Macromol. Rapid Commun. 16, 9-14 (1995) 9

Enantioselective isotactic alternating copolymerization of styrene and 4-methylstyrene with carbon monoxide catalyzed by a cationic bioxazoline Pd(I1) complex

Stefan0 Bartolini, Carla Carfagna, Alfredo Musco *

Istituto di Scienze Chimiche, Universita degli Studi di Urbino, 61029 Urbino, Italy

(Received: Septenber 12, 1994)

SUMMARY: The cationic bioxazoline Pd(I1) complex 2 catalyzes the alternating copolymerization of carbon

monoxide with styrene and 4-methylstyrene, respectively, to yield a highly isotactic optically active polymer at room temperature and low carbon monoxide pressure (1 - 4 atm).

Introduction

Alternating copolymerization of styrene and carbon monoxide to a syndiotactic polymer catalyzed by cationic 1,lO-phenanthroline and 2,2'-bipyridine Pd(I1) comple- xes was recently reported by several authors '1. A chain end control of the polymeriza- tion is considered to be operative with the above achiral ligands. Coordination of a chiral ligand to the metal may switch the chain end control to an enantiomorphic site control to yield an optically active isotactic alternating copolymer '). Indeed recently 4-tert-butylstyrene and carbon monoxide were copolymerized by Brookhart 3, to an optically active highly isotactic polymer in the presence of the cationic C, symmetric bis(oxazo1ine) Pd(I1) complexes 1 a and 1 b.

An enantioselective alternating copolymerization of a-olefins with carbon mon- oxide, although with a lower stereoregularity, was also reported by Sen4). These results recently appeared in the literature prompted us to report our own contribution in this subject. We report here that the cationic bioxazoline Pd(1I) complex 2 (Scheme I) catalyzes the alternating copolymerization of carbon monoxide respectively with styrene and 4-methylstyrene to yield a highly isotactic optically active polymer.

Results and discussion

Complex 2, which was prepared following a well-established route a) (Scheme I ) slowly dissolves in either 4-methylstyrene or styrene. The resulting yellow solution was reacted with carbon monoxide (1 - 4 atm) for 12 h a t room temperature (Eq. 1). As

3a R = 4-MeCsH4

3b R=CsHs

a) Experimental details and full spectroscopic characterization will be given in a forthcoming publication.

0 1995, Huthig & Wepf Verlag, Zug CCC 1022-1 336/95/$02.50

10

l+

S . Bartolini, C. Carfagna, A. Musco

/ \ Me NCMe

la R = C H M ~ >

lb R = M e

IC R =CH&sHs

extensive decomposition of the catalyst to metallic Pd occurred within this time the reaction yields are presently only moderate. The polymer was precipitated from the reaction medium by addition of methanol. In the case of 4-methylstyrene as monomer formation of some atactic poly(4-methylstyrene) also occurs. The homopolymer is readily separated from copolymer 3 a by short column S O , chromatography eluting first the homopolymer with methylene chloride and then the copolymer with ethyl acetate. The styrenekarbon monoxide copolymer 3 b was purified by the same route.

= -350 deg . dm-' . g- ' . cm3, 10,70 g .dm-3 , CH,Cl,) and 3b ([a]ti9 = -348 deg .dm- ' . g - ' *cm3 , 3,70 g .dm-3 , (CHCl,CHCl,) clearly suggests an isotactic microstructure. This was fully confirmed by ' H and I3C NMR spectra (Tab. 1). The presence of only one signal for each carbon in the I3C NMR spectrum as well as the sharpness of each peak is consistent with the stereoregularity of the polymer chain. The same conclusions are drawn by inspection of the 'H NMR spectrumb).

The high specific rotation of both 3a

Scheme I:

i I+

/ \ Me NCMe

2

b, The physicochemical characterization of polymers 3 is in progress. A preliminary X-ray investigation has shown that 3b is a crystalline material.

Enantioselective isotactic alternating copolymerization of . . . 11

h

v

W

N Ni

h

9 0

9 t- h

v,

00. W 0 N

W 8

v, d

d 0 . n .n m m m m

12 S. Bartolini, C. Carfagna, A. Musco

Assuming a square planar transition state in which the olefin double bond lies in the metal coordination plane in the required cis geometry for the insertion into the Pd-acyl bond, molecular modelling suggests that the Re face of the olefin is preferentially bound to the metal. Accordingly the styrene phenyl ring and the ligand isopropyl group are respectively above and below the coordination plane thus minimizing the steric repulsion. If this assumption is correct an R configuration may be anticipated for both the insertion product ' c ,6 ) and the asymmetric carbon atom of the polymer chain.

