Binuclear initiators for the telechelic synthesis of elastomeric polyolefins.

Novel binuclear complexes, 4,4'-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-κ(2)-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1'-biphenyl (2a) and 4,4'-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-4-methylpentamidato-κ(2)-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1'-biphenyl (2b), were synthesized by linking two nickel centers through a bis(benzyl) fragment. When activated with nickel bis(1,5-cyclooctadiene) (Ni(COD)(2)), 2a and 2b are capable of polymerizing ethylene in a quasi-living fashion, producing polymers with approximately twice the molecular weights relative to those obtained by using a structurally related mononuclear system. In addition, 2b/Ni(COD)(2) was utilized to synthesize a series of pseudo-triblock polyethylene (PE) macroinitiating copolymers, bearing atom-transfer radical polymerization (ATRP) initiators. Pseudo-pentablock copolymers were also prepared by taking advantage of a pressure-pulsing technique, wherein the ethylene pressure was increased from 100 to 500 psi in order to produce semicrystalline ethylene-rich end-blocks. Copolymers with elastomeric properties were synthesized by grafting n-butyl acrylate from the PE macroinitiators via ATRP. Examination using monotonic and step-cyclic stress-strain tests demonstrates that the materials exhibit large strains at break (1600-2000%) and excellent elastic recoveries at large strains (∼80%). That materials with such desirable properties could not be attained using a mononuclear initiator demonstrates the clear advantage of growing the polymer via a telechelic mechanism.

Ketene functionalized polyethylene: control of cross-link density and material properties.

The functionalization and cross-linking of polyethylene is synthetically challenging, commonly relying on highly optimized radical based postpolymerization strategies. To address these difficulties, a norbornene monomer containing Meldrum's acid is shown to be effectively copolymerized with polyethylene using a nickel α-iminocarbaxamidato complex, providing high-melting, semicrystalline polymers with a tunable incorporation of the functional comonomer. Upon heating the copolymer to common polyethylene processing temperatures, the thermolysis of Meldrum's acid to ketene provides the desired reactive group. This simple and versatile methodology does not require small molecule radical sources or catalysts, and the dimerization of the in situ generated ketenes is shown to provide tunable cross-linking densities in polyethylene. Subsequent rheological and tensile experiments illustrate the ability to tune cross-linked polyethylene properties by comonomer incorporation and elucidate valuable structure/property relationships in these materials. This study illustrates the power of well-defined and synthetically accessible functional groups in polyolefin synthesis and functionalization.

New polyethylene macroinitiators and their subsequent grafting by atom transfer radical polymerization.

A semicrystalline polyethylene (PE) macroinitiator was prepared by copolymerizing ethylene and an inititating monomer (5-norbornen-2-yl-2'-bromo-2'-methyl propanoate) (3) using [N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)isobutanamidato]-Ni(eta1-CH2Ph)(PMe3) (1) and Ni(COD)2 (bis(1,5-cyclooctadiene)-nickel) (2). Although 3 decomposes Ni(COD)2, if the initiating species (1/2) are exposed to ethylene for a period of time, t1, and then 3 is introduced for another period of time, t2, the polymerization takes place in a controlled manner. Variations in temperature, pressure, and [3] afford a great deal of control over the composition and architecture of the PE macroinitiator. Subsequent grafting with methyl methacrylate (MMA) yields PE-graft-PMMA with narrow polydispersities and increasing PMMA content at longer reaction times. Because the products are semicrystalline materials with high melting points, they are anticipated to function as blend compatibilizers for linear polyethylene.

Nuclear spin relaxation and molecular interactions of a novel triazolium-based ionic liquid.

Nuclear spin relaxation, small-angle X-ray scattering (SAXS), and electrospray ionization mass spectrometry (ESI-MS) techniques are used to determine supramolecular arrangement of 3-methyl-1-octyl-4-phenyl-1H-triazol-1,2,3-ium bis(trifluoromethanesulfonyl)imide [OMPhTz][Tf2N], an example of a triazolium-based ionic liquid. The results obtained showed first-order thermodynamic dependence for nuclear spin relaxation of the anion. First-order relaxation dependence is interpreted as through-bond dipolar relaxation. Greater than first-order dependence was found in the aliphatic protons, aromatic carbons (including nearest neighbors), and carbons at the end of the aliphatic tail. Greater than first order thermodynamic dependence of spin relaxation rates is interpreted as relaxation resulting from at least one mechanism additional to through-bond dipolar relaxation. In rigid portions of the cation, an additional spin relaxation mechanism is attributed to anisotropic effects, while greater than first order thermodynamic dependence of the octyl side chain's spin relaxation rates is attributed to cation-cation interactions. Little interaction between the anion and the cation was observed by spin relaxation studies or by ESI-MS. No extended supramolecular structure was observed in this study, which was further supported by MS and SAXS. nuclear Overhauser enhancement (NOE) factors are used in conjunction with spin-lattice relaxation time (T1) measurements to calculate rotational correlation times for C-H bonds (the time it takes for the vector represented by the bond between the two atoms to rotate by one radian). The rotational correlation times are used to represent segmental reorientation dynamics of the cation. A combination of techniques is used to determine the segmental interactions and dynamics of this example of a triazolium-based ionic liquid.

Living polymerization of ethylene and alpha-olefins using a nickel alpha-keto-beta-diimine initiator.

A nickel alpha-keto-beta-diimine initiator can be activated using treated methylaluminoxane (MAO) to generate active sites capable of the polymerization of ethylene, propene and 1-hexene under living conditions.