Synthesis of branched polystyrene

Synthesis of branchedpolystyrene W. A. J. BRYCE, G. MCGIBBON* and I. G. MELDRUM~ A new experimental technique for anionic polymerization has been devised and successfully used for the investigation of the coupling reaction between living polystyryl anions and 1,3,5-trichloromethyl benzene. This reaction was complicated by a dimerization reaction involving living polymer molecules.

INTRODUCTION THE TECHNIQUES of anionic polymerization first introduced by Swarc 1 are now well established, and such techniques have since been employed in the preparation of regularly branched polymers of predetermined molecular weight and molecular weight distribution. By coupling living polymers with suitable low molecular weight compounds, trifunctional z-4 and tetrafunctional 2, 4, 5 star branched polymers have been prepared and the coupling reaction has also been used to prepare polymers having six 6 or more v branches. The use of such model polymers allows careful investigation of the effect of branching on dilute solution behaviour 2, 3, mechanical and flow properties of polymers s, 9 This report describes a novel technique for the preparation of living polymers, and the method is adopted in preparing trichain star polystyrene by reaction of living polystyryl anions with 1,3,5-trichloromethyl benzene. The dilute solution properties of these polymers have been investigated, and will be described in later publications.

EXPERIMENTAL

Materials Since the successful production of monodisperse polymers by anionic techniques requires careful elimination of impurities, rigorous precautions were taken to remove impurities from all solvents and monomers. (a) Solvent. Tetrahydrofuran, (THF), was used as polymerization solvent throughout the investigation. Reagent grade THF was treated with potassium hydroxide pellets to remove inhibitors and distilled from sodium wire in an atmosphere of nitrogen. The solvent was then outgassed under vacuum and dried over a sodium film. Distillation onto a fresh sodium film, and the addition of a small amount of a-methyl styrene resulted in the formation of a-methyl styryl sodium, the red colour of which served as an indication of the *Present address: Arthur D. Little Research Institute, Inveresk, nr. Musselburgh, Midlothian tPresent address: BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex 394

SYNTHESIS OF BRANCHED POLYSTYRENE

absence of reactive impurities. T H F was distilled from this solution as required. (b) Monomer. Styrene, washed with 10~o sodium hydroxide solution to remove inhibitors, was partially dried over calcium chloride and distilled under reduced pressure. A centre fraction was stored over calcium hydride under vacuum. Further reduction of reactive impurities was achieved by storing the monomer, under vacuum, over the reactive anion mono-sodium benzophenone, which does not polymerize styrene TM, but should be reactive enough to reduce the water content further. Mono-sodium benzophenone is sufficiently soluble in styrene to give a pale green colour, which serves as an indication of the absence of reactive impurities. Styrene was distilled from this solution as required. (c) Catalysts. Benzyl sodium and cumyl potassium have been shown to be efficient anionic polymerisation initiators 11, and both were used in this work. Benzyl sodium was prepared by allowing a solution of dibenzyl mercury in THF to react for 3h with a sodium film. The problem of isomerisation ~ was overcome by addition of a small quantity of a-methyl styrene. A THF solution of cumyl potassium, prepared by reaction of cumyl ethyl ether with a potassium film was used as a polymerization initiator in the early stages of this work. Although it has been claimed 12 that quantitative yields of cumyl potassium can be prepared by this method, it was found by titration with palmitic acid that yields greater than 80 ~ were never obtained, even in the presence of a large excess of potassium. In some cases, the yield was as low as 5 0 ~ , despite the rigorous precautions taken to remove impurities. Thin layer chromatography, on a 'killed' catalyst sample showed the presence of unreacted cumyl ethyl ether. In later work, a commercial sample in heptane suspension, supplied by K. and K. Laboratories, Inc., was used. Catalyst solutions were filtered under vacuum into glass ampoules which were stored under liquid nitrogen until required. The catalyst solutions were standardized against known weights of palmitic acid. (d) Coupling Compounds. The low molecular weight materials used for coupling the living polystyryl anions to form trichain polystyrene were 1,3,5-tribromomethyl benzene and 1,3,5-trichloromethyl benzene. Their preparations are described below.

