Addition and Condensation Polymerization Processes - American

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch021

The Nature and Influence of Gegenions in Tetrahydrofuran Polymerization PATRICIA DREYFUSS and M. PETER DREYFUSS B. F. Goodrich Co., Research Center, Brecksville, Ohio 44141

The effects of gegenion on the polymerization of tetrahydrofuran (THF) indicated that the order of stability was PF = SbF > BF - ≥SbCl -. With all gegenions gel permeation chromatography indicated a rapid broadening of the molecular weight distribution before equilibrium conversion was reached. A narrowing of the distribution occurred after equilibrium conversion. These results suggested that reaction with polymer oxygen is very important in THF polymerizations. Low conversions with BF suggested termination. Decreased molecular weights with SbCl without a corresponding decrease in percent conversion suggested transfer. The conclusions are based on changes with time of conversion, intrinsic viscosity, and molecular weight distributions. Equivalent concentrations of each gegenion were introduced in the form of triethyloxonium salts. Polymerizations were run in methylene chloride at 30°C. 6

6

4

6

4

6

npetrahydrofuran ( T H F ) polymerizes by ring-opening polycondensa·*- tion to give a linear polymer usually called polytetrahydrofuran ( P T H F ) but frequently called poly(tetramethylene oxide). A l l its known polymerizations occur by a cationic mechanism. The generally accepted growing species is a tertiary oxonium ion, R 3 0 + X ~ , of the form: CHo —— CHo ~^CH2CH2OCH2CH2CH2CH20

+ CH2

'CHo

335 In Addition and Condensation Polymerization Processes; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

336

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES


As in all ionic polymerizations, the oxonium ion is associated with a gegenion or counterion, X " . The gegenion X " exerts a large influence on the course of the polymerization and affects the properties of the polymer that is formed. Meerwein and co-workers ( 1 6 ) reported long ago that simple anions such as CI" lead to unstable complexes that decompose to give the ether and the alkyl halide ( Reaction 1 ) (1)

R 3 0 + C 1 - ^ R 2 Q + RC1

They reported the first stable oxonium ion salts i n 1937. These salts all contained complex gegenions such as B F 4 " , S b C l 6 " , A l C L f , or F e C L f . They found ( 15 ) that stable oxonium ions, such as triethyloxonium tetrafluoroborate, could initiate the polymerization of T H F . Meerwein also recognized that the polymerization of T H F is reversible and goes to an equilibrium between monomer and polymer. It is now well established (JO) that with suitable choice of gegenion the conversion depends only on temperature and monomer concentration. Meerwein did not study systematically the effect of changing the gegenion. In 1965 B a w n and co-workers ( 2 ) reported a comparison of the effects of changing the gegenion from S b C l G " to P F 6 " using trityl and tropylium salts as initiators. These workers made their study in bulk polymerizations at 2 5 ° and 50 ° C . Some of their results at 50 ° C . on the effect of gegenion on conversion and rate are shown in Table I. Using Table I.

Effect of Gegenion on Conversion and Rate at 5 0 ° C . (2) Eq. Conv., %

56 43 54.5

Rp(PF6-)/Rp(SbCl6-)

1.5

the P F 6 " gegenion they obtained the 56% equilibrium conversion to polymer previously observed by Sims ( 2 1 ) and Dreyfuss and Dreyfuss (7). However, with S b C l 6 " gegenion, their percent conversion was lower —namely, 43% with the triphenylmethyl cation and 54.5% with tropylium cation. Bawn and co-workers attributed this difference to termination in the case of the S b C l 6 " gegenion. The rate of polymerization with the P F 6 " gegenion was 1.5 times greater than with the S b C l 6 " gegenion at equal catalyst concentrations. They concluded that this was not a significant difference. Under the right conditions ( 1 0 ) termination and transfer reactions are unimportant in T H F polymerization. By analogy to anionic systems ( 2 3 ) , one would expect that the degree of polymerization could be calculated from the relation shown i n Equation 2

In Addition and Condensation Polymerization Processes; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

21.

