Branching Ethylene-Higher Olefin Copolymers

Reproduced with permission from F.A. Bovey, F.G. Schilling, F.L. McCrackin and H.L. Wagner  

An introduction of branching, either long or short, will create additional resonances to those described above. 

De Footer and co-workers [19] applied C-NMR spectroscopy at sample temperatures of 130 C to branching studies of copolymers ethylene with 1-10 mole% of propylene, butene-, hexene-1, octene-1 and 4 methyl pentene-1. C-NMR spectra were recorded with proton noise decoupling to remove unwanted scalar 

This investigation includes the chemical shift assignments (relative to that of the isolated methylene carbons at 30.0 ppm) and a set of experimental conditions for the quantitative analysis of branching in the commercial types of linear low-density polyethylenes. 

The integration limits given in Table 10.3 not only take into account the isolated branches, but also branches which are separated by one ethylene unit and branches next to each other. These latter two structures occur very infrequently in most commercial products but are significant to the calculations. 

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Short Chain Branching in Ethylene-Higher Olefin Copolymers... 

The xmiform branching distribution found in this new type of polyethylene, and the relatively high reactivity of these catalysts with higher 1 -olefins, has allowed the manufacture of ethylene/1-olefin copolymers containing very high levels of comonomer such as 1-butene and 1-hexene, i.e., 5-20 mol% comonomer. Consequently, the density range of the polyethylene... 

In order to improve the physical properties of HDPE and LDPE, copolymers of ethylene and small amounts of other monomers such as higher olefins, ethyl acrylate, maleic anhydride, vinyl acetate, or acryUc acid are added to the polyethylene. Eor example, linear low density polyethylene (LLDPE), although linear, has a significant number of branches introduced by using comonomers such as 1-butene or 1-octene. The linearity provides strength, whereas branching provides toughness. 

Carbon-13 nuclear magnetic resonance NMR) is important in imder-standing more detailed structural information in the backbone of the polyethylene sample. For example, l.T.DPE is produced commercially with either 1 -butene, 1 -hexene or 1 -octene as the comonomer. Copolymers with a low content of 1-olefin contain only isolated branches, however copolymers containing higher levels of comonomer (e.g., 2-20 mol%) contain a wide variety of complex sequence distributions making C NMR a particularly important characterization tool. Information into the sequence distribution of the comonomer in ethylene/1-olefin pentads provides data to determine the reactivity constants K, k, k and k, where e represents ethylene and h represents 1-hexene. 

VI. COPOLYMERS OF ETHYLENE WITH HIGHER OLEFINS AND BRANCHING IN POLYETHYLENE... 

Statistical copolymers of the types described in Chapter 8 tend to have broader glass transition regions than homopolymers. The two comonomers usually do not fit into a common crystal lattice and the melting points of copolymers will be lower and their melting ranges will be broader, if they crystallize at all. Branched and linear polyethylene provide a case in point since the branched polymer can be regarded as a copolymer of ethylene and higher 1-olefins. 

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