Modeling Branching in Polyethylene

Since ADMET offers a method of producing PE with precisely placed branches of a known and uniform length, it is well suited to model PE systematically to better understand LDPE, LLDPE, and metallocene PE [136]. 

Similarly, a study of ethyl-branched ADMET PE has also been reported [139]. This work showed that these polymers favored branch inclusion, resulting in a crystal structure similar to that obtained for the precisely methyl-branched ADMET polymers. This crystallization behavior, however, was dependent on branch concentration. When the branch content was 111 ethyl branches per 1000 carbons, the resulting polymer was amorphous, indicating that the steric demands of the ethyl branches precluded their participation in crystallization. Conversely, ethyl branch contents of 50-60 branches per 1000 carbons resulted in polymers able to include the ethyl branch defects in the crystal lattice, presumably due to the presence of kink defects able to accommodate the ethyl branches. 

ADMET has also been used to synthesize precise deuterated polymers, with CDg branches precisely placed along a PE backbone [140]. This allowed the study of the molecular motions in the amorphous and crystalline phases by NMR. 

Further work has resulted in ADMET copolymer models of ethylene/l-hexene [141], and ethylene/l-octene [142]. Commercial ethylene/1-octene copolymers represent a significant portion of the linear low-density PE market however, the ADMET model polymers, with hexyl branches on every 9th, 15th, or 21st carbon, displayed much better thermal profiles than those seen with ethylene/l-octene copolymers produced by other methods. Additionally, the thermal properties of these hexyl-branched ADMET polymers were similar to those of their methyl-branched ADMET polymer analogs. This observation points to the possible 

The effect of branch concentration on the material properties of ADMET PEs has also been studied. An initial study reported the precise placement of a butyl branch on every 39th carbon [144]. This was later expanded to a series of polymers synthesized with one of 13 different alkyl branches on every 39th carbon [145]. While all branched polymers displayed a decreased melting temperature relative to linear PE, the larger branches (ethyl through pentadecyl) all showed similar melting points between 70 and 78 °C, while the methyl-branched polymer melted at92°C. 

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