New Options via Chemical Modifications of Polyolefins: Part 1

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 18, 2013 | http://pubs.acs.org Publication Date: May 8, 1998 | doi: 10.1021/bk-1998-0704.ch014

Chapter 14

New Options via Chemical Modifications of Polyolefins: Part 1. Synthesis and Properties of Novel Phosphonium Ionomers From Poly(Isobutylene-co-4Bromomethylstyrene) 1

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P. Arjunan , H-C. Wang , and J. A. Olkusz 1

Exxon Chemical Company, 5200 Bayway Drive, Baytown, TX 77520 Exxon Chemical Company, 1900 E. Linden Avenue, Linden, NJ 07036

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A new ionomer, - poly[Isobutylene-co-(4-methylstyrenyl, triphenyl phosphonium tetraphenyl borate)] was synthesized by reacting the Exxpro elastomer, - poly[Isobutylene-co-(4-bromomethylstyrene)] with triphenyl phosphine and sodium tetraphenyl borate in tetrahydrofuran as solvent. This phosphonium ionomer had interesting mechanical properties that were in the range of typical thermoplastic elastomers : tensile strength = 1200 to 1600 psi and elongation at break = 275 652%. The thermomechanical behavior of these ionomers indicated the presence of strong ionic interactions which resulted in higher glass transition temperature and retention of modulus up to 70° C. A ten fold increase in melt viscosity of these ionomers with reference to the starting material indicated the presence of their strong ionic associations. The mechanical, thermal, and rheological properties of the above ionomers were controlled by their molecular structure characteristics such as the molecular weight, ion content, and type of the counter ion. These novel phosphonium ionomers could be useful as impact modifiers, tire-tread components, adhesives, polymer-bound catalysts, biocides, and flame retardants. Chemical modification of polyolefins is a broad and rapidly growing field of science. Such modification, often times, is done to introduce either subtle or gross changes that enhance the attributes of the original polymer. For example, introduction of ionic interactions in polymers provides a means of controlling polymer structure and properties. As would be expected, ion-containing polymers, otherwise known as ionomers", display properties which are dramatically differentfromthose of the parent polymer. Therefore, a broad spectrum of material properties may be created by varying the ion content, type of counter ion, and extent of neutralization. The Exxon Chemical Company, USA has developed a new elastomer, Exxpro via chemical modification of its precursor, XP50 - a copolymer of isobutylene and 4©1998 American Chemical Society In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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methylstyrene. Using free radical bromination, the 4-methylstryene moiety in the above precursor was selectively brominated to form 4-bromomethylstyrene derivative. This highly reactive and versatile benzyl bromide moiety was exploited as a precursor for many types of functional groups as well as for the synthesis of graft copolymers.(7) This paper describes our work on a novel ionomer, - poly [Isobutylene-co-(4-methyl styrenyl, triphenyl phosphonium tetraphenyl borate)] that was derived by a simple chemical modification of its precursor - the Exxpro elastomer.(2) Background Although much of early work on ionomers had focused on non-elastomeric materials, attention has recently been shifted to elastomeric ionomers as potential thermoplastic elastomers(TPE), i.e. elastomers which flow at high temperatures yet retain their network structure at ambient temperatures. For a material to function as a useful elastomer, the polymer chains must be interconnected in a three-dimensional network. Classically, such crosslinked elastomers cannot flow readily. However, if an elastomer is physically crosslinked via strong ionic bonds, this may lead to a potential TPE. The ionic bonds form physical crosslinks between the polymer chains and thus promote good elastomeric character, yet at higher temperatures they become sufficiently labile to allow the material to flow and be processed as a TPE. The pioneering work on elastomeric ionomers was done by Brown who first neutralized carboxylated elastomers with metal oxides resulting in ionically crosslinked elastomers - the first ionomers.(J) Using polystyrene, Lundberg and Makowski(^) also demonstrated that sulfonated polymers exhibited much stronger ionic association than the corresponding carboxylate material. Exxon patents in the mid-WO'sCJ-T) on sulfonated EPDM materials revitalized the efforts to probe the potential of elastomeric ionomers. Elastomeric ionomers based on polypentenamers were studied by MacKnight and coworkers.^) Another class of elastomeric ionomers consisted of the segmented polyurethanes.(P) There is, however, limited information on ionomers containing quaternary phosphonium salts. Poly(tributylvinylphosphonium bromide) was reported to be an effective flame retardant for thermoplastic products.(70) The salts of monomelic phosphonium cations with polyacrylate anions were reported to be useful for preparing nonflammable and heat resistant polymers.(77) Synthesis of water-soluble polyelectrolytes by quaternizing copolymers containing chloromethyl styrene with a phosphine were also reported.(72,73) Polymers containing phosphonium salts were developed for applications such as liquid toner for electrostatic images(74), dry electrographic developers(75), polymer-bound catalysts(7t>), and ion-exchange resins(77). However, work on elastomeric ionomers containing quaternary phosphonium salts is nonexistent to date. Experimental Materials. The commercial grade Exxpro elastomer, BrXP50 (Exxon Chemical Co., Baytown, TX), triphenyl phosphine(Aldrich F. Wt. 262.29, M.P.: 7981° C), sodium tetraphenyl borate(Aldrich, 99.5+ %), tetrahydrofiiran(Baker, reagent grade), methanol and isopropanol(Baker, reagent grade) were purchased and were used as such.

