The Internal Crosslinking of Styrene Copolymers

32 The Internal Crosslinking of Styrene Copolymers R. N. HAWARD, Β. M. PARKER, and E. F. T. WHITE Downloaded by UNIV OF ARIZONA on May 30, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch032

1

Department of Polymer and Fiber Science, The University of Manchester, Institute of Science and Technology, Sackville St., Manchester M60 IQD, England

The crosslinking of styrene-hydroxyethyl methacrylate copolymers by hexamethylene diisocyanate has been studied in toluene solution at 80°C. At high copolymer concentra tions the specific viscosity of the solutions increased during reaction owing to the coupling of polymer chains. However, below a certain concentration, which depends on the co polymer composition, the viscosity fell as the reaction proceeded. At very low concentrations, the change in vis cosity became independent of concentration. As the intrinsic viscosity decreased, the Huggins constant increased rapidly and the radius of gyration decreased, indicating a marked change in the polymer-solvent interaction. These results are explained on the basis of internal cyclization of the polymer molecule during the crosslinking reaction. Toluene solutions of α,ω-dihydroxy polystyrene on reaction with hexamethylene diisocyanate showed similar behavior.

Any molecule which contains two groups which are able to combine can react either with itself (cyclization) or with another molecule (polymerization). In the general case these reactions can compete, and their relative proportions depend sharply on the concentration of the reactant. This principle was studied some time ago for small molecules by Salomon (26, 27) and Ziegler (38). Obviously, similar possibilities arise for polymers, but so far this subject has not been investigated systematically, although certain special areas have now been studied. Present address: Materials Department, Royal Aircraft Establishment, Farnborough, Hants, England.

1

489 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

Downloaded by UNIV OF ARIZONA on May 30, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch032

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One such area concerns the internal crosslinking which occurs during the preparation of certain thermosetting resins by free radical polymeriza tion. Here the work of Stockmayer (34) and Simpson and co-workers (31, 32) has laid the foundation for subsequent developments. Simpson showed that appreciable internal cyclization takes place during the poly merization of diallyl esters. Where the most favored 5- and 6-membered rings can be formed, Butler has demonstrated that the cyclization reaction can be dominant (7, 8). This field has been reviewed recently by Gibbs and Barton (12). On the other hand, there have been a few direct studies of the internal cyclization of a polymer with the formation of very large rings although Haward and Simpson (15) did provide some evidence for the occurrence of such a process in the copolymerization of styrene and divinylbenzene. However, up to the present, the knowledge we do have of internal crosslinking in polymers is largely due to the work of Kuhn, who originally calculated the probability of a cyclic configuration in a Gaussian chain (21). Similarly, Kuhn and Majer (23) calculated the expected change in intrinsic viscosity arising from intramolecular ring formation in a poly mer, and Kuhn and Balmer (22) went on to study the acetal formation reaction of polyvinyl alcohol by terephthaldehyde ( O C H · C H · C H O ) . In their experiments naturally there was a high probability of reaction between neighboring parts of the polymer chain, a situation which was taken into account in the calculation of Kuhn and Majer, but small changes in intrinsic viscosity were nevertheless observed when one molecule of aldehyde reacted with one polymer molecule. Kuhn and Balmer also showed that the observed reduction in intrinsic viscosity became constant at high dilutions. However, in view of the very limited range of these experiments, we felt there was need for further study of the intramolecular crosslinking reactions using polymers which had been prepared especially to contain a controlled number of reactive groups. For this purpose, some copolymers of styrene and hydroxyethyl methacrylate ( H E M A ) were prepared and characterized (see Appendix). These provide starting polymers with a range of hydroxyl content capable of reacting with bifunctional isocyanate to give internally crosslinked molecules. This paper describes the results of a study of these reactions. 6

4

Experimental Materials. Styrene-hydroxyethyl methacrylate copolymers were pre pared and characterized as described in the Appendix. The copolymers used in this work contained 0.7, 2.2,3.8, and 9.5 mole % H E M A . α,ω-Dihydroxy polystyrene, of molecular weight 1.2 χ 10 , was pre pared by terminating with ethylene oxide the polymer obtained by anionic 5

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

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Internal Crosslinking

polymerization of styrene and was kindly supplied by K. Riches (Shell Chemical Co., Ltd., Carrington). Hexamethylene diisocyanate was supplied as pure by W. Hopkins of this department (71), having a density of 1.045, and an average equiva lent weight of 83.7 ± 0.5 (theoretical equivalent 84.09). The weights of isocyanate used were calculated on the analysis of the particular sample. η-Butyl isocyanate (Aldrich Chemical Co.) ( n = 1.0461) was used as supplied without further purification. Toluene (reagent grade) was fractionated, and the middle fraction was stored over sodium wire. This dried toluene was degassed under high vacuum and distilled onto a sodium mirror at room temperature. All reagents were stored and handled only under anhydrous condi tions in a dry box. Glassware was dried at 175 °C. and cooled in the dry box. Standard solutions were prepared by weight, and all solvent and solutions were filtered before use.


