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We report here that C 2 -and C s -symmetric [(P∧P)Pd(H 2 O) 2 ](SO 3 -CF 3 ) 2 complexes, P∧P being either a rac-or a meso-diphosphine ligand, catalyze the copolymerization of propene and carbon monoxide to isotactic poly(1-methyl-2-oxo-1,3-propanediyl) (1a); the meso ligands are even more stereoselective than the racemic ligands and display much higher catalytic activity.The relationship between the geometric characteristics of the catalytic system and the microstructure of the produced polymer has received much attention in recent years, 1 particularly in the field of the polymerization of olefins by metallocenes. 2 Thus, C 2 -symmetric catalysts, 3,4 in which the alternative coordination sites are homotopic, 5,6 give isotactic polymers, whereas C s -catalysts with enantiotopic coordination sites give syndiotactic polymers. 7 Similarly, C 2 -symmetric palladium catalyst precursors enable the copolymerization of aliphatic or aromatic olefins with carbon monoxide to polyketones 1, 8 which have a highly isotactic microstructure. 9-13 With C 1 -symmetric catalyst precursors, isotacticity in the copolymerization was also achieved, 14-17 as was the case for poly-1-olefins with some C 1 -metallocene systems. 18,19 The role of square planar C s -catalysts in the copolymerization reaction is still unclear with respect to the possible synthesis of materials with an unknown microstructure as well as to the mechanistic implications for the catalysis. A first comparison was made with the [(P∧P)Pd(H 2 O) 2 ](SO 3 -CF 3 ) 2 20 complexes 2a-4a, modified with the corresponding ligands 2-4, for the copolymerization of propene (Table 1). Systems containing 2 and 4 had already been used as catalysts. 21 The results in Table 1 and the 13 C NMR spectra in the region of the carbonyl resonances of the synthesized copolymers ( Figure 1) clearly show that the meso ligand has much better stereochemical control and higher catalytic activity. In the related ethene copolymerization, the activity of the system modified by 3 was only 1.4 times higher than the corresponding system modified by 4. 22 Due to the possible implications of the incomplete regioselectivity exhibited by catalyst precursors 2a to 4a, however, we used systems that show better control of the regiochemistry of the insertion of propene, namely systems related to the recently investigated 5. 23 Ligands 6 and 7 were synthesized from 5 by using a slightly modified form of the procedure reported for their 1,3-isomers; 24 they were stereochemically identified by 31 P NMR (1) Kobayashi, S.

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