Functional Polymers - American Chemical Society

Downloaded by PENNSYLVANIA STATE UNIV on April 21, 2013 | http://pubs.acs.org Publication Date: May 8, 1998 | doi: 10.1021/bk-1998-0704.ch013

Chapter 13

A Novel Reactive Functionalization of Polyolefin Elastomers: Direct Functionalization of Poly(isobutylene-co-p-methylstyrene) by a FriedelCrafts Acylation Reaction Abhimanyu O. Patil Corporate Research Laboratory, Exxon Research and Engineering Company, Route 22 East, Clinton Township, Annandale, NJ 08801

Copolymers of isobutylene and p-methylstyrene [poly (IB-PMS)] represent a new type of elastomer with a saturated hydrocarbon backbone. This polymer has many desirable properties such as good environmental and aging resistance. However, these polymers have low reactivity and are incompatible with most other polymeric materials. We have developed a new method of directly modifying poly (IB-PMS) copolymer by the electrophilic substitution reaction of aromaticringsof the copolymer using Friedel-Crafts acylation chemistry. This paper deals primarily with synthesis and characterization of functionalized poly (IB-PMS) copolymers using succinic anhydride as an acylating agent and a Lewis acid, aluminum chloride, as a catalyst. This ring functionalization approach enables selective functionalization of the aromatic ring of p-methylstyrene in the copolymer. This is a general approach that can be extended to other catalysts or acylating agents to generate different functionalized polymers.

Butyl rubber consists mostly of isobutylene (95-98%) and about 2-5% isoprene units. The isoprene unit is halogenated by either chlorine or bromine to obtain the corresponding halobutyl rubbers. Despite the superior elastomeric properties of halobutyl, the elastomer can easily undergo dehydrohalogenation leading to crosslinking, and the isoprene unsaturation is subject to ozone cracking. To remedy these problems and to improve the halobutyl properties, a new class of elastomer poly(isobutylene-co-p-methylstyrene) [poly (IB-PMS)] was developed. Unlike butyl rubber, it contains no double bonds and therefore cannot be crosslinked unless otherwise functionalized. The chemical structures of butyl rubber and poly (IB-PMS) copolymers are shown below. 1

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©1998 American Chemical Society In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

185

CH I C H — CI CH3

CH I CH — C = C H — CH 3

3

2

2

X

(1-2 mole

Butyl Rubber

CH I I - C H — CI CH


3

2

C H , — CH-

3

Poly (JB-PMS) Copolymer Poly (IB-PMS) is a copolymer of isobutylene and p-methylstyrene with a saturated hydrocarbon backbone. The pairing of these two monomers produces an ideal copolymer that can readily be prepared by commercial low-temperature carbocationic polymerization in the presence of a Lewis acid catalyst The copolymer consists mostly of poly(isobutylene) backbone [isobutylene (IB): 97.7 mole %] and a very small number of p-methylstyrene (PMS) units (2.3 mole %). The introduction of functional groups to a few p-methylstyrene units can broaden the applicability of this material while preserving the desired mechanical properties of the initial material. The copolymer is brominated selectively at the p-methyl position of the aromatic unit, yielding a product free of unsaturation but stable to dehydrohalogenation. * The brominated poly (IB-PMS) can be converted to other desirable polar functional groups. Several derivatives of poly (IB-PMS) copolymers have been developed over the years. The nucleophilic displacement of benzylic bromide allows synthesis of new derivatives that include cationic and anionic ionomers, hydroxyl and carboxylic derivatives, and esters such as dithiocarbamates, acetate, acrylate and cinnamate. Similarly, poly (IB-PMS) copolymer can also be functionalized through metallation chemistry using a superbase and oxidation chemistry. Both of these methods allow functionalization at the methyl group of p-methylstyrene units. Friedel-Crafts acylation is a well-known reaction. For example, one can react benzene with succinic anhydride using a Lewis acid, aluminum chloride, to synthesize p-ketocarboxylic acid. Chemical modification of polystyrene-based resins has also been studied, because the aromatic rings can be readily modified through electrophilic aromatic substitution or other reactions. Although these reactions have been widely used in the preparation of crosslinked ion-exchange resins or polymer-supported catalysts and reagents, they need to be carefully considered if they are to be used for linear polymers where a well-defined structure is needed. In such derivatization reactions, side reactions, crosslinking, and/or degradation of the polymer chains are usually difficult to avoid. Our challenge is to introduce functional groups selectively and efficiently on a linear, high-molecular-weight polymer that contains very low levels of reactive PMS units. 3

3

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5

6

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In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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186 This paper deals with a novel approach of functionalizing poly (IB-PMS). copolymers using the Friedel-Crafts acylation reaction. Poly (IB-PMS) copolymer was reacted with an acylating agent, succinic anhydride, using aluminum chloride as a Lewis acid. The reaction involves the aluminum chloride-catalyzed substitution of an acyl group on the aromatic ring.