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

201


Methods. *H NMR spectra were obtained using a Varian XL 400 NMR spectrometer; the FTIR spectra were recorded using a Mattson Sirius 100 spectrometer; the Theological data were collected using Rheometrics System IV instrument; the DMTA data were obtained using a Polymer Labs instrument; DSC data (Tg and TGA) were gathered by using a Perkin Elmer instrument; the mechanical properties (tensile strength, % elongation and modulus) were measured using a Monsanto Tensometer 10; the Shore Hardness (ASTM D2240) measurements were made using a Durometer Shore instrument. Synthesis of PoIy[Isobutylene-co-(4-Methylstyrenyl, Tetraphenyl Borate)]

Triphenyl Phosphonium

General Procedure. A 1 liter, 3-neck, jacketed resin kettle (equipped with a thermometer, a mechanical stirrer and a condenser with N2 bubbler outlet) was charged with the Exxpro elastomer and the solvent tetrahydrofuran. The above mixture was stirred under N2 and was heated to 55° C(Lauder water circulating heating bath) to dissolve the polymer completely. The triphenyl phosphine was added with continuous stirring and the reaction was continued under the same conditions for another 24 hr, after which a solution of sodium tetraphenyl borate in tetrahydrofuran was added slowly. The reaction mixture was then kept under the same conditions for an additional 24 hr and was allowed to cool to ambient temperature. The mixture was poured into 750 ml IEC polypropylene screw cap centrifuge bottles and was centrifuged in an IEC Centra 8 model centrifuge equipped with an IEC 216 four place rotor for 7 minutes at 3400 rpm. The clear solution was decanted off and was coagulated with two volumes of 1:1 mixture of methanol and isopropanol. The coagulant was again kneaded into a fresh two volumes portion of the above alcohol mixture, was filtered off (150 mesh stainless steel screen), and was dried under vacuum (-32"Hg) at 80° C for 48 hr. The stoichiometric amounts of reactants were dependent upon the bromine content of the starting material, Exxpro elastomer, and typical examples (based on 50 g Exxpro elastomer) are listed below.

