Copolymerization of Ethylene and CO

As stated above, the discovery of efficient catalysts for the copolymerization of alkenes originated from a study of the alkoxycarbonylation of ethylene in methanol (MeOH) to methyl propionate (eq. (1)). 

The catalysts were cationic palladium-phosphine systems prepared from palladium acetate, an excess of triphenylphosphine (PPhs) and a Bronsted acid of a weakly or noncoordinating anion (e. g., p-tosylate (OTs ) methanol was used as both the solvent and a reactant. An unexpected change in selectivity was observed upon replacement of the excess of PPhs by a stoichiometric amount of the bidentate l,3-bis(diphenylphosphino)propane (dppp). Under the same conditions. 

Instead of the free acid it is sometimes advantageous to use a metal salt (e. g., of Cu of Ni ) to introduce the anions. Preformed complexes of the type L2 dX2 [18] and L2Pd(R)X [19] where L2 represents a chelating ligand, X a weakly coordinating anion, and R a hydrocarbyl group (e. g., methyl) have also been tested as catalysts. The results are, generally, very similar to those obtained with catalysts prepared in situ. 

Carbon monoxide insertion in a palladium-carbon bond is a fairly common reaction [21]. Under polymerization conditions, CO insertion is thought to be rapid and reversible. Olefin insertion in a palladium-carbon bond is a less common reaction, but recent studies involving cationic palladium-diphosphine and -bipyridyl complexes have shown that olefin insertion also, particularly in palladium-acyl bonds, appears to be a facile reaction [22], Nevertheless, it is likely that olefin insertion is the slowest (rate-determining) and irreversible step vide infra) in polyketone formation. 

End-group analysis by C-NMR of the ethylene/CO copolymer produced in methanol genertdly shows the presence of 50 % ester (-COOMe) and 50 % ketone (-COCH2CH3) groups, in accordance with the average overall stmcture of the polymer molecule as depicted in eq. (2). It is not clear a priori which group is the head and which is the tail of the molecules. Moreover, GC and MS analyses of oligomers produced with certain catalysts [13] show, in addition to the expected keto-ester product (Structure 2), the presence of diester (Stmcture 3) and diketone (Stmcture 4) compounds. 

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A nonlocal density functional study of the Pdn-assisted copolymerization of ethylene and CO has been published.490 491... 

Fig.l. Co60 y-copolymerization of ethylene and CO. % CO in copolymer vs. % CO in initial gas mixture. Taken from ref. 7 by permission of the publisher, John Wiley Sons, Inc. 

A further example of the application of an organometalhc palladium complex as polymerization catalyst was presented by Sheldon et al. [32]. The active complex can be obtained byreacting [PdCl(CH3)COD] (7) in methanol with a tetrasulfonated diphosphane, under abstraction of a chloro ligand by AgOTf The complex can be isolated as an air-stable white solid and shows a remarkable activity on the copolymerization of ethylene and CO in water. In fact, activities of 32.2 kg copolymer per gram Pd are reached, corresponding to TOFs > 61 000 h . ... 

Introduction of carboxyl groups by copolymerization of ethylene and CO, the carbonyl ions absorb UV radiation and facilitate break-up of chains into smaller parts... 

Despite numerous reports on the transition metal-mediated copolymerization of ethylene and CO, most of them afforded strictly alternating copolymers. The alternating nature can be attributed to (i) the formation of five-membered... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

The radical initiated copolymerization of C2H4 and CO in the presence of a polyolefin has been reported to result in the grafting of the C2H —CO copolymer onto the polyolefin backbone31). Polyolefins used included polyethylene, ethylene-propylene copolymer and polyisobutylene. 