Experimental Section Dr. H. C. Wang of Exxon Chemical Company provided the copolymers of poly(isobutylene-p-methylstyrene), [poly (IB-PMS)] designated XP-50. These copolymers had been prepared by carbocationic polymerization of isobutylene (IB) and p-methylstyrene (PMS) at various ratios to give a saturated copolymer backbone chain with randomly distributed pendant PMS units. The synthesis of the copolymer has been reported. The composition and molecular weight of the copolymer can be varied depending on the reaction conditions. Usually the composition is maintained at 2-5 mole % PMS, with a number average molecular weight ranging from 5,000 to a half a million. The copolymer of p-methylstyrene and isobutylene utilized for this derivatization and characterization were contained 15.0 wt % p-methylstyrene. It had a number average molecular weight (Mn ) of 149,600, a weight average molecular weight (Mw) of 377,500 with polydispersity of 2.52. Reagent grade chemicals were used as received unless otherwise stated. In the experiment, 5.00 g (0.00635 mole PMS) of copolymer was dissolved in 50 ml dichloromethane (Aldrich, anhydrous 99.8%) in a three-neck flask equipped with condenser, nitrogen bubbler, and a dropping funnel. Then 0.64 g (1 mole equivalent with respect to PMS) of succinic anhydride (Aldrich) was added, and the solution was stirred for 1 hour at room temperature under nitrogen. 1.70 g (2 mole equivalent with respect to PMS) of AICI3 (Aldrich) was then added and the mixture was stirred at room temperature. The colorless viscous solution turned into a redbrown gel in 5 minutes. The product polymer was isolated by adding the reaction mixture to a acetone, to precipitate the product. The functionalized polymer was purified by dissolving in THF/methanol mixture and reprecipitating with excess acetone. The product was dried under vacuum. Further characterization of the product required that it be soluble in solvents like THF and chloroform. Therefore, the acid-functionalized polymer was converted to an ester-functionalized polymer using ethanol/sulfuric acid. The general procedure is given below. The acid functionalized copolymer (1.00 g) was dissolved in 25 ml of toluene in a three neck, 100ml flask equipped with condenser, nitrogen bubbler, and a dropping funnel. 25 ml of ethyl alcohol was added, followed by 1 ml of concentrated sulfuric acid. The solution was heated in an oil bath at 85°C for six hours under nitrogen. The product was isolated by adding the reaction mixture to 200 ml of water. The product was washed with water three times, then with acetone, then was dried under vacuum. The ester-functionalized polymer was completely soluble in solvents such as THF, chloroform, and toluene. 4

Results and Discussion The functionalization reaction of the poly (IB-PMS) copolymer involves the aluminum chloride catalyzed substitution of an acyl group on the aromatic ring. Unlike alkylation reactions, acylation reactions tend to give monosubstituted products. The reaction scheme for acid functionalization of the copolymer is shown below.


187


CH

CH

3

3

The infrared spectrum of the product was similar to that for the starting polymer with additional peaks due to carboxylic acid and ketone carbonyl functional groups. Figure 1 shows the infrared spectra of the starting poly (IB-PMS) copolymer (bottom spectrum), the p-ketocarboxylic acid-functionalized poly (IB-PMS) copolymer (middle spectrum). The infrared spectrum of the product showed a broad peak at 3500-3100 cm" due to a carboxylic acid group and two characteristic carbonyl peaks, a sharp peak at 1690 cm" and a broad peak jat 1610 cm" . The Pketocarboxylic acid functionalized poly (IB-PMS) polymer (5 wt % functionalized polymer) was insoluble in THF but was soluble in THF/methanol (95:5) solution. This solubility behavior was similar to that of mildly associated polymers. To determine whether this association is due to carboxylic acids or whether it also involves the aluminum salt of carboxylic acid, the acid-functionalized polymer was evaluated using X-ray photoelectron spectroscopy (XPS). Figure 2 shows that there is no metal aluminum. Thus the association is most likely due to dimerization of carboxylic acids, as is commonly seen in smaller organic molecules. The XPS spectrum also shows the incorporation of oxygen functionality into the copolymer by the acylation reaction. The reaction scheme is as follows: 1

