The kinetics of 1-hexene polymerization using a family of five zirconium amine bis-phenolate catalysts, Zr[tBu-ON(X)O]Bn2 (where X = THF (1), pyridine (2), NMe2 (3), furan (4), and SMe (5)), has been investigated to uncover the mechanistic effect of varying the pendant ligand X. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction specific rate constants involved in a given polymerization process. The mechanism of polymerization for 1-5 was similar and the necessary elementary reaction steps included initiation, normal propagation, misinsertion, recovery from misinsertion, and chain transfer. The latter reaction, chain transfer, featured monomer independent β-H elimination in 1-3 and monomer dependent β-H transfer in 4 and 5. Of all the rate constants, those for chain transfer showed the most variation, spanning 2 orders of magnitude (ca. (0.1-10) × 10(-3) s(-1) for vinylidene and (0.5-87) × 10(-4) s(-1) for vinylene). A quantitative structure-activity relationship was uncovered between the logarithm of the chain transfer rate constants and the Zr-X bond distance for catalysts 1-3. However, this trend is broken once the Zr-X bond distance elongates further, as is the case for catalysts 4 and 5, which operate primarily through a different mechanistic pathway. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure-activity relationships.

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The dehydrocoupling of organosilanes is an efficient method for producing polysilanes. Although this is traditionally done with group 4 metallocene catalysts, there are a few examples of nickel catalysts that are effective for this reaction. We report the dehydrocoupling of phenylsilane and phenylmethylsilane with [(dippe)Ni(μ-H)] 2 (1) (dippe=1,2-bis(diisopropylphosphino)ethane). As expected from thermodynamic and steric evidence, the primary silane is more active toward dehydrocoupling than the secondary silane. This catalyst compares favorably in required reaction conditions, molecular weight of polysilane product, and selectivity for linear oligomers. Possible mechanisms for the dehydrocoupling of silane are discussed. The hypothesized intermediate is a hydrido silyl nickel complex. We report the isolation and single-crystal X-ray structure of a stable analogue of the proposed catalytic intermediate, (dippe)Ni(SiCl 3 )Cl (2).

Thermoreversible polymeric biomaterials are finding increased acceptance in tissue engineering applications. One drawback of the polymers is their synthetic nature, which does not allow direct interaction of mammalian cells with the polymers. This limitation may be alleviated by grafting arginine-glycine-aspartic acid (RGD) containing peptides onto the polymer backbone to facilitate interactions with cellsurface integrins. Toward this goal, N-isopropylacrylamide (NiPAM)-based thermoreversible polymers containing amine-reactive N-acryloxysuccinimide (NASI) groups were synthesized. Conjugation of RGD-containing peptides to polymers was demonstrated with 1 H NMR spectroscopy and reverse-phase high-pressure liquid chromatography. The conjugation reaction was optimal at 4°C and pH of 8.0, and increased with the increasing NASI content of polymers. With a peptide grafting ratio of 0.25 mol %, there was no significant change in the lower critical solution temperature of the polymers. Finally, the NASI-containing polymers, cast as films, on tissue culture polystyrene, were shown to conjugate to RGD-containing peptides and support C2C12 cell attachment. We conclude that NASI-containing thermoreversible polymers are amenable for grafting biomimetic peptides to impart cell adhesiveness to the polymers.

Bone Morphogenetic Proteins (BMPs) in combination with biomaterials are being explored for clinical bone regeneration. The current biomaterials used for BMPs delivery are not specifically designed to support BMP-induced osteogenesis. Towards this goal, we designed synthetic N-isopropylacrylamide (NiPAM)-based thermosensitive polymers and investigated their ability to support osteogenic transformation of pluripotent C2C12 cells. Cell attachment to the polymers was limited as compared to attachment to the plastic surfaces optimized for cell culture. Short-term (<7 days) studies indicated relatively little cell growth on the polymer surfaces. However, C2C12 cells retained their ability to respond to BMP-2, as determined by alkaline phosphatase (ALP) induction, when cultured on thermoreversible polymers. Some polymers supported ALP induction that was far superior ($10-fold) to cells grown on tissue culture surfaces. We conclude that thermosensitive polymers, although limited in their ability to support cell attachment and growth, did support the pluripotent cells' ability to be transformed under the influence of BMP-2. The ALP induction was dependent on the compositional details of the polymers, suggesting that in vivo osteoinduction was likely to be influenced by the physicochemical properties of the polymers.