Polypropylene Long-chain branching

Grafting to create long chain branches employs well-known organic reactions to incorporate polymer chains that have reactive end groups. When there is no suitable reactive group on the backbone that can be attacked directly (as is the case with polymers such as polypropylene,... 

Nam GJ, Yoo JH, Lee JW (2005) Effect of long-chain branches of polypropylene on rheological properties and foam-extrusion performances. J Appl Polym Sci 96 1793-1800... 

Chain branching is a common occurrence during radical polymeriza ] tions and is not restricted to poiyethyiene. Polypropylene, polystyrene, andt poly(methyl methacrylate) all contain branched chains. Studies have shown that short-chain branching occurs about 50 times as often as long-chain branching. 

Molar mass distribution is a dominant microstracture parameter that, in copolymers, needs to be measured with additional information to account for long chain branching, comonomer incorporation, or ethylene propylene combinations (in the case of EP copolymers). The combination of GPC and IR spectroscopy has been shown to be of great value in the characterization of copolymers. The importance of automation and sample care, especially in the case of polypropylene, has been discussed as well as the significant improvement in sensitivity by the use of IR MCT detectors. There are big expectations for the analysis of ultrahigh molar mass polyolefins by the new AF4 technology. 

Among the most commonly utilized synthetic polymers are polyolefins, such as polyethylenes (PEs), polypropylenes (PPs), and ethylene-propylene copolymers (P(E-co-P)s). Despite their simple elemental compositions, consisting of only carbon and hydrogen, it is well known that their physical properties are quite dependent on the microstructural features, such as short- and long-chain branchings, stereoregularities, chemical inversions of monomer enchainment, sequence distributions, etc. ... 

The mechanism on long-chain branch (LCB) formation with coordination catalysts was discussed briefly in Section 2.2 and illustrated in Figure 2.18. LCB formation with coordination catalysts is nothing more than a copolymerization reaction with macromonomers made in the reactor through -hydride elimination and transfer to monomer reactions for polyethylene, and -methyl elimination for polypropylene (Scheme 2.2). At this point, the population balances could be re-derived to include LCB-formation reactions and solved by the method of moments. For brevity, however, only the final results of this derivation will be shown, leading to an analytical solution for the instantaneous distribution of MWD for chains containing LCB derivation details are available in the literature [45-47]. 

Therefore, it is more suitable to measure the shear sensitivity of polypropylene. Capillary rheometers or oscillatory shear flow rheometers are widely used for that purpose. Moreover, an investigation of elongational flow properties of molten PP can be used to check, for example, the presence of long-chain branching in some speciality grades of PP (to study the strain-hardening effect). 

The best foamabOity of PP was found with long-chain branched polypropylenes with only low gel contents in the melt. The first technical process to produce long-chain branched PP was developed by Montell [4]. 

Amorphous polypropylene (aPP) of low molecular weight has been know for over 100 years as an oily or waxy substance of low viscosity. The isotactic polypropylene (iPP), discovered in 1954, has a regular structure, a high degree of order and crystallinity. Recently, two members were added to the industrial polypropylene family isotactic PP with long-chain branching, and syndiotactic polypropylene (sPP), both produced using metallocene catalysts. 

Krauz et al. [61] studied the effects of temperature during the electron beam irradiation of isotactic polypropylene. These workers noted a slight reduction in molar mess, an increase in long-chain branching, and an increase in crystallization temperature with increasing temperature. 

Langston JA, Colby RH, Shimizu F, Suzuki T, Aoki M, Mike Chung TC (2007) Synthesis and characterization of long chain branched isotactic polypropylene (LCBPP) via metallocene catalyst and T-reagent. Macromolecules 40 2712... 

Additionally, metallocene catalysts enable the design of catalysts for tailored polyolefins due to the intrinsic nature of the single site. Actually, new polymers which could never have been produced by conventional Ziegler-Natta catalysts, i.e., syndiotactic polypropylene, syndiotactic polystyrene, long chain branched polyolefins, cyclo-olefin polymer, and styrene copolymers, can be obtained by metallocene catalysts. This upcoming new S curve in the polyolefin development cycle means the evolution of new type of polyolefins. 

Stange, J. and Munstedt, H. 2006. Effect of long-chain branching on the foaming of polypropylene with azodicarbonamide. Journal of Cellular Plastics 42 445-467. 

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