( l ) 1,3,5-tribromomethyl benzene. Mesitylene was brominated with dibromodimethyl hydantoin 13 using carbon tetrachloride as solvent, and benzoyl peroxide as catalyst. Recrystallization of the product from petroleum ether gave pale yellow needles, m.p. 98°C (lit. 97-99°C 14) and bromine content 69 ~ (calc. 67 ~). (2) 1,3,5-trichloromethyl benzene. As far as is known, the preparation of this compound has not previously been reported and is described here in some detail. Mesitylene, (30g), was refluxed with 1,3-dichloro-5,5-dimethyl hydantoin (74g) and benzoyl peroxide (0.5g) in carbon tetrachloride (500ml) for 395

W. A. J. BRYCE, G. McG1BBON AND I. G. MELDRUM

l l0h. The mixture was filtered hot to remove the insoluble dimethyl hydantoin and removal of the solvent left a yellow oil and a white solid, which proved to be unreacted dichlorodimethyl hydantoin. The yellow oil was split into six fractions by vacuum distillation, and n.m.r, spectra showed the fifth fraction, boiling at 127°C, to be nearest the spectrum expected for 1,3,5-trichloromethyl benzene. After a further vacuum distillation, the oil crystallized slowly on standing, and the solid product was twice recrystallized from ethanol and petroleum ether, to give white needles, m.p. 57°C. The chlorine content, 46 ~ , agreed well with the calculated values of 47.6 ~. The n.m.r, spectrum of 1,3,5-trichloromethyl benzene is shown in

Figure 1.

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Figure 1 N.M.R. sp,ectrum of 1,3,5-trichloromethyl benzene Techniques assoc&ted with anion solutions All work involving anion solutions, i.e. solvent and monomer purification, catalyst preparation and monomer polymerization, was carried out under high vacuum. Break-seals were used rather than stop-cocks, and fragile glass bulbs were found to be particularly useful for introducing small quantities of materials. (a) Polymerization apparatus. A problem often encountered in anionic polymerization is that truly monodisperse polymers are not always obtained because of inefficient mixing of monomer and catalyst. This problem of mixing is not completely overcome by dropwise addition of monomer to the catalyst solution 15, and monomer distillation techniques have the disadvantage that local condensation of monomer may occur on the walls of the reaction vessel 16. The distillation technique of Cowie, Worsfold and Bywater 17 overcomes the problem of local condensation of monomer by incorporating a heating element, but this apparatus is not particularly suitable for manipulation of solutions. A satisfactory technique which has been devised to overcome these problems involved condensation of both solvent, from the initiator solution, and 396

SYNTHESIS OF B R A N C H E D P O L Y S T Y R E N E

monomer, from an ampoule, onto a cold finger, so that a dilute monomer solution was slowly added to the initiator solution. The polymerization vessel, which is constructed from a 500ml flask, is shown in Figure 2. Stirring is effected by a conventional glass stirrer in the Monomer

"Cold finger

(

Cord nitrogen

Figure 2 Polymerization apparatus top of which is a small permanent magnet. The stirrer is driven externally by a large permanent magnet, and mixing is further improved by indentations in the sides of the vessel. The cold finger is designed such that both monomer and solvent condense on the top, and it is cooled by gaseous nitrogen or air which has been passed through a long coiled tube immersed in liquid nitrogen. The temperature, and hence distillation rate may be adjusted by altering the gas flow rate. The rate of evaporation of solvent is sufficient to reduce the temperature of the initiator solution to about --20°C, thus decreasing the probability of side reactions. In practice, the use of this technique permitted a two hundred fold dilution of monomer while increasing the volume of solution only by the volume of monomer added. (b) Polymerization procedure. Ampoules of catalyst (cumyl potassium in THF), solvent (THFstored over sodium di-a-methyl styrene tetramer), monomer (styrene distilled from mono-sodium benzophenone), and a fragile glass bulb containing the coupling agent (1,3,5-trichloromethyl benzene), were attached to the polymerization vessel shown in Figure 2. The apparatus was evacuated at l0 4mmHg. for several hours and gently heated with a large flame to remove any remaining moisture. 397

