Olefin polymerization, chain termination

The preparation of PnBA-g-PE and PtBA-g-PE graft copolymers was reported using Fe-mediated olefin polymerization, chain shuttling with Zn and ATRP techniques [108]. Terminally hydroxyl PE was synthesized from Zn-terminated PE by oxidation and hydrolysis, as referred to above. It was converted to methacrylated PE, as PE macromonomer, using methacryloyl chloride. The resulting PE macromonomer was used for the copolymerization of nBA or tBA by ATRP using CuBr/tris((N,N-dimclhylamino)clhyl)aminc. The obtained graft copolymers were characterized by GPC, DSC, and JH NMR. 

The polymerization reaction is initiated hy either a (P P) Pd-H or a (P P)PdOMe species, which are formed during initiation or chain termination. Sequential insertion of ethene and carbon monoxide into a Pd-H bond leads to a Pd-C(=0) Et [K] group. Carbon monoxide insertion into a Pd-OMe bond results in formation of a PdC(=0)0Me [E] end group. After consecutive copolymerization of CO and olefin, the chain termination occurs hy either methanolysis or protonolysis upon reaction with the polar solvent. The ratio of [K] and [E] end groups is dependent on the actual employed reaction environment. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

The termination of the growing polymeric chain may occur through several different processes, mostly by chain transfer. Either the process of chain transfer with the monomer, or the reaction of dissociation to hydride, leads to the formation of terminal vinylidenic groups, whose presence was noticed in the olefin polymers, obtained with the previously described catalysts (22). 

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

The published values of related to olefin polymerization with soluble and heterogeneous catalysts are in the range from several seconds to hours 7). It should be noted that the value of is strongly dependent upon the polymerization conditions since the rates of chain-terminating reactions Rt are functions of the temperature and concentrations of chain-terminating reagents. In a living polymerization the value of is infinite. 

The concept of PO macroinitiators centers on the introduction of an initiation moiety into an olefinic polymer chain for polymerization. The most effective route for preparing PO macroinitiators is by employing functional polyolefins containing hydroxyl groups or other reactive groups. These functional POs are prepared by copolymerization of olefins with functional monomers and post-polymerization reaction, as mentioned above. In the case where an initiation moiety was at the chain-end of the polyolefins, a block type copolymer is produced. It has been reported that thiol-terminated PP was used as polymeric chain transfer agent in styrene and styrene/acrylonitrile polymerization to form polypropylene-b/odc-polystyrene (PP-b-PS) and polypropylene-btock-poly(styrene-co-acrylonitrile) (PP-b-SAN) block copolymer [19]. On the other hand, polymer hybrids with block and graft structures can be produced if initiation moieties are in the polymer chain. 

Recent examples of the first route have been described by Kobayashi and coworkers, who reported the synthesis and characterization of polymeric Pcs obtained through the olefin metathesis polymerization of terminal olefin groups in the side chains of unsymmetrical Pc monomers [159], X-ray analysis of the solid material indicates that the Pcs are ordered in stacks. 

The Catalyst System Eleven years ago, Kaminsky invented a novel olefin polymerization catalyst derived from Cp2ZrCl2 (Cp = 7 -5-C5H5) and methylaluminoxane (1), a result that has stimulated intense interest in synthesis and reactions of metallocenium ions. Important questions still remain, however, regarding the nature of the Kaminsky catalyst. These include (1) what is methylaluminoxane and how does it interact with Cp2ZrMe2 to initiate polymerization and (2.) what are the mechanisms of chain initiation, propagation, transfer and termination A collateral question is how these steps may be controlled. 

Recently, a number of a-olefins have been offered commercially. These olefins, made by ethylene polymerization or by wax cracking, are available either as relatively pure compounds or as mixtures of several adjacent members in chain lengths of about six to 20 carbon atoms. These products are largely straight-chain terminal olefins. 

Group 4 elements (e.g., Ti, Zr) are used as typical catalyst precursors for olefin polymerization and serve as potent cationic components for polymer chain growth with the aid of aluminum (e.g., MAO) or boron co-catalysts. It would be more efficient and convenient if organoaluminum cations were used to polymerize olefins. From this viewpoint, following an earlier precedent with two-coordinate cations (Equation (98)),319,320 some three-coordinate organoaluminum cations hold promise, and their ability to promote polymerization of ethylene or terminal olefins is now... 

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