High pressure free-radical

CSM products may be divided into three groups depending on the type of precursor resin low density (LDPE), high density (HDPE), and linear low density (LLDPE). LDPE is made by a high pressure free-radical process, while HDPE and LLDPE are made via low pressure, metal coordination catalyst processes (12) (see Olefin polymers). 

High pressure free radical process is not suitable for propylene due to extensive hydrogen transfer to free radical which results in resonance stabilised alkyl radicals with reduced tendency to... 

Comparison of High-Pressure Free Radical Process and Low-Pressure Gas-Phase Process... 

Polyethylenes produced commercially via high pressure, free radical processes have densities around 0.92 g/cc and are referred to simply as "low density" polyethylenes. It has been well established from infrared measurements that these low density polyethylenes possess appreciable quantities of ethyl and butyl branches (1-3) but it was not until C-13 NMR became available that an absolute identification, both qualitatively and quantitatively, of the short branches became possible (4-8). Long chain branching is also present in high pressure process low density polyethylenes and carbon-13 NMR was useful here also in establishing the identity and relative amounts of long versus short chain branches (9-11). 

Application To produce low-density polyethylene (LDPE) homopolymers and EVA copolymers using the high-pressure free radical process. Large-scale tubular reactors with a capacity in the range of 130-400 Mtpy, as well as stirred autoclave reactors with capacity around 100 Mtpy can be used. 

Historically, high-pressure free radical copolymerization has been used to produce highly branched, ill-defined copolymers of ethylene and various polar monomers. Although these materials are in production and extensively used throughout the world, the controlled incorporation of polar functionality coupled with linear polymer structure is still desired to improve material properties. Recent focus in this area has led to the development of new transition metal catalysts for ethylene copolymerization however, due to the electro-philicity of the metal centers in these catalysts, polar functional groups often coordinate with the metal center, effectively poisoning the catalyst. There has b een some success, but comonomer incorporation is hard to control, leading to end-functionalized, branched polyethylenes [44, 46]. These results are undesirable due to low incorporation of polar monomer into the polymer as well... 

If the monomer and polymer are not mutually soluble, the bulk reaction mixture will be heterogeneous. The high pressure free radical process for the manufacture of low density polyethylene is an example of such reactions. This polyethylene is branched because of self-branching processes illustrated in reaction (6-89). Branches longer than methyls cannot fit into the polyethylene crystal lattice, and the solid polymer is therefore less crystalline and rigid than higher density (0.935-0.96 g cm ) species that are made by coordination polymerization (Section 9.5). 

Although the density of the polymer can be varied by copolymerization with higher olefins to match that of polyethylene produced by high-pressure free-radical processes, the two types differ in branch frequency and character and in molecular weight distributions. As a result, they do not have comparable processing and mechanical properties. 

McCord, E.F. Shaw, W.H., Jr. Hutchinson, R.A. Short-chain branching structures in ethylene copolymers prepared by high-pressure free-radical polymerization an NMR analysis. Macromolecules 1997, 30, 246-256. 

Polyethylene is commercially produced either by a high-pressure free-radical polymerization or by a Ziegler-Natta or Unipol catalytic process. The polyethylene produced in these processes typically has not only a distribution of molecular weights, but also a distribution in backbone structure or branch content. Therefore, it should be possible to fractionate the parent material with respect to either molecular weight or backbone structure although, as subsequently described, the experimental techniques used to carry out these two types of fractionations are quite different. Let us first consider the properties of polyethylene that fix the operating philosophy used to fractionate with respect to backbone structure. 

Section 4.6.2 illustrates the experimental procedures that have recently been applied toward the study of high-pressure free-radical polymerization processes. Section 4.6.3 presents results of propagation, termination, chain-transfer (to monomer and to polymer), and P-scission rate coefficients for ethene homopolymerization. Recent results from experiments and modeling investigations into high-pressure copolymerizations (with ethene being one of the monomers) are reported in Section 4.6.4, together with information on homopolymerization rate coefficients of the comonomer species. 

Another family of low density polyethylenes (LDPE) can be obtained by high-pressure free radical polymerization, resulting in complex microstructures where side chain branches (mainly ethyl and butyl) are obtained through chain transfer reactions without the need of comonomer incorporation. The presence of long... 

The next major step, almost two decades after the discovery by ICI of then-high-pressure free radical polymerization of ethylene, was the discovery, from 1951, of (metal oxide and organo-metallic) catalysts that produced essentially linear high molecular weight polyethylene (and other polyolefins) under much lower pressures. These catalyst discoveries occurred almost simultaneously and independently in several laboratories in the USA and Europe [12]. 

Table 3. Typical free radical initiators used in the high temperature and high pressure free-radical po merization of PE ( from ref. 17).

To better understand the fundamental and practical differences between step-growth polymerization and chain-growth polymerization (see Table 1.4), consider the industrial chain-growth polymerization of ethylene (by either coordination polymerization or high-pressure free-radical polymerization) to produce polyethylene. 

The more conventional type branched polyethylenes that are formed by high pressure free radical polymerization, and are copolymers from a crystallization point of view, are also known to form gels.39>40 We have confirmed this phenomenon and a scanning electron micr< aph of such a gel is given in Figure 7b. ... 

Ehrlich, P. and Mortimer, G. A. (1970) Fundamentals of the free radical polymerization of ethylene,/4/v. Polym. Sci., 7, 386-448. (b) Buback, M., and Droge, T. (1997), High-pressure free-radical... 

The elastomers listed in Table 1 are typically prepared by high pressure, free-radical, aqueous emulsion polymerization (5,8,36,37). The initiators are organic or... 

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