Branching in Ethylene-propylene Copolymers

Sworen, J.C., Smith, J.A., Wagener, K.B., Baugh, L.S., Rucker, S.P., Modeling random methyl branching in ethylene/propylene copolymers using metathesis chemistry synthesis and thermal behavior, J. Am. Chem. Soc. 2003, 125 2228-2240. 

Natta and co-workers [5] determined the degree of alternation of ethylene and propylene units in ethylene-propylene copolymers from the infrared spectrum, using peaks at 13.35, 13.70 and 13.83 pm, the one at 13.70 pm being attributed to a sequence of three methylene groups between branch points, presumably due to the insertion of one ethylene between two similarly oriented propylene molecules ... 

Randall [56] has developed a C-NMR quantitative method for measnring ethylene-propylene mole fractions and methylene number-average sequence lengths in ethylene-propylene copolymers. He views the polymers as a snccession of methylene and methyl branched methine carbons, as opposed to a succession of ethylene and propylene units. 

Figure 19 (a) Peak melting temperature as a function of the branch content in ethylene-octene copolymers (labelled -O, and symbol —B (symbol, ) and -P (symbol, A) are for ethylene-butene and ethylene-propylene copolymers, respectively) and obtained from homogeneous metallocene catalysts show a linear profile, (b) Ziegler-Natta ethylene-octene copolymers do not show a linear relationship between peak melting point and branch content [125]. Reproduced from Kim and Phillips [125]. Reprinted with permission of John Wiley Sons, Inc. 

From Mark s RIS model for ethylene-propylene copolymers (J. Chem. Phys. 1972, 57, 2541) it is determined that P(t) = 0.380, P g+) = 0.014, and Pig") = 0.606 in 2,4-dimethylhexane (2,4-DMH). Using this RIS model, furthermore, for all the branched alkanes considered whose isopropyl groups are separated by at least one methylene carbon from the next substituted carbon and the RIS model developed by Asakura et at. (Makromol. Chem, 1976, 177, 1493) for head-to-head polypropylene to treat 2,3-dimethyl pentane, AS s are calculated for a large number of branched alkanes. The agreement between the observed and the calculated nonequivalent 13C NMR chemical shifts is quite good, including the prediction that separation of the isopropyl group from the next substituted carbon by four or more methylene carbons removes the nonequivalence. 

Ethylene/propylene copolymers made with these nickel catalysts contain up to 6 mol % propylene. When the nickel catalysts were combined with supported chromium catalysts, branched polyethylene (5.0 methyl-ended branches per 1000 carbon atoms) was produced by the chromium copolymerizing ethylene with the a-olefins that were produced in situ by the nickel catalyst. Like the catalysts above, nickel catalysts with anionic ligands may themselves be supported on inorganic supports and polymeric supports. " ... 

More recently, this group has also reported the identification of new structures formed by radiolysis of fluorinated ethylene-propylene copolymers The changes to the spectra are similar to those reported for irradiated PTFE by Fuchs and Scheler, and thus the radiation chemistry of these two polymers is similar. Irradiation at low temperatures resulted in the formation of new -CF3 chain ends, while irradiation in the melt (523 K) resulted in the formation of long-chain branches. However, it was clear that chain-scission reactions were still dominant at these higher temperatures. 

If the suitable functionality (halogen) is not present originally in the polymer molecule, it can be introduced by suitable post-polymerization techniques. For example, polystyrene branches can be grafted onto ethylene-propylene rubber after chlorinating the rubber. Ethylene-propylene copolymer contains tertiary hydrogens which can be readily exchanged for chlorine. Subsequently the tertiary chlorines are easily activated by complexation with A1(C2H5)2C1 and the macro-cation formed is eminently suitable for the polymerization-initiation of, say, styrene. 

It can be seen that major differences occur in the products of thermal degradation that are obtained for these three similar polymers. PE produces major amounts of normal to Cg alkanes and minor amounts of 2-methyl and 3-methyl compounds such as isopentane and 3-methylpentane, indicative of short-chain branching on the polymer backbone. For PP, branched alkanes predominate, these peaks occurring in regular patterns, e.g., 2-methyl, 3-ethyl, and 2,4-dimethylpentane and 2,4-dimethylheptane, which are almost absent in the PE pyrolysate. Minor components obtained from PP are normal paraffins present in decreasing amounts up to -hexane. This is to be contrasted with the pyrogram of PE, where n-alkanes predominate. The ethylene-propylene copolymer, as might be expected, produces both normal and branched alkanes. The concentrations of 2,4-dimethylpentane and 2,4-dimethylheptane are lower than those that occur in PP. 

Figure 4. 15) M. Brookhart s new late transition metal systems using Pd and Ni. 16) Ethylene/Propylene copolymers from the work of Wunderlich. 10) J. E. O Gara and K. B. Wagener PE produced by ADMET polycondensation. 13, 14) Kaminsky, Cecchin, and Zucchini s work reviews on metallocene PE catalysis. 11, 12) J. D. Hoffman s equilibrium values derived for an infinitely long PE chain. 9) Work in this study-PE model polymers made by ADMET with precise placement of methyl branches along the backbone.

In 1948, Dr. Natta concluded that "a revolution will be marked by the development of processes that lead to tiie formation of macromolecules having a predetermined structure. They will make some branches of industry independent of agriculture and increase the area of land used for tiie production of food." His contributions to pol3rpropylene plastics, film and fibers as well as ethylene-propylene copolymers and polyisoprene elastomers have... 

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