Exxpro elastomer

P93

NaBq>4

THF

Mn = 60K (BrPMS = 1.3 mol % Br= 10.5 mmol)

2.75g (10.5 mmol)

3.59 g (10.5 mmol)

500 ml

Mn=100K (BrPMS = 1.3 mol % Br = 10.5 mmol)

2.75 g (10.5 mmol)

3.59 g (10.5 mmol)

500 ml

Mn = 280K (BrPMS = 3.5 mol % Br = 28 mmol)

7.34 g (28 mmol)

9.58g (28 mmol)

750 ml

Mn = 280K (BrPMS = 3.5 mol % Br = 28 mmol)

3.67 g (14 mmol)

4.79 g (14 mmol)

750 ml


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Results and Discussion Synthesis. Polymers containing the vinyl-benzyl phosphonium unit can be made by either direct synthesis of the monomers and subsequent polymerization or phosphonation of chloromethyl styrene containing polymers. The latter is preferred due to the difficulty of obtaining the vinyl benzyl phosphonium monomer in good yield and purity without spontaneous polymerization. In either case, synthesis of elastomers containing quaternary phosphonium salts have not been reported to date. The conversion of the Exxpro elastomer into an ionomer, - the quaternary phosphonium salt (tetraphenyl borate) was accomplished by using a one-pot reaction that involved a two-step reaction sequence(Scheme 1). Tetrahydrofuran was the preferred solvent to carry out the above sequence of reactions in the same pot without isolating the intermediate. The above synthesis involved a nucleophilic displacement of 'Br"fromthe benzylic carbon of BrPMS by triphenylphosphine. Analogous reactions involving simple organic compounds such as benzyl bromide or other active halogenated hydrocarbons were well known and served as precursors for the so called Wittig reaction.(7#)

Scheme 1

CH -PPh 2

3

BPIu + NaBr J White Powder


203 Characterization. The conversion of Exxpro elastomer to its phosphonium tetraphenyl borate ionomer was followed by analyzing the HNMR and FTIR spectral data. The benzylic protons in Expro elastomer had an unique signal at 4.47 ppm (Figurel). Formation of phosphonium ionomer from Exxpro elastomer involved the nucleophilic displacement of 'Br" by P93 and created phosphonium ion, vicinal to the benzylic protons. Due to the proximity of the phosphonium ion, the signal for benzylic protons was shifted downfield from that of the starting material, Exxpro elastomer. The benzylic protons of the ionomer were found to have a doublet signal at 5.35 ppm (Figure 2). This doublet was due to the geminal proton (CH2) coupling, J . = 15 Hz. The ionomer formation was almost quantitative as there was no signal at 4.47 ppm in *H NMR spectrum of the ionomer. 1


2

H

H

e

CH2-PP113

CH Br 2

XP50

BrXP50

2-3 ppm (s)

Ionomer

5.35 ppm (d)

4.47 ppm (s) l

l

U- H = 15 H Z

Analysis of FTIR spectra of the ionomer(Figures 3 and 4) was also informative about the structure. It was reported previously that IR absorption bands at 1893 cm" and 1905 cm" were characteristics of 4-methylstyrene (of XP50) and 4-bromomethyl styrene (of Exxpro elastomer, Figure 5). Such absorption bands were not found in the FTIR spectra of the above ionomers. However, new absorption peaks, characteristics of the quaternary phosphonium salts, at 1580, 1100, 100, & 670-730 cm" were observed. The presence of both phosphorous and boron in the above ionomers was also confirmed in terms of the elemental analysis data, collected from the inductively coupled plasma/atomic emission spectroscopy(ICP/AES) technique. These results were in good agreement with the expected convertion of the respective phosphonium salts. 1

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Properties Mechanical Behavior. The ionomers, - poly[Isobutylene-co-(4-methyl styrenyl, triphenyl phosphonium bromide or tetraphenyl borate)] were found to be different in physical appearance(hard and strong) and tougher than the starting material, the Exxpro elastomer. The mechanical properties of these quaternary phosphonium



204

'I

J 1

V I

1

7.5

1

1

7.0

6.5

6.0

Figure 1.

5.5

5.0

4.5 PPM

NMR Data, Exxpro Elastomer.

0 O C H — P0 B0, 2

3

^ 5.35 ppm

1

I

8.0

1

1

7.5

7.0

1

—l— 6.5

6.0

5.5

5.0

J

4.5 PPM

Figure 2. H NMR Data, Polypsobutylene-co-(4-methylstyrenyl, triphenyl phosphonium tetraphenyl borate)]


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1.60

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800

600

Wavenumber

Figure 3. FTIR Data, Exxpro Elastomer.