1

CH

1

CH

3

3

Figure 1 shows the infrared spectra of the p-ketocarboxylic acid-functionalized poly (IB-PMS) copolymer (middle spectrum) and the ester-functionalized poly (IBPMS) copolymer (top spectrum). The infrared spectrum of the ester product shows the disappearance of a broad peak at 3500-3100 cm" due to the carboxylic acid group and the appearance of new characteristic ester peaks at 1738 cm" and 1718 cm' , in addition to the carbonyl peak at 1610 cm" (Figure 1). This suggests complete conversion of the acid-functionalized polymer to the ester-functionalized polymer. Figure 3 shows the C NMR spectra of the starting poly (IB-PMS) copolymer (bottom spectrum) and the ester-functionalized poly (IB-PMS) copolymer (top spectrum). The NMR spectrum of the product resembled that of the starting polymer, with additional peaks in the carbonyl region and a peak at 14 ppm due to a methyl group. Figure 4 shows the NMR spectra of expanded aromatic and carbonyl regions of the starting poly (IB-PMS) copolymer (bottom spectrum) and the 1

1

1

1 3


1

188


2

Wavenumbers Figure 1. a) FTIR spectrum of starting poly (IB-PMS) copolymer (bottom spectrum), b) FTIR spectrum of acid functionalized poly (IB-PMS) copolymer (middle spectrum), c) FTIR spectrum of ester functionalized poly (IB-PMS) copolymer (top spectrum). CO

J i ni i

0

n it III n T M f

100

200

1i I i

300

iii

I

l

400

500

Binding Energy / ev

..

J

600

Figure 2. XPS survey scan of acid functionalized poly (IB-PMS) copolymer.


l

700


189 ester functionalized poly (IB-PMS) copolymer (top spectrum). The product ester shows carbonyl peaks at 173 ppm and 202 ppm due to ester and ketone carbonyl, respectively. The changes due to functionalization in the aromatic region are also clearly seen in the product. Integration of the carbonyl region and the aromatic region can be used to calculate the proportion of the aromatic rings that were functionalized with ester groups. In this reaction, approximately one third number of the rings have been derivatized with ester croups. Figure 5 shows the NMR spectra of the expanded aliphatic methyl region of the starting poly (IB-PMS) copolymer (bottom spectrum) and the esterfunctionalized poly (IB-PMS) copolymer (top spectrum). The starting poly (IB-PMS) copolymer shows only one methyl absorption peak at 21.2 ppm, while the product ester-functionalized poly (IB-PMS) copolymer shows methyl peaks at 21.2 and 20.9 ppm as well as 14.4 ppm; due to the two kinds of methyl groups. The integration of the 20.9 ppm absorption peak is the same as 14.4 ppm absorption peak, suggesting that both of these peaks are due to methyl groups on the substituted aromatic rings. The peak at 21.2 ppm could be due to a methyl group on the unsubstituted aromatic ring of the functionalized polymer. Thus, relative integration of just the methyl peaks at 21.2 and 20.9 ppm can be used to calculate the extent of ester functionalized aromatic rings. In this reaction, approximately one third of the rings have been substituted with ester groups; this agrees with the results from integration of the aromatic and carbonyl peaks. These spectroscopic data are useful in characterizing derivatization of the copolymer, but they do not give information about the effect of molecular weight change of the copolymer as a result of functionalization. Molecular weights and molecular weight distributions were measured by gel permeation chromatography (GPC) relative to polystyrene standards. Figure 6 shows GPC analysis in THF solution of the starting poly (IB-PMS) copolymer and the ester-functionalized poly (IB-PMS) copolymer using both refractive index (RI) and ultraviolet (UV) detectors. The GPCs indicates a molecular weight breakdown of the ester-functionalized copolymer (RI detector) . The GPCs also indicates a uniform distribution of pmethylstyrene over the entire molecular weight range in both the starting poly (IBPMS) copolymer and the ester-functionalized poly (IB-PMS) copolymer (RI and UV detector). The intensity of the GPC peak of the ester functionalized poly (IB-PMS) copolymer is much higher than that of the starting poly (IB-PMS) copolymer. To understand this result, the UV-visible spectra of the starting poly (IB-PMS) copolymer and the ester-functionalized poly (IB-PMS) copolymer were recorded in hexane solutions (Figure 7). The UV-visible spectra of the product had almost 20-fold larger absorption at wavelength 254 nm (UV detector position) than the starting polymer (Figure 7). This explains the greater intensity of the GPC peak of the ester functionalized poly (IB-PMS) copolymer compared to that of the starting poly (IBPMS) copolymer. Summary We have shown that we can introduce functional groups selectively and efficiently on a linear, high-molecular-weight poly (IB-PMS) copolymer containing a very low level of reactive aromatic groups of PMS monomer. The functionalization occurs by an electrophilic substitution reaction of aromatic rings of the copolymer using FriedelCrafts acylation chemistry. In the copolymer, the functionalized p-methylstyrene unit was quite uniformly distributed over the entire molecular weight range. This ring functionalization approach enables selective functionalization of the aromaticringof pmethylstyrene in the copolymer and is a general approach that can be extended to other catalysts or acylating agents to generate different functionalized polymers.