W. A. J. BRYCE~ G. McGIBBON AND I. G. MELDRUM

After sealing from the vacuum line, a filtered solution of a-methyl styryl sodium was used to rinse the apparatus, and this solution was returned to the solvent ampoule. Remaining a-methyl styryl sodium anions were removed by repeated solvent distillation until the rinsing solution was colourless, when all the solvent, (250ml), was distilled from the ampoule and used as the solvent for polymerization. The empty ampoule was removed from the apparatus. Initiator was added and the refluxing system put into operation. When the temperature of the initiator solution had decreased to about --20°C, the break-seal of the styrene ampoule was opened, and the monomer, (6ml), was allowed to distil onto the cold finger and into the initiator solution, over a period of 6h. After addition of all the monomer, the solution was warmed and a portion of the polymer solution was removed in an ampoule and terminated with methanol. This sample constituted the linear precursor. To the remainder, the coupling agent was added to form the branched star polystyrene. Polymers were precipitated from solution by pouring into a large excess of methanol, and dried in vacuo for 72h at 40°C.

PHYSICAL MEASUREMENTS

Weight and number average molecular weights of linear and branched polymers prepared by the above technique have been determined by methods of light scattering and osmometry. Dilute solution viscosities have also been determined, the results of which will be described in a later paper. The techniques of gel permeation chromatography and ultracentrifugation gave a measure of the monodispersity of the samples. The latter techniques were particularly useful in identification of the species produced by the coupling reaction, and also in following progress of fractionations.

(1) Light scattering Weight average molecular weights ()Qw) were obtained from light scattering measurements performed on a Sofica Photo Gonio Diffusometer. Benzene was used as a solvent and scattering intensities were determined at nine angles, between 30 ° and 150 °. Zimm plots were constructed and molecular weights determined by extrapolation to zero angle and zero concentration.

(2) Osmometry Number average molecular weights (/On) were determined from osmotic pressure measurements performed on a Mechrolab 501 high speed membrane osmometer. The instrument was operated at a temperature of 37°C with toluene as solvent. Measurements were carried out on solutions of different concentration and molecular weights were determined from the zero concentration osmotic pressure. 398

SYNTHESISOF BRANCHED POLYSTYRENE (3) Gel permeation chromatography (g.p.c.) Molecular weight distributions were determined using a Waters Associates Gel Permeation Chromatograph Unit, Model 200. Toluene was used as solvent throughout, at a temperature of 80°C and a flow rate of 1 ml/min. Polymer samples were injected at a solution concentration of 0-25 % over a period of 2 rain. The instrument was calibrated by means of a series of narrow molecular weight distribution polystyrenes of known molecular weight and the calibration curve is shown in Figure 3. The use of narrow molecular weight distribution polystyrenes as calibration standards proved particularly useful in 10 ~

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Figure3 G.P.C. calibration curve this study, since peak molecular weights of the anionic polymers prepared could be rapidly derived from the calibration curve. (4) Ultracentrifugation Sedimentation patterns of the polymers were obtained using a Beckman Model E Ultracentrifuge. Measurements were made at - - 7 ° C using n-butyl formate as solvent. This constitutes a theta-solvent for polystyrene at 9°C ts and the low theta temperature was convenient in that solutions could be prepared at room temperature. Polymer concentrations were 0.6 % and runs were carried out at 60000 revs. per rain.

RESUL'I-S AND DISCUSSION Using the techniques described above, a series of linear polystyrenes has been prepared and the reaction between living polystyryl anions and suitable

399

W. A. J. BRYCE, G. MCGIBBON AND I. G. MELDRUM

low molecular weight coupling compounds has been investigated. The symbols L and Y are used here to describe the linear and branched polymers respectively and subscripts are used to differentiate polymers within each group. The linear polymers described here are not necessarily the precursors of corresponding branched polymers.

