POLYMER LETTERS VOL. 2, PP. 689-692 (1964)

UNUSUAL ASPECTS OF THE CATIONIC POLYMERIZATION OF N-VINYLCARBAZOLE*

The cationic polymerization of N-vinylcarbazole has several unusual features which can account for the various results which Chapiro and Hardy ( l ) , Breitenbach and Srna (2), Scott and Labes (3), Ellinger (4), and Breitenbach and Olaj (5) have reported for the polymerization in car- bon tetrachloride. The polymerization is retarded by water and there is no consumption of water or catalyst. When the polymerization is slow, it is difficult to a s s e s s whether a compound present in a relatively large quantity, such a s a solvent, has catalytic properties. The problem is further complicated by the usual methods of purification of solvent in- volving simultaneous removal of possible catalytic and retarding impuri- ties and purification of monomer involving removal of more retarding than catalytic impurities. The net effect of general purification is that N- vinylcarbazole becomes susceptible to polymerization with concentra- tions of catalyst in the parts per billion range, a s indicated by the fol- lowing experiment.

By using 1 ml. of a solution of 0.25 g. monomer in acetonitrile, in which polymer is insoluble, polymerization was observed to begin repro- ducibly in 13 to 14 min. as indicated by precipitation of polymer on addi- tion of 0.04 ppm acid. For another experimental purpose, the catalyst used was an equimolar mixture of diethylphosphoric and hydrochloric ac ids , the catalytic effect of which is generally the s a m e a s that of sin- gle acids, which, incidentally, include such weak acids as phenol and acetic acid. Control solutions of monomer in acetonitrile to which no catalyst solution was added became cloudy after about 60 min., indicat- ing that even less , or, possibly, no catalyst is required to bring about polymerization of dry monomer in dry acetonitrile.

recrystallized from a carbon-black-treated solution of ethanol containing 10% water (hydroxylic solvents minimize polymerization), ground, and vacuum-oven-dried overnight at room temperature to give a product mel t - ing at 64.OoC. It contained about 700 ppm water a s determined by the Karl Fisher titration using a Beckman KF-3 Aquameter and a blanket of Airco prepurified nitrogen dried by passage through 4 f t . by 2 1/2” col- umn of Linde 3A molecular sieves. The monomer used in our previously reported experiment (3) was not dried beyond th is stage. For the present experiment, however, the monomer was then freeze-dried for a week a t -2OOC. (to minimize a tendency to polymerize in the solid state) on a

The monomer used in this experiment was obtained from Matheson Co.,

*Supported by the U. S. Army Chemical Center under Contract DA-18- 108AMC-169.

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vacuum line with liquid nitrogen traps at < able to determine the water content of freeze-dried monomer reproduc- ibly, possibly because the large sample of oxidizable monomer required to determine a low water content interferes with the redox reactions in- volved in the titration. The acetonitrile (spectroquality grade, Mathe- son Co.) was distilled from P,O, to reduce the water content from ca. 600 ppm to < 5 ppm and passed slowly through a 100 x 25 mm. column of powdered CaH, to remove an acidic impurity which was occasionally produced, possibly a s a result of an H3P0,-catalyzed hydrolysis of acetonitrile. In th i s c a s e , distillation from CaH2 was a less effective drying procedure than distillation from P,O,. The catalyst was added in the form of a 0.1 ml. solution in methylene chloride. The solution was prepared by repeated dilution in small, capped beverage bottles. The methylene chloride (reagent grade, Fisher Scientific Co.) was disti l led from P2O5 to reduce the water content from 270 ppm to < 5 ppm. It yielded no precipitate when treated with aqueous silver nitrate and it did not cause polymerization of monomer for 2 hr. Purified solvents were stored under nitrogen pressure in beverage bottles baked for sever- al days a t 15OoC., cooled under nitrogen, and capped with crown caps having a hole in the center and an extracted self-sealing rubber liner for withdrawal of solvent with a hypodermic syringe. No increase in wa- ter content was noted after a month's storage. The monomer was quick- ly transferred from the vacuum line and covered with acetonitrile, mini- mizing adsorption of moisture from the atmosphere which makes monomer less susceptible to polymerization. After establishing that oxygen and carbon dioxide do not affect cationic polymerization and that no signifi- cant increase in the water content of a monomer solution in acetonitrile occurs within a 15-min. interval by comparing results obtained with baked, unbaked, stoppered, and unstoppered test tubes in the absence and presence of air, oxygen, and carbon dioxide at various catalyst con- centrations and initial water levels, the polymerization test was perform- ed in an open, unbaked test tube.

