Some steric regulation on radical polymerization of perfluorobutadiene

Some steric regulation on radical polymerization of perfluorobutadiene* Madeline S. Toy and d. C. DiBari Stanford Research Institute, Menlo Park, Calif. 94025, USA (Received 16 January 1973; revised 10 April 1973) Polymer initiation by irradiation alone exerts only a minor effect on the steric regulation of polyperfluorobutadiene. Under ultra-violet irradiation the extent of stereoregularity for 1,2-polymerization increases with increasing concentration of the free radical catalyst, while the solvent effect under ultra-violet initiation is not as significant in steric regulation nor in improving the yield of the homopolymer. This study demonstrated some stereoregularity of polyperfluorobutadiene through variation of the experimental conditions under ultraviolet irradiation. INTRODUCTION Previously it was found that radical polymerizations of perfluorobutadiene under low pressure and ambient temperature formed 1,2- and 1,4-polyperfluorobutadiene 1, z. With the presence of higher concentrations of radical initiator during bulk polymerization, the polymer was reported to consist of higher 1,2-moiedes ~. In the present investigation, experimental conditions are being sought that will demonstrate that radical polymerization of perfluorobutadiene can allow polymerization to favour one or both olefinic bonds of the monomer. In other words, the objective is to show that a high ratio of 1,2- or 1,4-polymer can be selectively obtained under some polymerization conditions. The molecule perfluorobutadiene, unlike butadiene monomer, adopts a cis-bent conformation with the dihedral angle being 42 +15 degrees or 47.60+0.58 degrees, in keeping with later electron diffraction data 4, 5. The non-planar conformation of perfluorobutadiene does not allow the p orbitals to overlap effectively, and thus extensive delocalization of this monomer is precluded. Because of the less conjugated nature of the two double bonds of the monomer and its comparatively slow rate of polymerization under low pressure and ambient temperature, radical polymerization of this monomer was undertaken to demonstrate some steric orientation effects of the polymer by varying the radiation dose rate, polarity of the aprotic solvent, and concentration of the free radical catalyst under gamma and electron irradiation6. This paper reports the continuing work on the solvent and catalyst effects under ultra-violet (u.v.) irradiation and the various results so far achieved.

EXPERIMENTAL

Materials and polymerization techniques Perfluorobutadiene and CF3OOCFa (from Peninsular ChemResearch, PCR) were checked by methods described previouslyZ, 7. The solvents (n-C4Fg)3 (from 3M Co.), CC13F (from duPont de Nemours), and n-CaFls from PCR were checked by infra-red spectra only. * Paper presented at the Joint Pacific Conference on Chemistry and Spectroscopy,San.Francisco, California,October 1972.

The polymerizations of perfluorobutadiene were carried out in bulk, in solution, and in the presence of CFaOOCFa catalyst in evacuated sealed quartz tubes under autogenous pressure of about 0"8 atm (1 a t m=101.33 kN/m2). They were then placed in the polymer tumbling apparatus under external ultra-violet irradiation at ambient temperature for a week. At the end of the polymerization period, the sealed polymerization tubes were cooled to solid, opened, and the unconverted monomer was removed through the vacuum system. The polymer samples in the presence of solvents were centrifuged and the decanted liquids were combined with the CClaF extracts from the fractionations of the solid polymers. The combined liquids were evaporated to dryness and evacuated over anhydrous MgSO4 as CClaF-soluble fractions. The fractionations of various solid polymer samples were carried out by CCIaF solvent extractions under identical conditions.

Polymer&ation apparatus and intensity of the u.v. source A Hanovia medium pressure mercury arc lamp was introduced at the centre of the previously described polymer tumbling apparatus a in a double-walled quartz cooling jacket and at 4tz in distance from the sealed quartz polymerization tubes. The light intensity for the u.v.induced polymerization was determined by the liquid chemical actinometer using a potassium ferrioxalate system 9-11. In experiments, the K3Fe(C204)3 solution was placed in two quartz polymerization tubes at the same distance for polymerization and irradiated in the polymer tumbling apparatus. The unirradiated K3Fe(C204)3 solution was used as a reference to set the zero reading. The measured optical densities from the Beckman DU spectrophotometer (log10 I/Io, 1.23 and 1.29) and the calculated light intensities (I~, 6.005 and 6.250× 1016 quanta/sec) give average I~0 as 6.127× 1016 quanta/sec. RESULTS AND DISCUSSION Polymer initiation by irradiation alone seems to polymerize the unsaturated perfluorobutadiene monomer with only limited specific control of configuration. When irradiation is combined with another factor, however, such as the presence of free radical catalyst CFaOOCFa under ultra-violet irradiation or the presence of a non-

POLYMER, 1973, Vol 14, July

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Steric regulation on polymerization of perfluorobutadiene : M. S. Toy and J. C. DiBari Table 1 Solvent effect on polymerization of perfluorobutadiene in the presence of catalyst under u.v. irradiation (monomer, 4.43 g) After CClaF extraction

Solvent

Catalyst

Yield of product (g)

0

0

0.30

0

5% CFsOOCF8

0.89

(n-CaFg)aN, 1 ml

5% CFaOOCFa

0"88

3 ml

0

0.23

3 ml

2~/o CF3OOCFB

0.47

3 ml

5% CFaOOCFs

0.75

6 ml

5% CFaOOCFa

0.63

1 ml

5 ~ CFaOOCFa

0.64

3 ml

5% CF3OOCFa

0-58

6 ml

5% CFaOOCF~

0"58

n-CaFza,

Fraction

Yield (g)

