Chain End-Groups Reveal Two States for Palladium-Based Polyketone Catalyst Species

Abstract

Over the past decades several new families of well-defined transition metal-based polymerization catalysts have been discovered. 1 Although these systems are generally referred to as "single-site" catalysts, this seems not to hold in all cases. 2 Deviations of the polydispersity (M w /M n ) of the produced (co)-polymers from the theoretical Flory-Schulz value of 2 have been attributed to polymer precipitation and heterogenization of the catalyst giving rise to "multiple-site" catalysis. 2a,c-f The copolymerization of ethene and carbon monoxide to a perfectly alternating polyketone, [CH 2 CH 2 C(O)] n , which is catalyzed by a cationic palladium catalyst, [L 2 PdR] + (X) -(L 2 ) bidentate ligand; X ) weakly or noncoordinating anion; R ) [CH 2 CH 2 C(O)] n OCH 3 or [C(O)CH 2 CH 2 ] n H (n g 0)), generally affords materials with M w /M n ≈ 2. 5-5. 3 This suggests that the initially single-site catalyst becomes a heterogeneous, multiple-site system during copolymerization, in line with the slurry nature of the polyketone manufacturing process. In this communication, evidence is provided from polymer end-group analysis that the running catalyst species, [L 2 PdR] + , exhibits two-stage kinetic behavior. This is ascribed to residence of the catalyst in two different states. Depending on the length of the growing copolymer chain R, the catalyst is either dissolved during copolymerization (at the shorter chains) or polyketone-supported (at the longer chains).Termination during CO/ethene copolymerization proceeds via two competing mechanisms, protonolysis, leading to the formation of a ketone end-group (K; eq 1) and alcoholysis, giving an ester end-group (E; eq 2). 1h,4 After termination via protonolysis a Pdmethoxy species is obtained that gives rise to the formation of an E end-group at the start of the next copolymer chain it produces (eq 3), whereas after alcoholysis a Pd-hydride species is obtained, giving a K end-group in the next chain (eq 4).According to this two-catalytic cycle model, three types of copolymer chains may be produced, those containing: (i) both an ester and a ketone end-group (EK), (ii) two ester end-groups (EE), and (iii) two ketone end-groups (KK). This model also predicts the formation of equal amounts of E and K and, consequently, equal amounts of EE and KK copolymer chains. 1h,5 However, in several cases the E/K ratio in the methanol-soluble oligomer fraction (filtrate) deviated from unity (GC-analysis). 6 For example, when Pd(bdompp)(TFA) 2 (bdompp ) 1,3-bis-(di-(o-methoxyphenyl)phosphino)propane) is employed as the catalyst, the E/K ratio in the filtrates of polyketone produced at 50 bar CO/ethene and 90°C amounts to 0.14, while no oligomeric EE-chains are detected (see Table 1). The E/K ratio in the precipitated polyketone did not significantly deviate from unity. As the latter fraction contains the large majority (>98%) of the chains formed, the E/K ratio of the total products is about unity, indicating that chain transfer occurs in a perfect fashion, that is, only via eq 1-4. These obser...

Influence of the operating conditions on the catalytic activity of [PdCl2(dapp)] in the CO–ethene copolymerization in the H2O–CH3COOH as a solvent (dapp=1,3-bis(di(2-methoxyphenyl)phosphino)propane)

“…15 atm and declines at higher pressure. It was suggested that CO impedes coordination of E [31]. The previous experiments have shown that high LVN can be achieved under high total pressure and at low temperature, whereas in order to achieve a high productivity it is necessary to increase the temperature.…”

Polyketones

“…Because of growth of the copolymer chain, the solubility of these species gradually decreases and at a certain chain length n, the running catalyst precipitates and becomes heterogeneous. Copolymer chain end-group analysis revealed that this phase transition occurs when n is about 13-20 and is accompanied by a change in the extent to which the two termination reactions, alcoholysis and protonolysis (vide supra), occur while the ratio of chain transfer vs propagation (k t /k p ) increases (115).…”

In Situ Preparation of Palladium Diphosphane Catalysts

“…45:55) deviated from the theoretical value of 1. [7,8] Formation of an excess of ketone end-groups is probably caused by increased partial initiation via a Pd-H species instead of via Pd-OCH 3 . This can occur if initiation via the reactions given in Equations (7) to (10) efficiently competes with those given in Equation (5) and (6).…”