Fig. 1 . Molecular model of the enantioselective coordination of styrene in a square planar intermediate complex

Fig. 2. Molecular model of the insertion of styrene in the Pd-acyl bond

We have also investigated the catalytic properties of complex l c which is similar to complexes 1 a and 1 b used by Brookhart for the 4-tert-butylstyrene/carbon monoxide copolymerization3). In the same reaction conditions as catalyst 2, l c gave much lower yields, e. g., polymer 3 b was obtained only in traces, although again as a highly isotactic material. This is very likely due to the different electronic properties of the ligands, bioxazoline being a better n acceptor ligand than bis(oxazo1ine).

Enantioselective isotactic alternating copolymerization of . . . 13

In conclusion our results show that the C, symmetric (4S, 4's)-( -)-4,4',5,5'-tetra- hydro,4,4'-bis(l-methylethyl)-2,2'-bioxazole '1 coordinated to Pd(I1) in the cationic complex 2 is a valuable ligand for the highly isotactic alternating copolymerization of styrene and 4-methylstyrene with carbon monoxide. Further work is in progress on the catalytic properties of cationic Pd(I1) complexes containing C, symmetric chelating nitrogen ligands with different n acceptor properties.

Experimental part

NMR were recorded with a Bruker AC 200 spectrometer. The copolymerization reactions were performed under nitrogen atmosphere. Optical rotation measurements were performed on a Perkin Elmer 241 polarimeter usig a sodium lamp. Molecular modelling has been performed with Alchemy I11 software of Tripos Associates, Inc. Styrene and 4-methylstyrene were dried over CaH, and distilled prior to use. Poly(l-oxo-2-(4-methyl)phenyltrimethylene) (3a): 4-Methylstyrene (10 mL, 76 rnmol) and 2

(0,107 g, 0,086 mmol) were introduced in a magnetically stirred stainless steel reactor (70 mL). Carbon monoxide was introduced at 4 atm. After 12 h at room temperature methanol (50 mL) was added to the reaction mixture. The resulting grey material was extensively washed with methanol and dried under high vacuum (1,67 8). The reaction product (0,600 g) was purified through SiO, (30-75 micron) chromatography (20 cm x 2.5 cm column) to yield poly(4-methylstyrene) (0,260 g) and then pure 3a (0,280 g) eluting with ethyl acetate. I. R. (film): 1711 cm-' v ~ = ~ .

(C ,OH loo), (14619, Calc. C 82,16 H 6,89 Found C 82,02 H 6.46

Poly(2-0x0-2-phenyltrimethylene) (3b): Styrene (1 1 mL, 96 mmol) and 2 (0,400 g, 0,32 mmol) were magnetically stirred under 1 atm of CO for 12 h. Precipitation with methanol yielded 0,440 g of raw material which was purified as 3 a to yield 0,120 g of pure 3b.

I. R. (film): 171 1 cm-' vce0.

(C&@), (1 32,16), Calc. C 81,79 H 6,lO Found C 81,65 H 6,07

Financial support from the Italian Ministry for University and the National Research Council of Italy is gratefully acknowledged.

(a) Eur. Patent Application 229408 (1986), inv.: E. Drent; Chem. Abstr. 108, 6617b (1988); (b) M. Barasacchi, G. Consiglio, L. Medici, G. Petrucci, U. W. Suter, Angew. Chem., Int. Ed. Engl. 30, 989 (1991); (c) M. Brookhart, F. C. Rix, J. M. DeSimone, J. C. Barborak, 1 Am. Chem. SOC. 114, 5894 (1992); (d) A. Sen, 2. Jiang, Macromolecules 26, 91 1 (1993); (e) V. Busico, P. Corradini, L. Landriani, M. Trifuoggi, Makromol. Chem., Rapid Commun. 14, 261 (1993)

2, A. Batistini, G. Consiglio, U. W. Suter, Angew. Chem., Int. Ed. Engl. 31, 303 (1992); A. Sen, Ace. Chem. Res. 26, 303 (1993)

3, M. Brookhart, M. I . Wagner, G. G. A. Bolavoine, H. A. Haddou, 1 Am. Chem. Soc. 116, 3641 (1994)

14 S. Bartolini, C. Carfagna, A. Musco

4, 2. Jiang, S. E. Adams, A. Sen, Macromolecules 27, 2694 (1994) ’) R. van Asselt, E. E. C. G. Gielens, R. E. Riilke, K. Vrieze, C. J. Elsevier, J. Am. Chem. Soc.

116, 977 (1994) and references quoted therein 6 , J. S. Brumbaugh, R. R. Whittle, M. Parvez, A. Sen, Organometallics 9, 1735 (1990) ’) I. Butula, G. Karlovic, LiebigsAnn. Chem. 1455 (1976); M. Onishi, K. Isagawa, Inorg. Chim.

Acta 179, 155 (1991); A. Pfaltz, Arc. Chem. Rex 26, 339 (1993)