DREYFUSS

AND

DREYFUSS

Gegenions

in

DP = M e - M i / C


where M 0 centration effects on tained are

337

Polymerization

(2)

e

is the initial monomer concentration, M i is the monomer con at time t, and C 0 is the initial catalyst concentration. The the degree of polymerization that Bawn and co-workers ob shown i n Table II.

Table II. Catalyst

Effect of Gegenion on the Degree of Polymerization [Catalyst'],

(/>3C PF6 )4 Ο

Dreyfuss et al. have shown previously that alkyl ethers are reactive i n T H F polymerizations, and the oxygens of the polymer chain are alkyl ether oxygens. The only outcome of this reaction is a randomization of the molecular weight distribution. The number of polymer molecules is not changed, and the number of active sites stays the same. After equilibrium conversion is reached, a rather surprising decrease i n the molecular weight distribution occurred (see Table V I ) . Although


21.

DREYFUSS A N D DREYFUSS

Geeenions

in

Polymerization

347


the differences appear small, they are outside the normal experimental error for a series of polymers run at one time. Note that after equilibrium there is a 13% drop i n Mw and only a 5 % drop i n Mn and that the trend in both Mw and Mn is down. W e said earlier that the decrease i n viscosity was caused by transfer. This transfer reaction w i l l result i n the formation of new polymer chains. If the relatively small chains now react with polymer oxygen as previously shown, something more than just redistribution occurs. Reaction 8 shows that since R is small, an effective chain cleavage is possible. This should result i n a more rapid breakdown of the larger molecules because the greater number of oxygens i n the longer chains would increase the proba b i l i t y ^ their reacting. Hence, Mw would decrease more rapidly than Mn, and Mw/Mn would tend to decrease. CH2CH2'v/'^ +

Ο N

RO(CH.,)4

^ Ο+

• ^CH2CH2/\~v

.CHoCHo< -

THF

*

(8)

/^CH2CH2* CHoCH..^^ RO(CH2)4

Ο

0 G e l permeation chromatograms were not obtained on the S b F e " polymers. However, there is no reason to suspect that these results would differ substantially from the P F 6 " results. For the B F 4 ~ gegenion the initial narrow distribution formed was again followed by a rapid broadening. The viscosity and conversion data suggested that nothing much was happening after 8 hours, and the poly merization died. The G P C data also reflects this. G P C data from the study with the S b C V gegenion are interesting. Here the effects of reaction with polymer oxygen i n the presence of a relatively large concentration of new chains are visible. As one would

American Chemical society Library 1155 16th N.W.Platzer, Norbert A. J.; In Addition and Condensation Polymerization St, Processes; Advances in Chemistry; American Chemical U.C. Society: 20036 Washington, DC, 1969. Washington,


348

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES

expect from the viscosity data, both Mn and Mw are substantially lower than in previous cases. This occurred despite the fact that each experiment initially contained the same number of growing centers. Again a rapid broadening of the initial narrow molecular weight distribution occurs, but i n this case, the Mw/Mn ratio never seems to reach that of a normal distribution. Again this can be explained by the more probable chain cleavage of the longer polymer molecules by the products of the transfer reaction. Reaction with polymer oxygen seems to occur readily i n T H F polymerizations. This reaction makes it difficult to prepare narrow distribution polytetrahydrofuran at significant conversions. The calculations of Hermans (12) and Rozenberg et al. (19) show that this kind of reaction should lead to the formation of polymer with a random distribution. W h e n the additional complication of transfer reactions leading to new short chains is introduced, reaction with polymer oxygen tends to give a molecular weight distribution somewhat narrower than the normal. This reaction also seems to speed up the effect of transfer reactions, especially those occurring after equilibrium is achieved. Conclusions

