Ethylene chain transfer agent

MMA can function as a chain-transfer agent for late transition metal-catalyzed ethylene polymerization. Gibson and Tomov revealed that Ni complexes with bulky phosphino-enolate ligands, F13-17, were capable of polymerizing ethylene in the presence of MMA to afford MMA-end-capped PEs through immediate /3-H transfer after MMA 337... 

Typical heterogeneous Ziegler catalysts operate at temperatures of 70 100°C and pressures of 0.1 2 MPa (15 300 psi). The polymerization reactions are carried out in an inert liquid medium (e.g, hexane, isobutane) or in the gas phase. Molecular weights of LLDPE resins are controlled by using hydrogen as a chain-transfer agent. Reactivities of a-olefins in copolymerization with ethylene depend on two factors the size of the alkyl groups attached to their double bonds and the type of catalyst,... 

Molecular hydrogen has been known for a long time as an effective chain-transfer agent in the coordination polymerization of ethylene and a-oiefms with Ziegler-Natta catalysts 99-101,50). The mechanism for the reaction of a growing polymer chain with H2 has not been established, The living coordination polymerization system is well suited for an elucidation of the mechanism, since the reaction with H2 can be studied independently of any interference from other chain-terminating processes. 

The incorporation of TEMPO alkoxy amine at the end of a PE chain has been achieved [99]. The dialkylmagnesium compound in ethylene polymerization was adopted as a chain transfer agent, as mentioned above. It was also reported that PE-TEMPO and terminally N-(2-mcthyl-2-propyl)-N-(l-diethylphosphono-2,2-dimethylpropyl)-N-oxyl PE were synthesized by the reaction with di-polyethylene magnesium produced in ethylene polymerization. They were used for CRP of n-butyl acrylate, leading to PE-h-PnBA. 

FIG. 14 Platelet adhesion to the PU films grafted with methoxy polyethylene glycol) methacrylates at different concentrations of chain transfer agent (c-cysteine). Ethylene glycol units in monomers ( ) 4, (O) 9, and ( ) 23. (From Ref. 74.)... 

Gaseous ethylene is fed to a primary compressor (1) with recycled ethylene from an LP system and chain-transfer agent to control density and molecular weight. The mixture leaves the primary compressor at 250 bar and is subsequently compressed by the secondary compressor (2), together with the unconverted ethylene from the HP recycle (3) and VAM to produce EVA copolymers, to a polymerization... 

The copolymerization between trioxane and suitable comonomers (ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butane diol formal in amounts of 2-5% by weight) is performed using cationic initiators. The cationic initiators could be Lewis acids, such as BF3 or its etherate BF3Bu20 which was used, for example by Celanese (the mechanism of this reaction was studied in detail [163,164]) or protic acids such as perchloric acid, perfluoroalkane sulfonic acids and their esters and anhydrides. Heteropoly acids were used and also a series of carbenium, oxocarbenium salts, onium compounds, and metal chelates. To regulate the molecular weight chain-transfer agents, such as methylal and butylal, are added. 

Theonly important current application of tubular reactors in polymer syntheses is in the production of high pressure, low density polyethylene. In tubular processes, the newer reactors typically have inside diameters about 2.5 cm and lengths of the order of I km. Ethylene, a free-radical initiator, and a chain transfer agent are injected at the tube inlet and sometimes downstream as well. The high heat of polymerization causes nonisothermal conditions with the temperature increasing towards the tube center and away from the inlet. A typical axial temperature profile peaks some distance down the tube where the bulk of the initiator has been consumed. The reactors are operated at 200-300°C and 2000-3000 atm pressure. 

In ethylene—propene copolymerization the former monomer is greatly favoured and a value for r, of 72 was found. Hydrogen is particularly active as a chain transfer agent for this catalyst, a value of fetr,H,/ tr,M 3.8 X 10 being quoted, some ten times greater than that for a conventional Ziegler system [133b]. The active species in both these systems was ascribed to a low valence Cr complex. 

Recently, diphenylphosphine has been shown to be an efficient chain-transfer agent in organolanthanide-catalyzed ethylene polymerization, yielding phosphine-terminated polyethylenes. This reaction is a versatile, efficient way of incorporating an electron-rich functional group into an otherwise inert polymer.929... 

Fig. 4.13 H nmr spectrum of low molecular weight poly(norbornene) made using ethylene as a chain transfer agent.

Ni(fJ,l r)2. In an effort to lower the molecular weight of the polymer chain and simplify analytical characterization, homopoiymerization of norbornene was carried out in the presence of ethylene pressure. Norbornene polymerizations catalyzed by cationic nickel catalysts such as (// -crotyl)Ni(1,5-fX)l)) ld, are quite susceptible to acyclic olefins as chain transfer agents and produce substantially reduced molecular weight vinyl-terminated poly(norbornenes) (Section 4.2.3.5). 

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