Chain transfer to hydrogen

Termination occurs primarily through chain transfer to hydrogen, that is, hydro-genolysis of the R -Ti bond as in eq 3.7. The titanium hydride may add ethylene to produce another active center for polymerization. 

Active sites produced after different chain transfer steps may differ slightly. For instance, a metal hydride site is obtained after chain transfer to hydrogen, while a metal-C2H5 site is obtained after chain transfer to polymer. For the sake of simplicity, these sites are not differentiated in Table 5.2 for most modeling applications, these differences are also irrelevant. 

Published data [78-81] show that a first order rate of chain transfer to hydrogen in the polymerization of ethylene with a ZN catalyst is usually observed on the other hand, a number of studies [82] show a rate order of 0.5. 

The presence of n-butyl end groups in polymer chains formed during propene polymerization in the presence of hydrogen supports this hypothesis [85, 91-93]. The possibility of formation of the inactive centers Mt-CH(CH3>2 by the reaction of chain transfer with the monomer in a secondary 2,1-orientation or by a secondary insertion in the Mt-H bond, formed as a result of chain transfer to hydrogen, has been considered [76] ... 

A kinetic model of polyolefins copolymerisation by Ziegler-Natta catalysts has been proposed in [81]. The model is based on a mechanism assuming a reaction pathway with different types of AC in the catalyst particle. The kinetic chart describes the formation and deactivation of AC, as well as the spontaneous reactions of chain transfer to hydrogen, monomer, or metal-organic compound. The model is suitable for calculations of copolymerisation rate, composition, and MWD of copolymers. 

The tendency towards chain transfer to hydrogen seems to be the highest with metallocenes prone to unimolecular p-H elimination. Metallocenes with high selectivity towards bimolecular p-H elimination are less reactive towards hydrogen. 

Chain initiation reaction with ethylene after chain transfer to hydrogen (El) ... 

Chain transfer to hydrogen leads to the production of a dead chain with a saturated chain end, Dr, and a metal hydride active site that can be initiated with monomer according to Eq. (5). 

Chain transfer to hydrogen - creates saturated chain end " H2... 

Chain transfer to polymer is reported as a major complication and is thought to be unavoidable in the polymerization of alkyl acrylates.200 202 The mechanism is believed to involve abstraction of a tertiary backbone hydrogen (Scheme 6.32). It has been proposed that this process and the consequent formation of branches may contribute to the early onset of the gel or Norrish-Trommsdorff effect in the polymerization of these monomers. At high temperatures the radicals formed may undergo fragmentation. 

The proposal that PVAc also has non-hydrolyzable long chain branches stems from the finding that PVA also possesses long chain branches. No/akura et a/.171 "07 suggested, on the basis of kinetic measurements coupled with chemical analysis, that chain transfer to PVAc involves preferential abstraction of backbone (methine) hydrogens (ca 5 1 v,v the acetate methyl hydrogens at 60 °C). 

Termination occurs when the active sites of two growing chains meet, as shown in Fig. 2.3 d). The unpaired electrons form a bond that couples the ends of the chains. Alternatively, disproportionation may occur. This happens when one chain transfers a hydrogen atom to the other and the electrons on both species rearrange themselves to satisfy the octet rule. 

Thus, in one cycle, eight hydrogen atoms (H+ + e ) are transferred to hydrogen-transmitting coenzymes and later oxidized to water in the respiratory chain. This process is linked to oxidative phosphorylation, i.e., the synthesis of ATP from ADP and inorganic phosphate. 

Reactivity Is typical of an acrylamide. For example, compound 1 shows essentially 1 1 copolymerizablllty with butyl acrylate. Copolymerizablllty has also been demonstrated with styrene, other acrylates and methacrylates, vinyl acetate (VAc), VAc/ethylene and vinyl chlorlde/ethylene. High molecular weight polymers and copolymers remain soluble. Indicating any chain transfer to polymer, e.g. through abstraction of the acetal hydrogen. Is minor. 

In certain situations, termination occurs by disproportionation. This termination process involves chain transfer to a hydrogen atom from one chain end to the free radical chain end of another growing chain, resulting in one of the dead polymer chains having an unsaturated chain end (Equations 6.19 and 6.20). 

Not only the case of vinyl chloride but also styrene shows that the observed chain transfer to monomer is not the simple reaction described by Eq. 3-112. Considerable evidence [Olaj et al., 1977a,b] indicates that the experimentally observed Cm may be due in large part to the Diels-Alder dimer XII transferring a hydrogen (probably the same hydrogen transferred in the thermal initiation process) to monomer. 

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

Chain transfer to an active hydrogen compound such as molecular hydrogen ... 

Chain transfer to molecular hydrogen not only affects polymer molecular weight, but unlike other transfer agents, also affects polymerization rate. Hydrogen often decreases the rate of ethylene polymerization, but increases the rate of propene polymerization [Chadwick,... 

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