I.r. absorbance ratio 5.6/5.8

Soluble Insoluble Soluble* Insoluble* Solublet Insolublet Soluble Insoluble Soluble Insoluble Soluble~ Insoluble Soluble Insoluble Soluble Insoluble Soluble Insoluble Soluble Insoluble

0" 19 0.11 0.65 0.24 0.68 0.20 0.14 0.09 0-34 0.13 0.64 0.11 0-54 0.09 0.56 0.08 0.50 0.08 0.50 0.08

1.1 1.2 2-7 1.5 2.6 1.6 1.1 1.0 2-1 1.8 3-0 1-9 2.4 1.7 2.5 1.9 2.4 2.2 1 "8 1 "8

* Mol. wt. of CCIzF solution fraction=6200 and insoluble fraction=6300 "f Mol. wt. of CCI3F solution fraction=6000 and insoluble fraction=6800 5; Mol. wt. of CCIzF solution fraction=5730

polar aprotic solvent under g a m m a irradiation in the absence of catalyst 6, the incoming m o n o m e r unit seems in some way to favour 1,2-polymerization for the former and 1,4-polymerization for the latter. Table 1 shows that under u.v. irradiation both the stereoregulation for 1,2-polymerization and the polymer yield increase with increasing concentration of the free radical catalyst. Table 1 also shows that bulk polymerization in the presence of CFaOOCFa catalyst under u.v. irradiation gave polyperfluorobutadiene (molecular weight around 6000) with higher 1,2-moieties and better yield than in the absence of catalyst. For solution polymerizations with the same amount of catalyst, the presence of more polar aprotic solvent, (n-C4Fg)aN, is preferred over non-polar solvent (n-CsFls). The pendant perfluorovinyl bonds ( - - C F = C F z ) from 1,2-addition polymerization appear at 5-6/zm and the internal perfluorovinylene bonds ( - - C F = C F - - ) from 1,4-addition polymerization appear at 5.8/zm. Thus, in Tables 1 and 2, a large infra-red absorbance ratio (5.6/5.8) indicates more 1,2-polymerization. Table 2 shows that the solvent effect under u.v. initiation is not a significant factor in steric regulation nor in improving the yield of the homopolymer. The converse is true for homopolymerization under g a m m a irradiation, where the extent of stereoregularity for 1,4-polymerization and the yield of the homopolymer increase with decreasing polarity of the aprotic solvents 6. In summary, under ultra-violet irradiation the predominance of 1,2-moieties is in the lower molecular weight fraction and is enhanced by increasing the concentration of the free radical catalyst. For the preparation of high 1,4-moieties, see ref. 6. ACKNOWLEDGEMENTS This work is sponsored in part by Stanford Research Institute and in part by National Aeronautics and Space Administration under Contract NAS3-15577. 328

P O L Y M E R , 1973, V o l 14, d u l y

Table2 Solvent effect on polymerization of perfluorobutadiene under u.v. irradiation (monomer, 44.43 g) i

i

After CClaF extraction Solvent

Yield of product (g)

0

0.80

(n-C4Fg)aN, 1 ml

0.31

n-CsFzs,

3 ml

0.23

6 ml

0.20

1 ml

0.34

3 ml

0.28

6 ml

0.18

Fraction

Yield (g)

I.roabsorbance ratio5.6/5.8

Soluble* Insoluble* Soluble Insoluble Soluble Insoluble Soluble Insoluble Soluble Insoluble Soluble Insoluble Soluble Insoluble

0.19 0.11 0.21 0.10 0.14 0.09 0.13 0.07 0.19 0.15 0.17 0.11 0-14 0.04

1-1 1.2 1.1 1.0 1.1 1.0 1.1 1.0 1.1 1.2 1.2 1.3 1.3 1.2

* Mol. wt. of CClaF solution fraction = 2700 and insoluble fraction = 9800

REFERENCES 1 Toy, M. S. 'Photochemistry of Macromolecules', (Ed. R. F. Reinisch), Plenum Press, New York, 1970, pp 135-144 2 Toy, M. S. and Lawson, D. D. J. Polym. Sci. (B) 1968, 6, 639 3 Toy, M. S. Polym. Prepr. 1971, 12(1), 385 4 Brundle, C. R. and Robin, M. B. J..4m. Chem. Soc. 1970, 92, 5550 5 Andreassen, A., Chang, C. H. and Bauer, S. H. Paper presented at 160th Am. Chem. Soc. Meeting, Chicago, Sept. 1970 6 Toy, M. S. and DiBari, J. C. Ind. Eng. Chem. (Prod. Res. Develop.) 1972, 11, 404 7 Toy, M. S. and Newman, J. M. J. Polym. Sci. (,4-1) 1969, 7, 2333 8 Toy, M. S. and Newman, J. M. Polym. Prepr. 1970, 11(1), 121 9 Hatchard, C. G. and Parker, C. A. Proc. R. Soc. 1956, A235, 518 10 Calvert, J. G. and Pitts, J. N. 'Photochemistry', Johrt Wiley, New York, 1967, pp 769-814 I1 Skoog, D. and West, D. M. 'Fundamentals of Analytical Chemistry', Holt, Rinehart and Winston, New York, 1966