at 180.degree. C. under high vacuum 
(0.15 torr) to remove residual solvent. 
A sample of the polymer is placed in the Rheometrics RMS-605 Mechanical 
Spectrometer using the conditions used in Example 1. A plot of the tan 
delta against time for a 360.degree. C. isothermal experiment indicates a 
gel point in 50 minutes. A similar experiment at 320.degree. C. isothermal 
indicates that no gel point is reached by this polymer in the 150 minute 
time span of the experiment. These rheological experiments indicate that 
crosslinking of this polymer is much slower.