(1) Linear polymers In preparing the linear polymers, an interesting phenomenon was discovered which has not hitherto been reported. It was found that a 'dimerization' reaction occurred between the living polystyryl anions if termination was not effected immediately after addition of all the monomer to the catalyst solution. Figure 4a shows the molecular weight distribution as determined by g.p.c.

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2j3

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Figure 4 G.P.C. of linear polymers: (a) polymer terminated immediately after addition of all monomer; (b) polymer terminated after/7 h of a typical linear polymer which was terminated immediately after addition of all the monomer, and Figure 4b shows the chromatograph of a sample of the same living polymer, which was terminated 17h after the end of the polymerization. A high molecular weight shoulder is obvious, and from the g.p.c, calibration curve shown in Figure 3, it may be seen that the peak molecular weight of the polymer represented by this shoulder is approximately twice that of the polymer represented by the main peak. The difference between the two polymer samples is particularly well defined in the sedimentation patterns shown in Figure 5a and Figure 5b, 400

SYNTHESIS OF BRANCHED POLYSTYRENE

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A b Figure 5 Traces of schlieren patterns: (a) polymer terminated immediately after addition of monomer; (b) polymer terminated after 17 h

In view of the precautions taken to eliminate impurities it is unlikely that the high molecular weight material results from the coupling of the living polystyrene with a difunctional impurity. Das, Feld and SzwarO 9 reported that living polystyryl lithium/THF solutions became more viscous on standing. They attributed this to association. In an article, Cubbon and Margerison ~° report that later work suggested that the increase in viscosity was due to alcoholates. (The authors would like to thank the referee for this information). Fetters 21 reported that the association was due to lithium alkoxide chain-ends. Spach, Levy and Szwarc z2 gave evidence for the following reactions occurring -V~AF-CH,~-CHPh-Na + - - ÷ - V ~ - C H ~ C H P h + Nail - ~ / v - C H 2 - C H P h - N a + + -VVV~--CH~CHPh - - ~ -~A/v-CH2-CH2Ph + Na+CHPh CH-CPh--CH,~-~AA/L Neither of the above reactions would give a large increase in molecular weight. The formation of an alcoholate destroys the activity of a carbanion with only a small increase in molecular weight 2a. Since our evidence (Figure 4 and Figure 5) shows a large increase in molecular weight it is suggested that the following reactions may be responsible. After hydride abstraction: -Vvv-CH2-CHPh-Na + +-vw~CHPh-CH~CHPh - - - - ~ -~Apc-CHz-CHPh-CHPh-CH-Na ~ \\ [ ~. CHPh-~/V-~A/v-CHz-CH P h - C H - C H P h - N a + CHPh-~AA?Both reactions would double the molecular weight in one step. The production of the dichain polymer was obviously undesirable and in 401

W. A. J. BRYCE, G. McGIBBON AND I. G. MELDRUM

order to suppress its formation, linear polymers were terminated as quickly as possible after addition of all the styrene. Termination was effected by addition of either methanol or the trifunctional coupling agent. The molecular weights of the linear polymers used in solution property measurements were determined as previously described and are shown in Table 1. Molecular weight distributions determined by g.p.c, showed that the polymers listed in Table 1 did not contain any dichain polymer. Table 1 M o l e c u l a r weights d a t a for linear polymers Polymer 1L 2L 3L 4L

-Mn × 10 -5

Mw × 10 -5

-~/~/Mn

0'744 1"283 2"29 5"30

0"795 1"374 2"55 5'84

1"07 1"07 1" 11 1"10

(2) Branched polymers A series of trifunctionally branched polymers, suitable for investigation of solution properties, has been prepared, but initial experiments were carried out in order to determine the optimum conditions for obtaining the maximum yield of star polymer. In early experiments, the coupling reaction between polystyryl anions and 1,3,5-tribromomethylbenzene was investigated, and the sedimentation pattern of a typical product of the reaction is shown in Figure 6. It may be seen from

