Kenneth B. Wagener

ACYCLIC DIENE METATHESIS (ADMET) POLYMERIZATION

1,9-Decadiene has been converted quantitatively to poly(octenylene), and 1,5-hexadiene has been converted to exclusively 1,4-polybutadiene by acyclic diene metathese polymerization, using a catalyst free of Lewis acids. Poly(octenylene) exhibits a minimum Mw of 108000 and is more than 90% trans in its stereochemistry. Exclusively 1,4-polybutadiene is of a minimum Mw of 28000 and is more than 70% trans in its stereochemistry. The stereochemistry appears to be controlled thermodynamically due to the equilibrium nature of the polymerization

Nanoscale Morphology in Precisely Sequenced Poly(ethylene-co-acrylic acid) Zinc Ionomers

The morphology of a series of linear poly(ethylene-co-acrylic acid) zinc-neutralized ionomers with either precisely or randomly spaced acid groups was investigated using X-ray scattering, differential scanning calorimetry (DSC), and scanning transmission electron microscopy (STEM). Scattering from semicrystalline, precise ionomers has contributions from acid layers associated with the crystallites and ionic aggregates dispersed in the amorphous phase. The precisely controlled acid spacing in these ionomers reduces the polydispersity in the aggregate correlation length and yields more intense, well-defined scattering peaks. Remarkably, the ionic aggregates in an amorphous, precise ionomer with 22 mol % acid and 66% neutralization adopt a cubic lattice; this is the first report of ionic aggregate self-assembly onto a lattice in an ionomer with an all-carbon backbone. Aggregate size is insensitive to acid content or neutralization level. As the acid content increases from 9.5 to 22 mol % at approximately 75% neutralization, the number density of aggregates increases by approximately 5 times, suggesting that the ionic aggregates become less ionic with increasing acid content.

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.

Synthesis and Catalyst Issues Associated with ADMET Polymerization

Acyclic diene metathesis (ADMET) is a flexible approach for the production of diverse polymeric materials. The advent of well-defined catalysts and the simplification of laboratory techniques has made the ADMET reaction useful for many applications, such as polyolefin model studies and the synthesis of organic/inorganic hybrid polymers, telechelics, copolymers, conjugated polymers, liquid crystalline polymers, and amino acid-based chiral polymers. Many of the polymer architectures that have been produced using ADMET cannot be made by other means.

Ionic Aggregate Structure in Ionomer Melts: Effect of Molecular Architecture on Aggregates and the Ionomer Peak

We perform a comprehensive set of coarse-grained molecular dynamics simulations of ionomer melts with varying polymer architectures and compare the results to experiments in order to understand ionic aggregation on a molecular level. The model ionomers contain periodically or randomly spaced charged beads, placed either within or pendant to the polymer backbone, with the counterions treated explicitly. The ionic aggregate structure was determined as a function of the spacing of charged beads and also depends on whether the charged beads are in the polymer backbone or pendant to the backbone. The low wavevector ionomer peak in the counterion scattering is observed for all systems, and it is sharpest for ionomers with periodically spaced pendant charged beads with a large spacing between charged beads. Changing to a random or a shorter spacing moves the peak to lower wavevector. We present new experimental X-ray scattering data on Na(+)-neutralized poly(ethylene-co-acrylic acid) ionomers that show the same two trends in the ionomer peak, for similarly structured ionomers. The order within and between aggregates, and how this relates to various models used to fit the ionomer peak, is quantified and discussed.

Decreasing the alkyl branch frequency in precision polyethylene: pushing the limits toward longer run lengths.

A symmetrical α,ω-diene monomer with a 36 methylene run length was synthesized and polymerized, and the unsaturated polymer was hydrogenated to generate precision polyethylene possessing a butyl branch on every 75th carbon (74 methylenes between branch points). The precision polymer sharply melts at 104 °C and exhibits the typical orthorhombic unit cell structure with two characteristic wide-angle X-ray diffraction (WAXD) crystalline peaks observed at 21.5° and 24.0°, corresponding to reflection planes (110) and (200), respectively.

The polymerization of dicyclopentadiene: an investigation of mechanism

Abstract This contribution presents further information about the mechanism of the cross-linking reaction which occurs during the polymerization of dicyclopentadiene with the classical catalyst, WCl6/Et2AlCl (1), and the well-defined preformed alkylidene, Mo(N-2,6-C6H3-i-Pr)(CHC(CH3)2Ph)(OCCH3(CF3)2)2 (2). When the classical system 1 was used as a catalyst, insoluble polymer was formed in all cases. However, when molybdenum catalyst 2 is employed, solution concentration determines whether soluble or insoluble polymer will form. The formation of insoluble material is attributed to an olefin addition process catalyzed by the heat released upon ring-opening metathesis polymerization of the norbornene subunit of the monomer. If the heat is removed from the polymerization system through dilution or by cooling the solution, soluble linear polymer is formed. These results suggest that an olefin addition process is at least partly responsible for the cross-linking reaction that occurs during the polymerization of dicyclopentadiene. All attempts to cross-link oligomers of linear polydicyclopentadiene with the well-defined molybdenum alkylidene 2 resulted in only the recovery of soluble polymer. With the classical catalyst system 1, insoluble material was obtained, which was assumed to be cross-linked through olefin addition. These results disprove the idea that metathesis cross-linking can be induced by a critical chain length or concentration of polydicyclopentadiene. On the contrary, no indication of metathesis cross-linking was observed whatsoever for these polymerization systems.

Local and Collective Motions in Precise Polyolefins with Alkyl Branches: A Combination of 2H and 13C Solid-State NMR Spectroscopy†

Branching out: The mobility of linear polymers changes upon branching, which has a pronounced effect on processability and drawability. Regularly branched model polyolefins were studied by advanced solid-state NMR spectroscopy, and twist defects around the branches in the crystalline regions are identified. For lower branch content, the twisting motions are decoupled; for higher content, collective motion is found (see picture).

Cyclic polymers from alkynes

Cyclic polymers have dramatically different physical properties compared with those of their equivalent linear counterparts. However, the exploration of cyclic polymers is limited because of the inherent challenges associated with their synthesis. Conjugated linear polyacetylenes are important materials for electrical conductivity, paramagnetic susceptibility, optical nonlinearity, photoconductivity, gas permeability, liquid crystallinity and chain helicity. However, their cyclic analogues are unknown, and therefore the ability to examine how a cyclic topology influences their properties is currently not possible. We have solved this challenge and now report a tungsten catalyst supported by a tetraanionic pincer ligand that can rapidly polymerize alkynes to form conjugated macrocycles in high yield. The catalyst works by tethering the ends of the polymer to the metal centre to overcome the inherent entropic penalty of cyclization. Gel-permeation chromatography, dynamic and static light scattering, viscometry and chemical tests are all consistent with theoretical predictions and provide unambiguous confirmation of a cyclic topology. Access to a wide variety of new cyclic polymers is now possible by simply choosing the appropriate alkyne monomer.

Precision sulfonic Acid ester copolymers.

Linear ethylene copolymers containing sulfonic acid ethyl esters precisely spaced on every 21st carbon have been synthesized using metathesis polycondensation chemistry. These precision structures with one directly attached and one aromatic spaced sulfonic acid ester are synthesized with the goal of tailoring layered higher order morphologies in contrast to conventional clustered ionic polyolefins. Primary structural characterization confirms the precision polymer structures. Additional secondary microstructural analysis by DSC shows a recoverable endothermic melt transition of polyethylene-like lamellae crystallites of the directly attached ester while completely amorphous behavior is observed when the ester is spaced away from the backbone with an aromatic group.