Methyl-branched polyethylene

Scheme 2 Proposed mechanism of ethylene polymerization by late-tiansition metal pre-catalysts, indicating the formation of linear polyethylene, methyl-branched polyethyltmes or various alkyl-branched polyethylenes. [C] coordination of ethylene, [7] chain propagatirai through ethylene insertion into M-polymeric alkyl group, [5] chain ismneiizatirai by p- or other (y, 5, e.., )-hydrogen elimination...

Random methyl branch placement on the polyethylene backbone... 

This thermal behavior is completely opposite from the model polyethylenes that we have synthesized via ADMET. All the endotherms for the methyl-branched ADMET polyethylenes are sharp in comparison with their chain-made... 

As shown earlier, the melting point for perfectly linear ADMET polyethylene (Table 8.1) approaches the range of melting point values exhibited by industrially produced, highly linear polyethylenes.40 However, for ADMET polyethylene with a methyl branch placed on every ninth carbon, the 7m is lowered significantly. The melting point of — 14°C and enthalpy of 28 J/g is 150°C and approximately... 

Fig. 5. 13C NMR spectrum of methyl-branched polyethylene generated by ADMET... 

A fourth mechanism has been suggested by several researchers in order to explain the formation of near-linear polyethylene type oligomers produced on modified[22] and unmodified1231 ZSM-5 zeolite from olefins such as propene, isobutene and 1-decene. They propose a mechanism which proceeds by losing methyl branches via protonated cyclopropyl intermediates as presented in Scheme 6.4. 

Complex 17a displayed moderate catalytic activity toward the polymerization of ethylene (3.3 x 105 g/molh 1). In addition, higher molecular weight distributions were observed (Mw/Mn = 12.8). The 13C NMR analysis of the polyethylene showed that methyl branches predominate (with ca. 3.4 methyl branches per 1000 carbon atoms), suggesting that chain walking does not affect polymerization to a high degree. When only the pyridine moiety (and not the imidazolium salt) is ligated (17b) [48], ethylene polymerization occurs twice as effectively (6 x 105 g PE/(mol of Ni) h 1) under similar conditions (only 30 min rather than 60 min). 

Scheme 4 Precise methyl branching in polyethylene (from [73])...

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). 

These materials can be considered linear copolymers of ethylene and propylene or precisely methyl-branched polyethylene. In addition, copolymerizations of the methyl-containing monomers with 1,9-decadierie yield polymers with lower propylene content [50]. These materials are of great interest to the polyolefin community, especially in the physical understanding of the effects of branching on physical properties. Polyethylenes with a variety of main chain functionality have also been synthesized and analyzed [51-54]. 

Some methods, originally developed for the determination of the degree of branching in polyethylene, were extended to C2-C3 copolymers with low C3 content. One such method, proposed by Slovinski et al. (71), involves measuring the absorbance ratio of the bands at 7.25 and 7.30 p. The calibration curve reported by these authors, however, is not correct4, since they used three hydrocarbons (octacosane, hexatriacontane and octapentacontane) which contain only terminal methyl groups and these have a much lower absorptivity (a little more than one half) than methyl branches (39). A similar, but more reliable method was proposed by Rohmer (64) she uses the same... 

Silica supported chromium catalysts that polymerize ethylene to polyethylene with as many as 12 methyl branches/1000 carbon atoms have been reported. The small amount of branching observed in the ethylene homopolymers prepared by these supported chromocene catalysts was attributed to a chain isomerization process (a) Karol, F. J. Karapinka, G. L. Wu,... 

Smith JA, Brzezinska KR, Valenti DJ, Wagener KB (2000) Precisely Controlled Methyl Branching in Polyethylene via Acyclic Diene Metathesis (ADMET) Polymerization. Macromolecules 33 3781-3784. 

In this study, [9] model polyethylenes were made with precise placement of methyl branches on every 9, 11, 15, 19, and carbon atom of the backbone (Figure 2,... 

Figure 3. DSC visual overlay of melt transitions for methyl branched ADMET polyethylenes. Mn data were obtained by GPC using chloroform as solvent and polystyrene standards. Reproduced with permission from Macromolecules 2000,33, 3781-3794. Copyright 2000 Am. Chem. Soc.

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