Figure 6 Traces o f schlieren patterns o f Yr. 1

this figure that the yield of trifunctional star polymer is very small, and the main products are mono- and dichain polymer. It is also obvious from the sedimentation pattern that high molecular weight, possibly tetrachain, polystyrene is a product of the reaction. Since metalation reactions are considered to be unlikely to occur 5 when potassium is used as a gegen-ion, it is probable that the 'dimerization' reaction described above produces living dichain polymer, and reaction of the latter with the coupling agent is responsible for the polymer species represented by the fourth peak in the sedimentation pattern. In the reaction between living polystyrene and 1,3,5-tribromomethyl benzene as the coupling agent. The technique was fully investigated and two conditions were found necessary for the production of maximum yields of 402

SYNTHESIS OF BRANCHED POLYSTYRENE

trifunctional star polymer: (a) The coupling agent should be added as quickly as possible after addition of all the monomer, otherwise the 'dimerization' reaction described above is competitive. (b) Sufficient coupling agent must be added to the linear polymer to terminate all chains, as evidenced by the complete disappearance of the red colour due to the polystyryl anions. If a longer time is allowed for reaction of a slight excess of polystyryl anions with the coupling agent, the yield of star polymer is decreased, rather than increased. Under these conditions a series of four polymers has been prepared in which the yield of trifunctional star polymer is high, and sedimentation patterns of these polymers are shown in Figure 7. The number average

/k

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Figure 7 Traces of schlieren patterns (a) IY, (b) 2Y, (c) 3Y, (d) 4Y molecular weights of the linear precursors are listed in Table 2, and it may be seen that the yield of star polymer is decreased as the molecular 403

W. A. J. BRYCE, G. MGclBBON AND I. G. MELDRUM

weight of the linear precursor is increased. This is undoubtedly due to increased shielding of both living ends and reactive coupling groups by the larger polymer chains. Figure 8 shows the gel permeation chromatogram of one of the products of the coupling reaction (4Y) and it may be seen that the three species indicated in the sedimentation pattern (Figure 7d), are not obvious in the chromatogram. Due to the smaller size in solution of the branched polymer,

_J Figure 8 G.P.C. of 4Y (0"5%)

complete separation of the latter and the dichain polymer has not been possible. From Figure 3, it is estimated that the low molecular weight shoulder in the chromatogram corresponds to monochain polystyrene. The presence of species other than trichain polymer required that before any investigation of the properties of the branched polymers could be made, it was necessary to isolate monodisperse samples of the trifunctional polystyrenes from the parent mixtures, by fractionation. Fractionations were carried out by precipitation from butanone solution using ethanol as non-solvent, and it was found convenient to follow the progress of the fractionations by ultracentrifugation of the centre fractions, and by gel permeation chromatography of all fractions. Although somewhat complicated by the fact that the dichain and trichain species were eluted together, the latter technique was useful in providing a rapid estimate of the efficiency of the fractionation. This is demonstrated in Figure 9 which shows a gel permeation chromatogram of a typical product (3Y) of a coupling reaction, and the chromatograms of the three fractions obtained by precipita404

SYNTHESIS OF BRANCHED POLYSTYRENE

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Ft~gure 9 G.P.C.: (a) 3Y (0"570); (b) high M.W. fraction of 3Y; (c) centre fraction of 3Y (i.e. 3YO; (d) low M.W. fraction of 3Y

405

w. A. J. BRYCE, G. McGIBBONAND I. G. MELDRUM tion from butanone solution. It is obvious that the fractionation procedure has been successful in removing high and low molecular weight material. In the case of the low molecular weight branched polymers (1Y, 2Y and 3Y) sedimentation patterns and chromatograms indicated that only one fractionation was required to yield pure trichain polymers (1Y1, 2Y1 and 3Y1) but the higher molecular weight polymer 4Y was subjected to two fractionations to yield star polymer 4Y2. The sedimentation patterns of the four truly trifunctional star polymers are shown in Figure 10 and number and weight average molecular weights of these polymers are shown in Table 2.