D

20

Q20

f

O.I5

> $αιο Ο

2500

5000

Reaction Time

7500 mins

Figure 1. Variation in specific viscosity of a toluene solution of a styrenehydroxyethyl methacrylate copolymer (2.2 mole % HEMA) on reac tion with hexamethylene diisocyanate at 80°C. Polymer concentration 0.047%, (gram/dl) [NCO] : [ O H ] = 9.5 0

0

Methods. V I S C O S I T Y M E A S U R E M E N T S . Crosslinking reactions and re actions with butyl isocyanate were carried out in an Ubbelohde sus pended-level viscometer modified to enable reactions to be carried out in an enclosed atmosphere. Viscometers were immersed in an oil bath at 80°C., and flow times were measured over periods of up to 10 days. Specific viscosities, η, were then calculated. In general, runs were fol lowed until the change in viscosity with time had become very small. Flow times were measured frequently enough to allow a smooth curve to be drawn when η was plotted against reaction time; a typical curve is


492

ADDITION A N D C O N D E N S A T I O N

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given in Figure 1. Values of η at particular times were interpolated from such curves. The initial viscosity, η , could not be measured directly owing to the finite time taken for the viscometer and solution to reach the reaction temperature, and it was therefore determined by extrapo lating the specific viscosity-time curve to zero time. Since η is a function of polymer concentration, runs of different concentrations were compared 0

0

through the function —

which to a first approximation may be conVo


sidered independent of concentration. The reproducibility of the viscosity change is illustrated in Figure 2.

Ο

500 Reaction

Time

IOOO mins

Figure 2. Reproducibility of the viscosity change for replicate crosslinking runs. Styrene-hydroxyethyl methacrylate copolymer (2.2 mole % HEMA) concentration in toluene 0.09% (gram/dl.) [NCO] :[OH] = 1 0

0

In many ways it would have been desirable to have obtained a con tinuous record during the reaction of the intrinsic viscosity, [77], which can be found by extrapolating to zero concentration of the function η/c. However, no convenient experimental procedure was devised for doing this, and no extrapolation formula could be used since the Huggins con stant for the polymer changed during the reaction. With the styrene-hydroxyethyl methacrylate copolymers some depo sition of polymer on the walls of the reaction vessel was generally ob served after 7-8 days of reaction. It was difficult to estimate the amount of gel, but in no case was the amount found as much as 4% of the total polymer. Thus, the effect of polymer loss on intrinsic viscosity was small.


32.

H A W A R D

R A T E

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O F R E A C T I O N

493

Internal Crosslinking O F T H EC O P O L Y M E R S

W I T H

I S O C Y A N A T E S .

The

re

actions were carried out in a three-necked flask open to the atmosphere through a drying tube. Samples were removed at intervals. The isocyanate concentrations were determined using variable space cells in an Infracord 137 spectrophotometer by measuring the intensity of the band at 4.4 μ. The extinction coefficients were determined using samples of known con centration. Light scattering measurements were made on polymer solu tions in toluene at 2 5 ° C . using a Sofica-gonio-diffusiometer.


Results and Discussion Reactions of Styrene Hydroxyethyl Methacrylate Polymers. The effect of reaction with hexamethylene diisocyanate on the specific vis cosities of toluene solutions of the copolymers was studied over a wide range of isocyanate and polymer concentrations. The first feature noted was that the sign of the viscosity change depended on the polymer con centration (Figure 3). Although at high concentrations the viscosity increased, at low concentrations it decreased. Increased viscosity was undoubtedly caused by an increase in molecular weight, but we cannot, at this stage, be certain that the decrease was caused by intramolecular crosslinking. It has been shown that an increase in the polarity of the polymer molecule owing to an increase in the number of hydroxyl groups caused

Figure 3. Effect of polymer concentration on the viscosity changes on reaction between styrene-hy droxyethyl methacrylate copolymer (2.2 mole % HEMA) and hexamethylene diisocyanate in toluene at 80°C. [NCO] :[OH] = 1. Polymer concen trations 0.047% (gram/dl) (%), 0.134% (gram/dl.) (O), 0.85% (gram/dl.) (+) 0