The order of stability of the gegenions we have studied i n T H F polymerizations at 3 0 ° C . is: PF 6 - c~ SbF 6 " > B F 4 " ^ SbCl 6 " From a chemical standpoint there is little evidence at present to prefer P F 6 " over S b F 6 " or vice-versa. Both gegenions seem considerably more stable than B F 4 " or SbCl 6 ". The B F 4 " polymerization contains considerable termination, and the SbCl 6 ~ polymerization contains considerable transfer and some termination. It is interesting to note that both results can be explained i n terms of a similar reaction with gegenion. A l l our studies were made at 30 ° C . Data i n the literature suggest that as the temperature is lowered, these differences w i l l become less because reaction with gegenion becomes much slower. O n the other hand the differences w i l l become even more pronounced as the temperature is raised above 30 ° C . Thus, for high temperature studies the use of P F G " or S b F 6 " is clearly indicated. Acknowledgment

W e thank J. C . Westfahl for making the N M R measurements and D . Harmon for carrying out the G P C measurements and calculations.


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DREYFUSS AND DREYFUSS

Literature

Gegenions in Polymerization

349

Cited


(1) Alliet, D. F., Proc. Intern. Symp. Polymer Characterization, Battelle Mem. Inst., Columbus, Ohio, IV-1 (1967).

(2) Bawn, C. Ε.H.,Bell, R.M.,Fitzsimmons, C., Ledwith, Α., Polymer 6, 661 (1965). (3) British Industrial Plastics Ltd., Netherland Patent Application 6,509,888 (1966). (4) Brown, W. B., Szwarc,M.,Trans. Faraday Soc. 54, 416 (1958). (5) Burrows, R.C.,Crowe, B. F., J. Appl. Polymer Sci. 6, 465 (1962). (6) Distillers Co., Ltd., Netherland Patent Application 6,612,244 (1967). (7) Dreyfuss, M. P., Dreyfuss, P., J. Polymer Sci., Pt. A-1 4, 2179 (1966). (8) Dreyfuss, M. P., Westfahl, J.C.,Dreyfuss, P., ACS, Div. Polymer Chem., Polymer Preprints 7 (2), 413 (1966).

(9) Dreyfuss, M. P., Westfahl, J. C., Dreyfuss, P., Macromolecules 1, 437 (1968). (10) Dreyfuss, P., Dreyfuss, M. P., Advan. Polymer Sci. 4, 528 (1967). (11) Furukawa, J., Saegusa, T., "Polymerization of Aldehydes and Oxides," Interscience, New York, 1963. (12) Hermans, J. J., J. Polymer Sci. C (12), 345 (1966). (13) Kuntz, I., J. Polymer Sci. B4, 427 (1966). (14) Meerwein,H.,Battenberg, E., Gold,H.,Pfeil, E., Willfang, G.,J.Prakt. Chem. 154, 83 (1939). (15) Meerwein,H.,Delfs, D., Morshel,H.,Angew. Chem. 72, 927 (1960). (16) Meerwein, H., Hinz, G., Hofmann, P., Kroning, E., Pfeil, E., J. Prakt. Chem. 147, 257 (1937). (17) Miyake, Α., Stockmayer, W.H.,Makromol. Chem. 88, 90 (1965). (18) Ofstead, Ε. Α., ACS, Div. Polymer Chem., Polymer Preprints 6 (2), 674

(1965). (19) Rosenberg, Β. Α., Irzhak, V. I., Enikolopyan, N. S., Proc. Acad. Sci. USSR 170, 888 (1966). (20) Rosenberg, Β. Α., Ludvig, Ε. B., Gantmakher, A. R., Medvedev, S. S., J. Polymer Sci. C (16), 1917 (1967). (21) Sims, D., J. Chem. Soc. 1964, 864. (22) Sorenson, W. R., Campbell, T. W., "Preparative Methods of Polymer Chemistry," p. 255, Interscience, New York, 1961. (23) Szwarc,M.,Advan. Polymer Sci. 2, 275 (1960). (24) Vofsi, D., Tobolsky, Α. V., J. Polymer Sci. A3, 3261 (1965).

RECEIVED

March 25, 1968.


Addition and Condensation Polymerization Processes - American

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