s


1

200

' 1' ' ' ' 1 ' ' ' I ' ' ' ' I ' '

220

1

180

'I

140

120

100

80

60

r~t •

Figure 3. C NMR (CDCI3) spectra of starting poly (IB-PMS) copolymer (bottom spectrum) and ester functionalized poly (IB-PMS) copolymer (top spectrum).

1 3

160

I I I I i I I I I i I I I I ! i • • • » i I i I I I i i i i i i I i ! -R-T-T-T-i 1 , • , - p n - r r p - r r-r-


40


192



1 3

Figure 4. C NMR (CDCI3) spectra of starting poly (IB-PMS) copolymer (bottom spectrum) and ester functionalized poly (IB-PMS) copolymer (top spectrum) showing aromatic region of the spectra.



194



24

24

22

1 3

20 19 18

3

17

16

15

14

13

Figure 5. C NMR (CDCI3) spectra of starting poly (IB-PMS) copolymer (bottom spectrum) and ester functionalized poly (IB-PMS) copolymer (top spectrum) showing only methyl carbons.

21

CH


12

11

ppm


22

25

28

31

34

37

Elutlon Time (min) Figure 6. GPC chromatograms of the starting poly (IB-PMS) copolymer and ester functionalized poly (IB-PMS) copolymer using both RI and UV detectors.



197

200

250

300 350 400 Wavelength Figure 7. UV-visible absorption spectra of starting poly (IB-PMS) copolymer (bottom spectrum) and ester functionalized poly (IB-PMS) copolymer (top spectrum) in hexane solvent


198 Acknowledgment I would like to thank Greg Springstun for experimental help and Dr. Hsien Wang of Baytown Polymer Center for poly (IB-PMS) samples. I would also like to thank Debbie Sysyn for NMR spectra. Literature Cited 1.


2. 3. 4. 5.

6. 7. 8.

9.

Kresge, E.; Wang, H. C. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Vol. 8. Wiley, New York, 1993, p.934.] Wang, H. C.; Powers, K. W. paper no. 85, presented at a meeting of the Rubber Division, American Chemical Society, Toronto, Canada, May 21-24, 1991. Wang, H. C.; Powers, K. W. Elastomerics, Jan. & Feb., 1992. Powers, K. W.; Wang, H. C.; Chung, T. C.; Dias, A. J.; Olkusz, J. A. U.S. Patent, 5,162,445, 1992. Merrill, N. A.; Powers, K. W.; Wang, H. C. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1992, 32, 968. Haque, S. A.; Powers, K. W.; Wang, H. C.; Frechet, J. M. J.; Steinke, J. H. G. Polym. Mater. Sci. Eng. 1997, 76, 87. Arjunan, P.; Wang, H. C.; Olkusz, J. A. Polym. Mater. Sci. Eng. 1997, 76, 310. Olah, G. A. in Friedel-Crafts and Related Reactions, Vols. I-IV, Wiley, New York, 1963-1964, Olah, G. A. in Friedel-Crafts Chemistry, Wiley, New York, 1973. Grummitt, O.; Becker, E. I.; Miesse, C. in "Organic Synthesis" Collective Volume 3, p. 109. Frechet, J. M. J.; Darling, G. D.; Itsuno, S.; Lu, P. Z.; Meftahi, M. V.; Rolls, W. A. Pure Appl. Chem. 1988, 60, 353. Hodge, P.; Sherrington, D. C. Eds., Polymer-Supported Reactions in Organic Synthesis, Wiley, New York, 1980. Kresge, E. N.; Schatz, R. H.; Wang, H. C. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, vol. 8, p 423, 1987.


Functional Polymers - American Chemical Society

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