0


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ADDITION A N D C O N D E N S A T I O N

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a decrease in viscosity (14). It is possible that a similar effect might be the cause of the viscosity decreases observed. Reaction of the polymer molecule with hexamethylene diisocyanate attaches a free polar isocyanate group to the chain and creates a urethane grouping — Ν — C — which H Ô might be expected to be more polar than the hydroxyl groups owing to the resonance


_N^-C—

I

_N=C—

Ik

II

Η Ο H O — which would increase electron density on the carbonyl carbon. This could cause a contraction of the molecule owing to increased attraction between polar groups. Therefore, we studied the reaction of the polymer with a monoisocyanate, butyl isocyanate, and showed that although this did cause a decrease in viscosity, it was very small compared with that pro duced by the diisocyanate under the same conditions (Figure 4). With butyl isocyanate, of course, no crosslinking could occur. The reactions of the polymer with isocyanates ( Figure 5 ) show that the two reactions do indeed take place at approximately the same rate.

0.4

Ίο α Ίο

Q

2

Ο Ο

ΙΟΟΟ Reaction Time

2000 mins

Figure 4. Comparison of viscosity changes on re action between styrene-hydroxyethyl methacrylate copolymer (2.2 mole % HEMA) and butyl iso cyanate (Φ) and hexamethylene diisocyanate (O) in toluene at 80°C. Although reaction with monoisocyanate cannot reproduce the vis cosity change caused by the attachment of an unreacted isocyanate to the polymer chain, in view of the small change caused by the formation of urethane, it is unlikely that this could cause an extra decrease in


32.

H AWARD E T A L .


495


viscosity large enough to equal that found for reaction with diisocyanate. It is therefore unlikely that viscosity decreases arose simply from changes in the polarity of the polymer molecule.

Figure 5. A comparison of decreases in isocyanate concentration during reaction between styrene-hy droxyethyl methacrylate copolymer (2.2 mole % HEMA) in toluene at 80°C. and butyl isocyanate (Φ) and hexamethylene diisocyanate (O). Polymer con centration 1.87% (grams/dl.) [NCO] :[OH] = 1 0

0

The simplest case of a polymer molecule which can undergo either cyclization or chain extension is that in which the groups are situated at each end of the polymer chain (which can react together). An example of this type of polymer is α,ω-hydroxy-terminated polystyrene which reacts with a difunctional isocyanate, and our first studies were carried out with this reaction system. Obviously, the chain extension reaction leads to an increased specific viscosity, and cyclization leads to the reverse. Since it was not possible to obtain a suitable dihydroxypolystyrene of high molecular weight, we were compelled to work with solutions of low specific viscosity. The observed changes in η on reaction with iso cyanate were therefore also small. Our measurements were limited to demonstrating the effect of dilution on this system. The rate of change of viscosity, — — Φ,

derived from the approximate viscosity-linear time

curve, was measured during the initial part of the reaction using a 1:1 ratio of isocyanate to hydroxyl. The results given in Figure 6 showed that the expected decrease in η could be observed at the lowest concentrations. For the styrene-hydroxyethyl methacrylate copolymers, the situation is more complex. To try to determine the nature of the reaction with


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ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N

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hexamethylene diisocyanate, we studied the effect of the copolymer composition and concentration in solution and of the initial ratio of isocyanate to hydroxyl [ N C O ] : [ O H ] , on the viscosity changes with time. 0

0


5D

a

0

d t

-SO Figure 6. Rate of change of specific viscosity as a function of polymer concentration for the reaction between hexamethylene diisocyanate and α,ω-dihydroxy polystyrene in toluene at 80°C. [ N C O ] : [ O H ] = 1 0

0

Figure 7 illustrates the effect of copolymer composition and of con centration, the curves obtained being at constant reaction time for runs of the same initial reactant ratio. Although different curves refer to dif ferent times and ratios, it is clear that increasing H E M A content of the polymer gave a larger viscosity change at both high and low polymer concentrations. Increasing the H E M A content also raised the polymer concentration at which the viscosity change becomes negative. At this point, the viscosity increase arising from the intermolecular reaction bal anced the decrease caused by the intramolecular crosslinking and the increase in polarity of the polymer molecule. Kuhn and Balmer (22) have shown that, as the polymer concentra tion is decreased, the change in viscosity becomes constant. At these concentrations it was assumed that no intermolecular reaction was taking place. A similar effect is demonstrated for the present system in Figures 7 and 8: for a given initial ratio of isocyanate to hydroxyl and a given reaction time, the polymer concentration had no effect on the viscosity change below a certain point. The effects of reaction time and initial ratio of reactant concentrations are illustrated in Figures 9-11. In all cases increasing the isocyanate concentration over a wide range increased the change in viscosity. Com parison of Curve 5 (copolymer 2.2 moles % H E M A ) in Figure 10 with



