Ind. Eng. Chem. Res. 1993,32, 986-988
986
Palladium-Catalyzed Alternating Copolymerization of Propylene and Carbon Monoxide. Formation of Poly(spiroketal/ketone) Pui Hwan Wong,I.+Johannes A. van Doorn, Eit Drent, Olaf Sudmeijer, and Hans A. Stil KoninklijkelShell Laboratorium, Amsterdam, Shell Research B. V., Badhuisweg 3, 1031CM Amsterdam, Netherlands
High molecular weight alternating copolymers of propylene and carbon monoxide were prepared using a cationic palladium complex modified with 1,3-bis(di-n-butylphosphino)propane.Solidstate l3C NMR analysis showed the presence of unexpected ketal repeat units in the isolated polymers which upon heating or dissolution in hexafluoro-2-propanol were converted into ketones. A mechanism involving intramolecular polymerization of ketone and depolymerization of poly(spiroketal) is proposed t o account for the observations.
Introduction Recent advances in the copolymerization of carbon monoxide with olefins have resulted in the discovery of efficient palladium catalysts for the preparation of perfectly alternating ethylene40 copolymer (Sen, 1982; Drent, 1984; Drent et al., 1991), s t y r e n e 4 0 copolymer (Drent, 1986; Barsacchi et al., 1991), and propylene40 copolymer (Drent and Wife, 1985; Van Doorn et al., 1989; Batistini et al., 1992; Batistini and Consiglio, 1992; Jiang et al., 1992). The copolymerization of propylene and CO has been reported to produce polymers containing unexpected ketal repeat units (Van Doorn et al., 1989), and a mechanism involving a carbene intermediate has been proposed recently to account for the formation of ketal (Batistini and Consiglio, 1992). We report here evidence supporting the formation of poly(spiroketal) and propose a mechanism involving the intramolecular polymerization of initially formed ketone repeat units.
Results and Discussion Attempts to prepare propylene-C0 copolymers using the catalyst system of Pd(OAc)n, 1,3-bis(diphenylphosphino)propane (BDPPP), and p-toluenesulfonic acid (PTSA),a combination which yields high molecular weight ethylene-C0 copolymers (Drent et al., 19911, produced only oligomers at rates significantly lower than those of ethylene40 copolymerization. The apparently slower propagation rate of propylene led to the investigation of a catalyst which would be sterically more accessible to propylene by replacing the bulky BDPPP with 1,3-bis(di-n-buty1phosphino)propane(BDBPP). The copolymerization of propylene and CO was carried out using a combination of Pd(OAc)z, BDBPP, and either Ni(C10.h or PTSA as the noncoordinating anion source at 42 "C in a mixture of methanol and tetrahydrofuran (THF). Reasonable rates and limiting viscosity number (LVN) were obtained provided that either naphthoquinone (NQ), an oxidant, or methyl orthoformate (MOF), a water scavenger, was present (Table I). In the absence of both NQ and MOF, the rate was reduced by more than a factor of 10 and the color of the product was gray, indicating possible reduction of palladium. The rate enhancement effects of NQ and MOF suggest that the catalyst is susceptible to deactivation by water. + Current address: Westhollow Research Center, Shell Development Company, P.O. Box 1380, Houston, TX 77251.
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Unlike the crystalline ethylene40 copolymer, which separates as powder during polymerization,the propyleneCO copolymer (I) formed a gel. It was broken into small pieces, dried, and characterized by both solution and solidstate NMR. The 13CNMR spectrum of I in hexafluoro2-propanol (HFIPA) as shown in Figure 1 is consistent with the formation of an alternating polyketone (Ia). The carbonyl resonances narrowly centered at 217 ppm suggest regioselective but nonstereoselective incorporation of propylene. Interestingly, in contrast to ethylene-C0 copolymer, which contains a 1:l ratio of ketone and ester end groups, I did not show any detectable ester carbonyl resonance indicative of ester end groups in the vicinity of 176 ppm. The small resonances at 12.3, 17.0, and 17.4 ppm may be assigned to probable n-propyl and isopropyl ketonic end groups. The absence of ester end groups suggests that initiation occurs predominately via the addition of a palladium hydride species to propylene and chain termination by protonation of palladium alkyl bonds is preferred over methanolysis of palladium acyl bonds. The palladium hydride initiator is probably formed by a water-gas shift reaction from water presence in the reaction medium. In mixture of THF and methanol containing methyl orthoformate, an optimal concentration of water is established by the equilibrium, methyl orthoformate + water == methyl formate + methanol, to maintain the supply of palladium hydride via the shift reaction. In the absence of methanol, the reaction medium is too dry and no polymerization occurs; whereas in the absence of methyl orthoformate, the higher concentration of water causes reduction of Pd(I1) to inactive Pd(0) and the presence of an oxidant such as naphthoquinone is required to maintain catalytic activity. Magic angle spinning (MAS) solid-state 13C NMR analysis revealed a surprising structural change upon heating as shown in Figure 2. In addition to the expected carbonyl and aliphatic resonances of a polyketone, the as-isolated I showed a strong and unexpected resonance at 114.3 ppm at room temperature. At 210 OC, the 114.3 ppm resonance disappeared to give the expected four-line spectrum of an alternating polyketone with carbonyl resonance at 210.7 ppm. After cooling to room temperature, the polyketone structure remained unchanged. Dissolution of I in HFIPA also caused irreversible conversion to the ketone structure. The resonance at 114.3 ppm is in the range of ketal carbon resonances. Assuming a mole per mole conversion to the ketone form, the concentration of ketone repeat units in as isolated samples ranged from 10 to 30% basis NMR measurements. 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No.5, 1993 987 Table I. Palladium-Catalyzed Copolymerization of Propylene and Carbon MonoxideP Ni(C10&, mmol 0.26 0.14 0.18 0.21
Pd(OAc)z, mmol
0.052 0.028 0.036 0.041 0.051
PTSA, mmol
NQ, mmol
HC(OMe)Z, mL
0 2.8 0 0 0
0 0 2 2 2
0
0 0.06 0.06 0.12
0
THF, mL 132 132 132 21 21
MeOH, mL 8 8 8 119 119
rate, g/(g of PdW 15
170 150 350 330
yield, g
11.5 33.7 30.5 25.5 31.1
LVNb dL/g 1.1 0.91 0.8 0.57 0.38
Polymerization carried out with equimolar amounts of Pd(0Ac)z and BDBPP in a mixture of 80 mL of propene and solvent at 42 “C and a total pressure of 40 bar maintained by a continuous supply of CO. Propene consumed during polymerization was not replenished. Limiting viscosity number was measured in rn-cresol at 60 “C.
Scheme I. Palladium-Assisted Formation of Poly(spiroketa1)
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.
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Figure 1.
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