Johannes Heinemann

Polyolefin nanocomposites formed by melt compounding and transition metal catalyzed ethene homo- and copolymerization in the presence of layered silicates

Nanocomposites of high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and highly branched polyethylene rubbers were prepared both by means of melt compounding and ethene homo- and copolymerization in the presence of layered silicates which were rendered organophilic via ion exchange with various quaternary alkyl ammonium cations. In comparison to melt compounding, in-situ ethene homo- and copolymerization, catalyzed with MAO-activated zirconocene (MBI), nickel (DMN) and palladium (DMPN) catalysts, proved more effective in nanocomposite formation, as evidenced by larger interlayer spacings and formation of exfoliated anisotropic nanosilicates with high aspect ratio.

Ethylene polymerization catalysts based on nickel(II) 1,4-diazadiene complexes: the influence of the 1,4-diazadiene backbone substituents on structure and reactivity

Abstract Thermally sensitive dialkyl nickel complexes {DAD(H,H)}Ni(CH2SiMe3)2 and {DAD(Me,Me)}Ni(CH2SiMe3)2 [DAD(X,X)=2,6-iPr2C6H4–NC(X)–C(X)N–C6H4iPr2-2,6] were synthesized and were characterized by X-ray diffraction studies on single crystals. The substituents X on the backbone of the α-diimine ligand significantly influence the conformation of the 2,6-diisopropylphenyl substituents. This effect is thought to be of crucial importance for the polymerization of ethylene when {DAD(X,X)}NiBr2/MAO is used as catalyst. The influence of the catalyst structure, pressure, and temperature on the polymerization activity, molar mass, glass transition temperature, melting temperature and branching of the polymers has been studied. The dialkyl complex {DAD(H,H)}Ni(CH2SiMe3)2 underwent rapid reductive carbonylation giving the dicarbonyl complex {DAD(H,H)}Ni(CO)2 along with both bis(trimethylsilyl)ethane and α,α′-bis(trimethylsilyl)acetone. The dicarbonyl {DAD(H,H)}Ni(CO)2 was characterized by X-ray crystallography.

Ni(II) and Pd(II) complexes of camphor-derived diazadiene ligands: steric bulk tuning and ethylene polymerization

Abstract Nickel(II) and palladium(II) centers were coordinated to a series of chiral 1,4-diazadiene ligands. The ligand backbones are camphor derivatives and the imine nitrogens are attached to independently varied 2- and 2,6-substituted aryl groups. Upon activation with methyl-aluminoxane, the dibromo nickel complexes polymerize ethylene and 1-hexene. The polymerization properties are dependent on the steric features of the aryl substituents on the imine nitrogens.

New molecular and supermolecular polymer architecturesvia transition metal catalyzed alkene polymerization

Superstructure formation during crystallization has been examined as a function of isotactic poly(propene) and poly(ethene) molecular architectures, tailored by means of metallocene catalyzed propene polymerization, metallocene catalyzed ethene/alk-1-ene copolymerization, and nickel-catalyzed migratory insertion polymerization of ethene to afford methyl-branched poly(ethene) without using comonomers. The role of steric irregularities in the chain resulting from false insertion in stereoselective polymerization or from short chain branching, respectively, was investigated. Randomly distributed regio- and stereo-regularities in isotactic poly(propene) chains and variation of crystallization temperature were the key to controlled poly(propene) crystallization and predominant formation of the γ-modification. Poly(propene) melting temperature increased with increasing isotactic segment length between stereo- and regio-irregularities. Superstructures of isotactic γ-poly(propene) were analyzed by means of light and atomic force microscopy. Both types of short-chain branched poly(ethene)s, prepared by ethene/oct-1-ene copolymerization and migratory insertion homopolymerization, showed similar dependence of melting temperature on the degree of branching, calculated as the number of branching carbon atoms per 1000 carbon atoms. Phase transitions were monitored by means of wide angle X-ray scattering and pressure–volume–temperature measurements. Atomic force microscopy was applied to image both lamella- and fringed micelle-type superstructures as a function of the degree of branching.

Energy transfer from phenanthrene to anthracene in a dye-labeled (ethylene-methyl acrylate) copolymer

We describe the synthesis of a branched polyethylene-based polymer containing randomly spaced fluorescent dyes (phenanthrene or anthracene) along the polymer backbone. The dyes were introduced via ester exchange into the ethylene-methyl acrylate copolymer. Fluorescence and fluorescence-decay experiments indicated that the dyes were distributed randomly in a purely amorphous matrix. There was no indication of dye aggregation or of an enhanced local dye concentration of the sort that might be expected in a semicrystalline matrix.

Correlations Between Chain Branching, Morphology Development and Polymer Properties of Polyethenes

Molecular architectures of polyethenes, in particular short and long chain branching, were varied over a very wide range by means of either metallocene-catalyzed ethene/1-olefin copolymerization or Ni- and Pd-catalyzed migration/insertion-type ethene homopolymerization. While short chain branches affected melting, glass transition, and blend compatibility, long chain branching represented the key to improved melt processability. Both the number of short and long chain branches depended upon the ligand Substitution pattern of dimethylsilylene-bridged bisindenyl complexes. The degree of branching increased with Variation of the substitution in 4-position, i.e., 4-napthyl > 4-phenyl > benzannelation. Variation of the 1-butene content of ethene/1-butene (EB) copolymers gave control of morphology development and properties of isotactic polypropene (iPP) blends with EB. Highly flexible, single-phase as well as stiff and tough two-phase iPP/EB (70 wt.-%/30 wt.-%) blends were obtained. Rheological studies on ethene/1-eicosene model polyethene revealed the presence of a positive comonomer effect with respect to molar catalyst activity, molecular weight, and long-chain branching. A new family of thermoplastic elastomers based upon highly branched polyethene was prepared via Pd-catalyzed ethene copolymerization with 2,2,6,6-tetramethyl-piperidineoxy (TEMPO)-functionalized 1-olefin as macromonomers to produce macroinitiators for the initiation of the controlled free radical graft copolymerization of styrene onto highly branched polyethenes. The Variation of the polystyrene block length gave control on nanophase Separation of the resulting branched polyethene - graft - polystyrene.

Branched Polyethenes Prepared via Olefin Copolymerization and Migratory Insertion

Branched polyethenes with variable alkyl side chains were prepared via three routes: (1) metallocene-catalyzed copolymerization of ethene with propene, 1-octene, 1-eicosene, (2) simultaneous ethene polymerization and copolymerization of in-situ formed 1-alkenes resulting from ethene oligomerization, using a blend of Ni- and Ti-based catalysts (“hybrid catalysts”), and (3) Ni- and Pd-catalyzed ethene homopolymerization with branching occurring due to migratory insertion. The resulting families of materials included high density, low and ultralow density semierystalline polyethenes as well as highly flexible and elastomeric polyethenes. The degree of branching (DB), as measured by the number of branched C/1000 C, was correlated with comonomer incorporation, catalyst structure, polymerization conditions, polyethene melting temperature and melting enthalpy. Polyethenes prepared by ethene/1-olefin copolymerization were compared with branched ethene homopolymers. Linear low density polyethenes with DB<50, produced with Ni-catalysts, resembled poly(ethene-co-propene). Highly branched polyethene elastomers were applied as toughening agents and blend components of isotactic polypropene in order to improve polypropene’s impact resistance.