Branching in polyethylene

A regression analysis of infrared (IR), differential thermal analysis and X-ray diffraction data by Laiber and co-workers [7] for low-pressure polyethylene showed that, as synthesis conditions varied, the number of methyl groups varied from 0 to 15 per 1000 carbon atoms and the degree of crystallinity varied from 84 to 61%. 

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How do chain transfer events generate long chain branches in polyethylene ... 

Branching in polymers plays a direct role in their crystallizability, as packing configurations are directly influenced by the nature and frequency of branches along a given polymer backbone. Much has been done in the past to understand the effect of randomly placed branches on the chain, yet no study has been performed on perfectly controlled branching in polyethylene. 

With regard to branching in polyethylene, I was wondering if you have arranged your experiments in such a way that NOE s and T] s are taken into account so you can assume... 

Casassa and Tagami (50) have given a theory for the partition of macromolecules, linear and branched, between a solution and cavities (such as the pores of a GPC gel). They have calculated elution volumes from their results, which agree closely with those derived from the hydrodynamic volume assumption this gives theoretical support to the use of hydrodynamic volume as a correlating parameter for branched polymers. This parameter has been used in studies of branching in polyethylene (Section 10). 

However, it was not until the appearance in 1953 of an important group of papers from the laboratories of Du Pont that any satisfactory evidence became available concerning the nature of the branches in polyethylenes of this type, the low-density polyethylenes (LDPE). Roedel (6) showed that the free-radical polymerization mechanism could be expected to lead not only to short branches containing a few carbon atoms (with which this review is not concerned) but by a mechanism first proposed by Flory (4) and involving the... 

Despite the considerable effort that has been expended on studying branching in polyethylene, it can hardly be claimed that it is at present possible to determine the degree of LCB in this polymer unambiguously. 

Characterization of Long-Chain Branching in Polyethylenes Using High-Field Carbon-13 NMR... 

Figure 15 Formation of branches in polyethylene by chain walking of diimine-nickel catalyst.

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

In terms of characterizing the microstrac-ture of polymer chains, the two most useful techniques are infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectroscopy. Commercial infrared spectrometers were introduced after the end of the second world war and quickly became the workhorse of all polymer synthesis laboratories, providing a routine tool for identification and, to a certain degree, the characterization of microstructure (e.g., the detection of short chain branches in polyethylene). In this regard it can no longer compete with the level of detail provided by modem NMR methods. Nevertheless, IR remains useful or more convenient for certain analytical tasks (and a powerful tool for studying other types of problems). So here we will first describe both techniques and then move on to consider how they can be applied to specific problems in the determination of microstructure. 

Moore, L. D. Relations among melt viscosity, solution viKosity, molecular weight, and long-chain branching in polyethylene. J. Polymer Sci. 36, 155 (1959). 

Randall, J. C. Characterization of long-chain branching in polyethylenes using high-held. C-NMR. ACS Symp. Ser. 1980, 142, 93-118. 

Gabriel, C. Milnstedt, H. Influence of long-chain 84. branches in polyethylenes on linear viscoelastic flow properties in shear. Rheol. Acta 2002,... 

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