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Figure 10 Traces of schlieren patterns: (a) 1Y1, (b) 2Y1, (c) 3Y1 (0"3~), (d) 4Y2 (0'3~) Table 2 Molecular weight data for star polymers

Polymer

19In × 10-5

AT/w× 10-5

1Y1 2Y1 3Y1 4Y2

0'797 1-445 2'84 3.51

0'925 1.48 3'26 4.10

)U4u,/~In

1.16 1"02 1.15 1.17 406

Precursor ]ffln × 10-5

0-315 0'48 1'02 1,31

Linking ratio

2'53 3.01 2.78 2.68

SYNTHESIS OF BRANCHED POLYSTYRENE

Also shown in Table 2 are the number average molecular weights of the linear precursors of the branched polymers described, and from this, the 'linking ratio' Mn(Y)//Qn(L) has been determined. Differences between the experimental value of this parameter and the theoretical value of three were expected, due to changes in molecular weight resulting from the fractionation procedure. CONCLUSIONS

A new experimental technique for anionic polymerization has been devised and successfully used for the investigation of the coupling reaction between living polystyryl anions and 1,3,5-trichloromethyl benzene. This reaction was complicated by a dimerization reaction involving living polymer molecules, but this complication has been overcome and coupling reaction products consisted largely of trichain polystyrene. By fractionation of these products, a series of pure trichain polymers of narrow molecular weight distribution has been produced, properties of which will be reported in later publications.

Chemistry Department, University of Aberdeen, OM Aberdeen, AB9 2UE, Scotland

(Received 21 January 1970) (Revised 18 May 1970) REFERENCES

1 Swarc, M. Nature 1956, 1"/8, 1168 2 Morton, M., Helminiak, T. E., Gaudkary, S. D. and Beuche, F. J. Polym. Sci. 1962, 57, 471 30rofino, T. A. and Wenger, F. aT. Phys. Chem. 1963, 6"7, 566 4 Zerlinsky, R. P. and Wofl'ord, C. F. J. Polym. Sci. (A) 1965, 3, 93 5 Yen, S. S. Makromol. Chem. 1965, 81, 152 6 Gervasi, J. A. and Gosnell, A. B. J. Polym. Sci. (A-l) 1966, 4, 1391 7 Altares, T., Wyman, D. P., Allen, V. R. and Meyerson, K. J. Polym. Sci. (A) 1965, 3, 4131 8 Wyman, D. P., EIyash, L. J. and Frazer, W. J. J. Polym. Sci. (A) 1965, 3, 681 9 Kraus, G. and Gruver, J. T. J. Polym. Sci. (A) 1965, 3, 105 10 Invoe, S., Tsuruta, T. and Furukawa, J. MakromoL Chem. 1960, 42, 12 11 Asami, R., Levy, M. and Swarc, M. J. Chem. Soe. 1962, p 361 12 Bhattacharyya, D. N., Lee, C. L., Staid, J. and Swarc, M. J. Amer. Chem. Soe. 1963, 85, 533 13 Reed, R. A. Chem. Prods. 1960, 23, 299 14 Reppe, W., Schlichting, O. and Meister, H. Ann. 1948, 560, 93 15 Litt, M. J. Polym. Sci. 1962, 58, 429 16 Wenger, F. Makromol. Chem. 1963, 64, 151 17 Cowie, J. M. G., Worsfold, D. G. and Bywater, S. Trans. Faraday Soc. 1961, 57, 705 18 Schulz, G. V. and Baumann, H. Makromol. Chem. 1963, 60, 120 19 Das, K. S., Feld, M. and Szwarc, M. J. Amer. Chem. Soc. 1960, 82, 1506 20 Cubbon, R. C. P. and Margerison, D. Progress in Reaction Kinetics Pergamon Press, 1965, Vol 3, p 419 21 Fetters, L. J. J. Polym. Sci. (B) 1964, 2, 425 22 Spach, G., Levy, M. and Szwarc, M. J. Chem. Soc. 1962, p 355 23 Szwarc, M., 'Carbanions, living polymers and electron transfer processes', lnterscience Publishers, 1968, p 658

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