A synthetic approach to attain precisely controlled methyl branching in polyethylene is described. Model polymers based on polyethylene have been created using acyclic diene metathesis (ADMET) chemistry as the mode of polymerization. Differential scanning calorimetry (DSC) was employed to examine the thermal behavior (melting point, heat of fusion, glass transition temperature) of five model polyethylene polymers wherein a methyl branch was placed on each 9th, 11th, 15th, 19th, and 21st carbon respectively along the backbone. Melting points and heats of fusion decrease as the frequency of precise methyl branching increases. On the other hand, the β glass transition and its change in specific heat are independent of branch frequency. Comparisons of these model polymers with industrial polyethylene samples demonstrate that this polycondensation approach will provide the basis for a better understanding of the morphology, crystalline structure, and thermodynamics of the crystallization process of the most abundant synthetic macromolecule in the world, polyethylene.

The structure of random ethylene/propylene (EP) copolymers has been modeled using step polymerization chemistry. Six ethylene/propylene model copolymers have been prepared via acyclic diene metathesis (ADMET) polymerization and characterized for primary and higher level structure using in-depth NMR, IR, DSC, WAXD, and GPC analysis. These copolymers possess 1.5, 7.1, 13.6, 25.0, 43.3, and 55.6 methyl branches per 1000 carbons. Examination of these macromolecules by IR and WAXD analysis has demonstrated the first hexagonal phase in EP copolymers containing high ethylene content (90%) without the influence of sample manipulation (temperature, pressure, or radiation). Thermal behavior studies have shown that the melting point and heat of fusion decrease as the branch content increases. Further, comparisons have been made between these random ADMET EP copolymers, random EP copolymers made by typical chain addition techniques, and precisely branched ADMET EP copolymers.

A structural investigation of precise ethylene/1-butene (EB) copolymers has been completed using step polymerization chemistry. The synthetic methodology needed to generate four model copolymers is described; their primary and higher level structure is characterized. The copolymers possess an ethyl branch on every 9th, 15th, and 21st carbon along the backbone of linear polyethylene. Melting points and heats of fusion decrease with increased branch frequency. Differential scanning calorimetry and infrared spectroscopy show highly disordered crystal structures favoring ethyl branch inclusion. On the other hand, the EB copolymers contain high concentrations of kink and gauche defects independent of branch frequency. These model copolymers are compared with random copolymers produced using traditional chain chemistry and previously synthesized ADMET EP copolymers.

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