Precision Polyethylene: Changes in Morphology as a Function of Alkyl Branch Size

Abstract

Metathesis polycondensation chemistry has been employed to control the crystalline morphology of a series of 11 precision-branched polyethylene structures, the branch being placed on each 21st carbon and ranging in size from a methyl group to an adamantyl group. The crystalline unit cell is shifted from orthorhombic to triclinic, depending upon the nature of the precision branch. Further, the branch can be positioned either in the crystalline phase or in the amorphous phase of polyethylene, a morphology change dictated by the size of the precision branch. This level of morphology control is accomplished using step polymerization chemistry to produce polyethylene rather than conventional chain polymerization techniques. Doing so requires the synthesis of a series of unique symmetrical diene monomers incorporating the branch in question, followed by ADMET polymerization and hydrogenation to yield the precision-branched polyethylene under study. Exhaustive structure characterization of all reaction intermediates as well as the precision polymers themselves is presented. A clear change in morphology was observed for such polymers, where small branches (methyl and ethyl) are included in the unit cell, while branches equal to or greater in mass than propyl are excluded from the crystal. When the branch is excluded from the unit cell, all such polyethylene polymers possess essentially the same melting temperature, regardless of the size of the branch, even for the adamantyl branch.

Synthesis and Crystallization of Precision Polymers with Repetitive Folding Elements

“…The presence of two glass transition temperatures already hints at the formation of two segregated levels of hierarchy, one from the packing of the methylene units, the other from the packing of the DAP units outside of a presumed lamellar crystal. [ 42,45 ] A similar behavior also was observed by Wagener and co-workers on their alkyl-branched polyethylene, whereby the branch size varied from methyl substituents to the pentadecyl substituent. The thermal data of their polymers are consistent with the experimental results we obtained from the thermal analysis of our polymer 4 .…”

“…The obtained solution was poured into a dialysis tube, which was placed in a beaker fi lled with methanol and stirred for 3 d. The methanol was exchanged every day, so to promote dialysis and removal of all low-molecular-weight reagents. Afterward, the content of the tube was collected, the solvent was removed in vacuum at 45 …”

Recent developments in the synthesis of sequence controlled polymers

“…Wagener and coworkers [66,67] synthesized a series of ,-dienes with pendant groups ranging from various alkyl to protected carboxylic groups. These monomers underwent ADMET to afford model polyethylene (after hydrogenation) with precisely located side groups ( Figure 16).…”

Morphology control in precision polyolefins

“…When the branch is excluded, the crystal lattice is orthorhombic. For the series of polymers with a branch on every 21st carbon, three branch types were included in the unit cell: methyl, gem-dimethyl, and ethyl [26]. The polymers with branches included in the unit cell display decreasing melting temperatures with increasing branch size.…”

“…To answer this question, a series of precise methyl-branched polymers was synthesized (Table from slide 30), ranging from having a methyl branch on every fifth carbon, to having a branch on every Table 1 Thermal and crystallographic data of alkylbranched precision polyethylene [26,28,29] Alkyl branch on every 21st carbon Alkyl branch Alkyl branch on every 39th carbon…”