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Figure 7. Viscosity changes and polymer concentration for the reaction between hexamethylene diisocyanate and sty rene-hydroxyethyl methacrylate copolymers in toluene at 80°C. Copolymer, mole % HEMA m

+ Ο Χ

Reaction Time, min.

[NCO] :[OH]o 0

200 200 100 100

50 1 10 10

o.7 2.2 3.8 9.5

0.3 ο +

O.I Ο

+

0.05

ΟΙΟ

O.I5

Polymer Concentration °/o Figure 8. Viscosity changes on reaction between sty rene-hydroxyethyl methacrylate copolymer (2.2 mole % HEMA) and hexamethylene diisocyanate in toluene at 80°C. At the ratios [ N C O ] : [ 0 H ] of 1 (%), 3 (+) and 10 (O), the reaction times were 200, 400, and 1000 minutes, respectively 0

0


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ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N


α

PROCESSES

0

Ο

400

800

Reaction Time

mins

Figure 9. Effect of the initial reactant ratio on the viscosity change during reaction between styrene-hydroxyethyl metha crylate copolymer (2.2 mole % HEMA) and hexamethylene di isocyanate in toluene at 80°C. Polymer concentration 0.134% (gram/dl.) [2VCO] : [OH] = 0.5, 1.1, 4.0, 18.4, 115, and 4000 for Curves 1-6, respectively 0

0

Figure 10. Change in viscosity over long times during reaction between styrene-hydroxyethyl methacrylate co polymer and hexamethylene diisocyanate in toluene at 80°C. Curves 1-4 (copolymer 0.7 mole % HEMA, 0.134% (gram/dl.) solution) had [ N C O ] : [ O H ] = 10 , 1.5 X 10 , 10 , and 10 , respectively. Curve 5 (copolymer 2.2 mole % HEMA, 0.047% (gram/dl) solution) had [ N C O ] ; [OH] — 10. All solutions have approximately the same [OH] 0

2

s

0

2

u

0

0

0


32.

HAWARD E T A L .

499


Curves 1-4 (copolymer 0.7 mole % H E M A ) shows clearly the difference in the viscosity changes between copolymers. For all curves [ O H ] was approximately the same; for Curves 1-4 increasing [ N C O ] gave an increased viscosity change, but the change for Curve 5 was even greater than that for Curve 4 despite [ N C O ] being l/1000th as great. At short reaction times ( < 1000 min. ) the viscosity decreased smoothly with time, and an increase in the initial isocyanate concentration gave a faster change in viscosity (Figure 9). However, the effect of increasing the isocyanate concentration was small at concentrations greater than 2 X 10" M (Figure 11). 0

0

0


3

0.4

^

02

Ο Ο

200 [NCQl

0

400

Χ ΙΟ" m II 4

Figure 11. Effect of initial isocyanate concentration on the viscosity changes after 200 minutes (O) and 1000 minutes (X) during reaction be tween styrene-hydroxyethyl methacrylate copolymer (2.2 mole % HEMA) and hexamethylene diisocyanate in toluene at 80°C. Polymer concentration /M

0.46 A «

Β

6.3 Χ 10" cc./gram

Z

w

4

The increase in Β is what might be expected for a solvent of higher dielectric constant. The most significant point in these results is that there is no evidence of a decrease in molecular weight. It is, therefore, unlikely that large hydrogen-bonded aggregates are present in the solutions. The alternative and, in our view, more probable explanation of the high value of R for polystyrene lies in the nature of the molecular weight distribution of the polymers. The radius of gyration measured by light scattering is a z-average quantity and is higher for a polymer of broad g


32.

H AWARD E T A L .