Sulfur dioxide reacts generally with transition metal alkyl, aryl, and a-allyl complexes to give sulfinate complexes. The reaction, first described in 1964 by Wojcicki and Bibler, resembles well-known insertion reactions of CO, C2F4, SnCl2, tetracyanoethy-lene, and other unsaturated species into metal-alkyl bonds, but there are important stereochemical and mechanistic differences Sulfur dioxide insertion into metal-alkene and metal-alkyne bonds have not been reported. However, PdCl2 has been used as a catalyst for copolymerization of ethylene and SO2 to polysulfones and insertion into a Pd-ethylene bond is a conceivable reaction step. 

Palladium complexes figure prominently as well in the copolymerization of Q -olefins with carbon monoxide. Unlike the low molecular weight photodegradable random copolymers of ethylene and CO produced from a free-radical process, olefin/carbon monoxide copolymers produced from homogeneous palladium catalysts are perfectly alternating, the result of successive insertions of olefin and CO (Figure 19). Consecutive insertion of two similar monomers is either slow... 

In 1998, Grubbs and co-workers [83, 84] reported on a new type of neutral Ni(II) complexes with salicylaldimin ligands (32). With these catalysts low-branched polyethylenes were obtained with a narrow molecular weight distribution. The copolymerization of ethylene and norbomene is possible. Fe and Co catalysts were used for the linear polymerization of ethylene by Gibson [85] and Brookhart [86] independently (33). Activities of 10 TONs were reported. The polyethylenes obtained are highly crystalline with a broad molecular weight distribution. 

Langer (13) has also disclosed the use of alkyllithium and dialkyl-magnesium tertiary diamine complexes as catalysts for copolymerization of ethylene and other monomers such as butadiene, styrene, and acrylonitrile to form block polymers. Examples are given in which polybuta-dienyllithium initiates a polyethylene block, as well as vice-versa. Random copolymers of these two were also prepared, and other investigators have used not only tertiary diamines but hexamethylphosphoramide (14) and tetramethylurea (15) as nitrogenous base cocatalysts in such polymerizations. Antkowiak and co-workers (11) showed the similarity of action of diglyme and TMEDA in copolymerizations of styrene and... 

Brookhart and co-workers recently reported tantalizing results that were close to constituting true copolymerizations of ethylene and methyl acrylate. ° ° The catalyst employed was the palladium version of the diimine complexes that were previously reported for ethylene and a-olefin homopolymerizations (complexes IV). °° The close qualification... 

The rates of insertion of ethylene into Pd-alkyl and Pd-acyl bonds have been evaluated for this type of systems, in particular [Pd(R)(C2H4)(L-L)]+ (R = alkyl, acyl L-L = phen, 1,3-diphenylphosphinopropane) [117,118], Lower activation barriers for the acyl complexes were consistently found with AAG (alkyl-acyl) about 2 kcal for the phen complexes and 4 kcal for the phosphino derivatives. Insertion barriers for CO insertion into the Pd-alkyl bond are even lower, making the alternating copolymerization of alkenes and CO possible and almost flawless (a perfect sequence of CO insertion into M-alkyl and alkene insertion into M-acyl with absence of alkene insertion into M-alkyl) [119]. 

The copolymerization of ethylene and carbon monoxide to give alternating copolymers has attracted considerable interest in both academia and industry over recent decades [1, 2]. Attention was focused on aliphatic polyketones such as poly(3-oxotrimethylene) (1) because of the low cost and plentiful availability of the simple monomers. The new family of thermoplastic, perfectly alternating olefin/ carbon monoxide polymers commercialized by Shell provides a superior balance of performance properties not found in other commercial materials the an ethylene/ propene/CO terpolymer is marketed by Shell imder the tradename Carilon . About the history of polyketones see Refs. [3-11],... 

The insertion of ethylene and a-olefins into acyl groups is one step of the remarkably selective copolymerization of alkenes and CO to form an alternating copolymer. This process was developed at Shell Chemicals and is discussed in Chapter 17. As depicted in Scheme 9.10, the relative rates for insertion of an alkene into an alkyl group and an acyl group are one factor that controls the selectivity. For high selectivity, the insertion of ethylene into the acyl group must be faster than insertion of ethylene into an alkyl group. ... 

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