Polyethylene chain termination

Triisopropan olamine is used in natural mbber cross-linking and as a color stabilizer for polyethylene formulations. Chain termination of polybutadiene with triisopropan olamine gives improved cold-flow properties. 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

Free-radical polyolefin reactions form polymers with many mistakes in addition to the ideal long-chain alkanes because of chain-branching and chain-termination steps, as discussed. This produces a fairly heterogeneous set of polymer molecules with a broad molecular-weight distribution, and these molecules do not crystallize when cooled but rather form amorphous polymers, which are called low-density polyethylene. 

Fig. 6. A primary means of regulating polyethylene chain length is through control of reactor temperature. Increasing the temperature enhances termination, probably by destabilizing the Cr-chain bond, resulting in shorter chains.

It may be interesting that chain termination with hydrogen, which is utilised in Ziegler-Natta polymerisations, does not operate in polymerisation systems with Phillips catalysts no influence of hydrogen to control the molecular weight of polyethylene in the latter case was achieved [37],... 

Effect of Substrate. Several comparisons were made to establish the effect of substrate on conversion of methylvinylpyridine and acrylonitrile. Relative effectiveness in the ethylene-propylene series were ethylene-propylene copolymer > polyethylene > polypropylene on a relativity scale of 4 3 2. The fact that chains terminated with polyethylene blocks are better substrates than polypropylene blocks suggests that random grafting on the backbone is not a major part of the reaction at low polymerization temperatures. The higher conversion on polyethylene... 

Molecular dynamics are time-consuming because the nonbonded interactions scale as n where n is the number of atoms. To save time, one may implement the united atom approach, substituting some atomistic detail with an imaginary entity that represents the essential features of what has been substituted. For example, it is common to substitute methylene groups with an imaginary spherical atom with mass 14. Therefore a polyethylene chain would look like a chain of spherical atoms, appropriately rescaled, terminated by similar entities with mass = 15 for the methyl groups. 

Yet Chien demostrated that it would be possible to obtain polyethylene with a Q value near the theoretical 2, with the homogeneous (CjH5)2TiCl2—A1(CH3)2C1 catalytic system, only if carefully controlled pseudosteady-state conditions are employed. In fact he showed mathematically that the relatively high experimental polydispersiiy (<,) tfom 2 to 5 in function of reaction time), is a natural consequence of a polymerization kinetic model based on non stationary first order initiation, chain propagation and bimolecular chain termination by recombination. 

The building unit or monomer in this case is ethylene. After n units of ethylene have been linked, the reaction may be terminated by the addition of a hydrogen to each end of the polyethylene chain. Such polymerizations are referred to as addition polymerization, in contrast to condensation polymerization in which molecules with diverse end groups react to give chains composed of units of varying size and chemical complexity. Nylon 66 is the condensation product of two monomers adipic acid and hexamethylenediamine. 

Diethylzinc is employed in several ways in polyethylene production. Its earliest application was as a chain transfer agent for molecular weight control (31, 32). Today, chain transfer in Ziegler-Natta catalyst systems is achieved chiefly by hydrogenolysis (previously discussed in connection with the mechanism of chain termination in section 3.7). Use of diethylzinc for molecular weight control for polyethylene is no longer significant. 

This paper examines some factors which affect not only the overall activity, but also the rate of termination of polyethylene chains growing on the Phillips Cr/silica polymerization catalyst. Although the theme of this symposium is not the termination but the initiation of polymer chains, the two aims are not inconsistent because on the Phillips catalyst the initiation and termination reactions probably occur together. They are both part of a continuous mechanism of polymerization. One possibility, proposed by Hogan, is shown below. The shift of a beta hydride simultaneously terminates one live chain while initiating another ... 

In the initiation reaction a carbonate group is fixed at the polyethylene chain-end, and in the termination reaction a saturated end-group is formed. Although these experiments failed to give telechelics they showed that primary radicals from percarbonates will give carbonate end-groups, and telechelics should be obtainable if this side reaction can somehow be avoided. 

Inherent viscosities of the polymer (Table) show that 10 and that polymerization at higher temperatures reduces the molecular weight (Figure 5). Infrared spectral analysis of the polyethylenes obtained in the Table showed no H(R)C=CR2 groups that would result from a-olefin formation then incorporation into growing polymer then chain termination via... 

The complex 1 is heterogeneous and is dark olive in color. The next step is the alkylation at the transition metal 2. Fixation of the ethylene takes place at the f ree co-or nation site ( ) of the titanium . Insertion into the Ti—C bond follows. High-density polyethylene (HDPE) is formed. Scheme 2 shows this chain propagation and the following chain termination steps (a). Hydrogenation terminates the chains, forming saturated polymers and metal-hydride, the latter adding olefin to reactivate the catalyst center (b). As a consequence, the MW of the polymers is decreased. 

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