511


molecular weight distribution than for a monodisperse polymer. The number average molecular weight, M was therefore determined for the copolymer containing 3.8 mole % H E M A by measuring osmotic pressure in toluene solution using an automatic membrane osmometer (Hallikainen Instruments) and was found to be 6.5 ± 0.5 Χ 10 compared with a weight average of 1.8 Χ 10 (see Table C ) . This gave a weightto-number average ratio of about 3.0 for the two copolymers measured, indicating that they have a relatively broad molecular weight distribution. n

5


6

We therefore conclude that the styrene-hydroxyethyl methacrylate copolymers used in this work have a relatively .broad molecular weight distribution and that the effect of increasing hydroxyl content on the properties of dilute toluene solution is to contract the polymer molecule and decrease the polymer-solvent interaction without causing appreciable aggregation by intermolecular hydrogen bonding.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

Alfrey, T., Goldberg, A. I., Price, J. Α., J. Colloid Sci. 5, 251 (1950). Baker, J. W., Holdsworth, J. B.,J.Chem. Soc. 1947, 713. Baker, J. W., Gaunt, J., J. Chem. Soc. 1949, 9, 19, 27. Baker, J. W., Davies, M. M., Gaunt, J., J. Chem. Soc. 1949, 24. Baker, W. O., Mason, W. P., Heiss, J. H.,J.Polymer Sci. 8, 129 (1952). Breitenbach, J. W., Cabler, H., Makromol. Chem. 37, 53 (1960). Butler, G. B., Ingley, F. L., J. Am. Chem. Soc. 73, 894 (1951). Butler, G. B., Angelo, R. J., J. Am. Chem. Soc. 79, 3128 (1957). Cragg, L. H., Manson, J. Α.,J.Polymer Sci. 9, 205 (1952). Fineman, M., Ross, S. D.,J.Polymer Sci. 5, 259 (1950). Flory, P. J., Fox, T. G., J. Am. Chem. Soc. 73, 1904 (1951). Gibbs, W. E., Barton, J. M., "Kinetics and Mechanisms of Polymeriza tion," Vol. 1, "Vinyl Polymerization," G. E . Ham, Ed., p. 59, Arnold, London, 1967. (13) Greenshields, J. N., Peters, R. H., Stepto, R. F. T., J. Chem. Soc. 1954, 5101. (14) Haward, R. N., Parker, Β. M., White, E. F. T., Appendix to this paper. (15) Haward, R. N., Simpson, W., J. Polymer Sci. 18, 440 (1955). (16) Heller, W., J. Colloid Sci. 9, 547 (1954). (17) Hopkins, W., Ph.D. Thesis, Manchester, 1967. (18) Huggins, M. L., J. Am. Chem. Soc. 64, 2716 (1942). (19) Krigbaum, W. R., Flory, P. J.,J.Am. Chem. Soc. 75, 1775, 5254 (1953). (20) Krigbaum, W. R., Carpenter, D. K.,J.Phys. Chem. 59, 1166 (1955). (21) Kuhn, W., Kolloid-Zeit. 68, 2 (1934). (22) Kuhn, W., Balmer, G., J. Polymer Sci. 57, 311 (1962). (23) Kuhn, W., Majer, H., Makromol. Chem. 18, 239 (1955). (24) Nottley, N. T., Debye, P.,J.Polymer Sci. 17, 99 (1955). (25) Rohm and Hass Co., Special Products Department, Bull. SP-261. (26) Ruziska, L., Salomon, G., Meyer, Κ. E., Helv. Chim. Acta 17, 882 (1934). (27) Salomon, G., Trans. Faraday Soc. 32, 153 (1936). (28) Sato, M., J. Am. Chem. Soc. 82, 3893 (1960). (29) Schulz, G. V., Blaschkle,J.Prakt. Chem. 158, 130 (1941). (30) Sigwalt, P., Bull. Soc. Chim. France 1959, 54.


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(31) Simpson, W., Holt, T., Zetie, R. J., J. Polymer Sci. 10, 489 (1953). (32) Simpson, W., Holt, T.,Proc.Roy. Soc. A238, 154 (1956). (33) Sorenson, W. R., Campbell, T. W., "Preparative Methods of Polymer Chemistry," p. 134, Interscience, New York, 1961. (34) Stockmayer, W. H., Weil, L. L., "Advancing Fronts in Chemistry," B. S. Twiss, Ed., Rheinhold, New York, 1945. (35) Tanford, C., "Physical Chemistry of Macromolecules," p. 210, Wiley, New York, 1961. (36) Trementozzi, Q. Α., Steiner, R. F., Doty, P., J. Am. Chem. Soc. 74, 2070 (1952). (37) Williamson, G. R., Carrington Plastics Laboratory, private communica tion. (38) Ziegler, K., Ber. A 67, 139 (1934). RECEIVED April 22, 1968.


The Internal Crosslinking of Styrene Copolymers

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