Giovanni Rojas

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

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.

Avoiding Olefin Isomerization During Decyanation of Alkylcyano α,ω-Dienes : A Deuterium Labeling and Structural Study of Mechanism

A two-step synthetic pathway involving decyanation chemistry for the synthesis of pure alkyl alpha,omega-dienes in quantitative yields is presented. Prior methodologies for the preparation of such compounds required 6-9 steps, sometimes leading to product mixtures resulting from olefin isomerization chemistry. This isomerization chemistry has been eliminated. Deuteration labeling and structural mechanistic investigations were completed to decipher this chemistry. Deuterium labeling experiments reveal the precise nature of this radical decyanation chemistry, where an alcohol plays the role of hydrogen donor. The correct molecular design to avoid competing intramolecular cyclization, and the necessary reaction conditions to avoid olefin isomerization during the decyanation process are reported herein.

Quantitative α-Alkylation of Primary Nitriles

Abstract A synthetic pathway that produces alkyl α,ω‐cyanodiolefins in quantitative yield is described, applying chemistry that is based on simple α‐alkylation of alkyl nitriles. Three amide bases, lithium 2,2,6,6‐tetramethylpiperidide, lithium diisopropylamide, and sodium amide, are used to create the α‐carbanions that undergo substitution with various alkylating agents. Optimization leads to essentially quantitative conversions for every substrate/example reported herein, which will prove useful in many synthetic schemes.

PRECISION POLYOLEFIN STRUCTURE: MODELING POLYETHYLENE CONTAINING METHYL AND ETHYL BRANCHES

Sequenced copolymers of ethylene and diverse species have been created using acyclic diene metathesis (ADMET) polymerization, a step growth, conden- sation-type polymerization driven to high conversion by the removal of ethylene. ADMET permits control over branch content and branch length, which can be predetermined during the monomer synthesis, allowing sequence control in the resultant unsaturated polymer. Monomers are symmetrical α,ω-dienes with a pendant functionality. Diverse functional groups are compatible with ADMET polymeri- zation when Schrocks or first-generation Grubbs catalysts are used. Saturation with hydrogen after ADMET polymerization affords a polyethylene (PE) backbone bearing specific functionalities in precise places. Varying both the pendant functional group and the spacing between functionalities alters the physical and chemical properties of the polymer. Incorporation of alkyl chains into the PE backbone via ADMET leads to the study of perfect structures modeling the copolymerization of ethylene with α-olefins such as 1-propene, 1-butene, 1-hexene, and 1-octene.