Termination reaction Ziegler polymerization

Problem 9.2 Termination reactions are more complex than described above. Thus polyethylene produced by Ziegler-Natta polymerization contains vinyl (CH2 =CHR), vinylidene (CH2 =CRR ), and frnns-vinylene (RCH=CHR ) end groups in amounts of approximately 60, 20, and 20%, respectively. Account for the formation of such groups. 

The mechanism of the Ziegler polymerization has been the subject of many experiments and much speculation. It is certainly not of free radical nature, since hydrogen acts as a chain transfer agent. When tritiated alcohols are added as chain-terminating agents, tritium is found in the polymer. If ( C2H5)3A1 is used as a starter, the polymer is radioactive. Therefore the overall reaction can be formulated only in terms of metal-carbon bond participation ... 

Various termination reactions are possible in Ziegler polymerizations. At polymerization temperatures below 60° C, every polymer chain contains a metal atom. Thus, no thermal termination takes place at this temperature, although it does at higher temperatures, where vinyl and vinylidene groups have been found ... 

No termination reactions occur in Ziegler polymerizations carried out at not too high a temperature (see Section 19.2.4). Thus, they are living systems. The total concentration of active centers must consequently be constant and time independent ... 

The termination reaction is facilitated by the p-hydride elimination. Thus, termination leads back to the reactive three-coordinate zirconium species which can initiate the polymerization again. Recall that this is similar to what we have seen with simple Ziegler-... 

The mechanism of polymerization by the transition-metal complexes is similar to what is known for conventional Ziegler-Natta catalyzed reactions (Fig. 2.28). Alkylation of the metal center followed by CHs" abstraction leads to a metal-centered cation. Olefin coordination followed by insertion leads to polymer growth. The termination of the polymerization can occur by a p-hydrogen transfer. Clearly the advent of these developments is an indicator that the last word in this area is far from over and the inge-... 

When ethylene reacts with triethyl- or tripropylaluminum, multiple carbometa-lation takes place, resulting in the formation of oligomers.509 Oxidation of the products followed by hydrolysis yields alcohols, whereas displacement reaction produces terminal alkenes that are of commercial importance.510 Transition-metal compounds promote the addition to form polymers (Ziegler-Natta polymerization see Section 13.2.4). 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

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)... 

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. 

A classical catalyst for metathesis reactions 1 reminiscent of a polymerization Ziegler catalyst it is essentially a combination of a transition metal halide (WCU. MoCl ) and an alkyl metal derivative (AIR). SnK. etc). It is noteworthy that a reduction step occurs during the constitution of the active center, since an elTicieni metathesis of terminal olefins has been achieved under electro-catalytic conditions [40]. (CI4 being the active carbenic entities in... 

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