C-Br

—1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

I

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

Wavenumber

Figure 4. FTIR Data, Poly[Isobutylene-co-(4-methylstyrenyl, triphenyl phosphonium tetraphenyl borate)]



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ionomers and some other commercial TPEs(fully compounded materials that had typically 20-25% rubber) are listed in Table I. Incorporation of the phosphonium salts in the 'Exxpro elastomer" backbone improved significantly the tensile properties, i.e. tensile strength has increased to almost 10 fold; also the tensile properties(tensile strength, % elongation at break, modulus) of these ionomers were in the range of typical ionic elastomer compounds.(iP)


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Thermomechanical Behavior. The starting material, Exxpro elastomer had a Tg at - 62° C(DMTA) andflowedreadily above its Tg with no signs of significant intermolecular attraction. However, incorporation of the phosphonium groups in the 'Expro elastomer" backbone brought out two significant changes in the thermomechanical behavior: 1) The Tg of the Exxpro elastomer was shifted to higher temperatures, = -37 to -27 °C, 2). A strong rubbery plateau was observed(Figure 6) which persisted up to about 70° C. These observations implied that strong ionic associations were present in the phosphonium salts of Expro elastomer. Also, it seemed that an optimum concentration of phosphonium ions was needed to form the ionic clusters which led to the strong rubbery plateau. The ionomers from low molecular weight Expro elastomer(Mn = 60 and, 100 K) did not exhibit strong rubbery plateau in the DMT A study. As expected, the rubbery plateau region for the high molecular weight (Mn = 280 K) phosphonium ionomer was extended to relatively high temperature, 70° C. However, the ionic associations were not permanent and their strength was dependent upon temperature. There were two opposing forces acting on the ionic crosslinks. First, the forces arising from the entropically driven molecular motions pull the chains away from the ionic aggregates. The opposing force was to coulombic force of attraction between oppositely charged ions. These forces lead the ionic crosslinking of different molecules unless intramolecular cyclization occurs. However, the coulombic forces of attraction were a weak function of temperature and decreased in strength with increasing temperature. Thus, above a certain temperature these ionic crosslinks could be sufficiently weakened to allow the polymer to flow readily. Rheological Properties. Incorporation of ions into a polymer invariably causes an increase in the melt viscosity of the polymer. This is mainly due to the strong association between the ionic segments (which lead to the so called 'bluster" formation in some cases!) which can best be described as time - dependent crosslinks. Such crosslinks are regarded as extra entanglements or as decreasing the segmental mobility which can be described byfreevolume considerations. Several authors who studied the rheological properties of ionomers reported a typical but unusual increase in the melt viscosity of ionomers. Canter(5) reported higher viscosity of sulfonated butyl rubber, i.e. almost three times that of the parent rubber at 90% neutralization. A significant difference between the rheological behavior of sulfonated and carboxylated polystyrenes was observed by Lundberg and coworkers.(20) At the level of 2 mole % sodium salt, the sulfonate was 100 times greater in viscosity than the carboxylate, the difference becoming greater with increased acid content. A dramatic increase in the viscosity (20 fold) of the ethylene-methacrylic acid ionomers (sodium salt, 2 mole %) was reported by Longworth and coworkers.(27)



1,583 3 1,217 19

Exxpro™ (100 K Mn, 1.3 mol. % Br)

Exxpro™ P 0 B 0(1OO K Mn, 1.3 mol. % Br)

Exxpro™ (280 K Mn, 3.5 mol. % Br.)

1,200 1,200

KratonG2705

Santoprene (EPDM/PP, DVA)

3

1,532

4

4

Exxpro™ P 0 B 0 (28O K Mn, 3.5 mol. % Br)

© ©

3

4

Exxpro™ P 0 B 0 (6O K Mn, 1.3 mol. % Br)

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New Options via Chemical Modifications of Polyolefins: Part 1

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