In connection with the question of whether purified carbon tetrachlo- ride catalyzes polymerization, it is significant that polymerization will a l so occur in similarly purified acetonitrile and methylene chloride in the absence of added catalyst . Water retards the cationic polymeriza- tion in acetonitrile and methylene chloride. The slower polymerization in methylene chloride is being studied in detail. Kinetic studies indi- cate the ra te of polymerization of N-vinylcarbazole with diethylphos- phoric acid in methylene chloride at conversions up to about 20% to be

mm. Hg. W e were not

[Cat.] [Man.]' [H201

proportional to where [Cat.] and [H,O] are initial concen-

trations of catalyst and water. The exponents of the catalyst and mo-

POLYMER LETTERS 69 1

nomer concentration dependencies are still somewhat uncertain, but all the data are consistent with the indicated water dependency and the lack of catalyst and water consumption. The polymerization can toler- ate high water levels if the catalyst concentration is high. A low yield of low molecular weight polymer was obtained within a half hour at room temperature when solid monomer was added to concentrated hydrochloric acid. W e have also found the cationic polymerization of N-vinylcarba- zole with n-acids or oxidants (4,6) to be retarded by water. Ellinger has reported that N-vinylcarbazole can be polymerized in an aqueous suspen- sion to high conversion within a half hour a t 60-70°C. with p-chloranil or tetranitromethane as the catalyst (4), indicating that water is also not consumed in the polymerization with n-acids. (The lack of water consumption would account for these heterogeneous polymerizations not being retarded to the extent of inhibition since it would cause the poly- merization rate to be retarded only by the water content of the organic phase.) Polymerizations of N-vinylcarbazole catalyzed by all types of acidic impurities likely to be present in most solvents can, therefore, be expected to be similarly affected by water.

Our earlier report (3) was based on the observation that no polymeriza- tion occurred in 1 to 2 days with monomer containing about 700-900 ppm water in carbon tetrachloride which yielded negative tests for the pre- sence of chlorine or chloride ion when treated with aqueous silver ni- trate. The polymer was precipitated in acetone instead of methanol which, although it is a poor solvent for monomer, has the advantage of dissolving somewhat less polymer than does acetone. W e had noticed polymer formation after several days standing, but since N-vinylcarba- zole is generally susceptible to polymerization on standing in the pre- sence and even in the absence of solvents we were not confident that this should be attributed to a catalytic effect of carbon tetrachloride. The formation of yellow color and polymerization were observed by pass- ing a few bubbles of chlorine g a s through the monomer solution. Chlo- rine catalyzes the polymerization in various solvents; in the case of acetonitrile it was only necessary t o expose the mouth of a bottle with monomer solution to chlorine gas briefly. Ellinger has also found chlo- rine to be a very effective catalyst (4).

tetrachloride occurs within a few hours, can be attributed to the monomer being very dry. The negligible effect of purification of solvent on the rate of polymerization can be attributed t o the simultaneous removal of water and catalytic impurities. The quantitative effect of chlorine addi- tion on the rate of polymerization would depend on the extent to which the resulting difference in catalyst concentration affects the rate in re- lation to other rate-determining factors. Unless the interaction of chlo- rine with monomer produces a catalytic species which is not consumed in the polymerization, a possible consumption of an oxidative catalyst

The finding of Breitenbach and Olaj ( 5 ) , that polymerization in carbon

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such as chlorine would tend to further minimize the effect of chlorine on the rate of polymerization. The intensive discoloration observed in the polymerization with carbon tetrabromide ( 5 ) suggests that a reaction not observed witl- the polymerization in carbon tetrachloride may be involved.

It seems highly improbable that acetonitrile, methylene chloride, and carbon tetrachloride should all have similar catalytic properties, and since, under very dry conditions, N-vinylcarbazole becomes susceptible to cationic polymerization by extremely minute traces of catalyst which would not be readily removable or detectable by ordinary means, it re- mains unlikely that carbon tetrachloride has catalytic properties.

References

(1) Chapiro, A., and G. Hardy, J . Chim. Phys., (2) Breitenbach, J. W., and Ch. Srna, J . Polymer Sci., B, 263 (1963). (3) Scott, H., and M. M. Labes, J . Polymer Sci., BJ, 413 (1963). (4) Ellinger, L. P., Chem. & Ind. (London), 1963, 1982. (5) Breitenbach, J . W., and 0. F. Olaj, J. Polymer Sci., E, 685

(1964). (6) Scott, H., G. A. Miller, and M. M. Labes, Tetrahedron Letters, 1z,

1073 (1963).

993 (1963).

Harvey Scott Thomas P. Konen Mortimer M. Labes

The Franklin Institute Laboratories Chemistry Division Philadelphia, Pennsylvania

Received April 10, 1964