George A. Mortimer

Science of Determining Copolymerization Reactivity Ratios

Abstract Copolymerization reactivity ratios were originally measured and tabulated for the purpose of describing and classifying the relative reactivities of various monomers toward various radicals in free-radical copolymerization. A great number of reactivity ratios have now been measured and tabulated for this purpose [1]. Within the last few years, the concept of relative reactivity has been extended to ionic and coordination addition polymerization mechanisms, and r1r2 values in these systems are appearing in the literature.

Synthesis and some characteristics of di-π-cyclopentadienyl-σ-organotitanium(IV) chlorides

Abstract A series of organotitanium compounds of structure (π-C 5 H 5 ) 2 Ti(R)Cl was prepared, where the R ligand was attached by a σ-Ti C bond. In studying the properties of these compounds, the following observations were made. The σ-Ti C bond absorbs in the infrared at about 460 to 465 cm −1 . It is apparently a weak, but otherwise normal, covalent bond. In contrast to many other metal atoms, the titanium atom has little effect on the C H bonds in the σ-bonded ligand. The Ti Cl bond has ionic character and, in compounds where steric crowding may be important, may form chloride ions oven in non-polar solvents.

Thermolysis of di-π-cyclopentadienyl-σ-organotitanium chlorides

Abstract The thermolysis of a series of compounds Cp 2 (R)TiCl (where Cpπ-cyclopentadienyl and R is a σ-bonded hydrocarbon moiety) was studied. After an initial unsteady period, first-order loss of the R group was observed. The activation energy of bond breaking was approximately 25 kcal/mole for all compounds. The products and kinetics are explained by postulating that the rate-determining step is the promotion of the R group to a non-ionic activated state in which it is still within and influenced by the titanium coordination sphere but mobile enough to migrate from one titanium atom to another.

Another Look at Long-Chain Branching

Abstract Long-chain branching can occur during radical polymerization and is especially important for polyethylene. An improved method of calculating the effect of long-chain branching on molecular weight distribution is presented. This method uses a probability treatment. The results are more consistent with both kinetic theory and experimental data than the results of previous long-chain branching calculations. In contrast to previous calculations, the present work shows that generation cannot occur from long-chain branching alone.

Titanium reduction in a soluble ziegler—natta catalyst

Abstract The spontaneous reduction of TiIV to TiIII in soluble Ziegler—Natta catalysts of type Cp2RTiCl·R’AlCl2 (Cp = h5 -cyclopentadienyl, R and R’ = methyl or ethyl) was studied both spectrally and chromatographically. Varied were R, R’, Al/Ti ratio, total concentration, solvent, and added olefin. Kinetic order in [Ti] could be varied from zero to second order by changing solvent. This can be explained by a mechanism in which a Cp2 RTiCl—R’AlCl2—olefin complex forms in the rate determining step and ligand R is expelled as half alkane half olefin. The expelled olefin may either polymerize or catalyze reduction by forming the rate-determining complex. Different apparent kinetic orders arise from differences in the olefin competitive reactions. The reaction products appear to form in a rapid bimolecular reaction following the rate-determining step. Evidence is presented that neither free radicals nor Cp2RR’Ti are reduction intermediates. The intermediate is postulated to be a TiIV transient hydride formed by a reverse insertion step.

Long-Chain Branching under Conditions of Nonuniform Branching Probability

Abstract Long-chain branching can occur during free radical polymerization and is especially important for polyethylene. An improved method of calculating the effect of long-chain branching on molecular weight distribution was presented in an earlier paper in which the assumption was made that the probability of branching at each monomer unit was constant throughout the polymerization. A method of including a nonuniform probability of branching in the calculations is presented. Calculation results show that the predictions of the two mathematical models are similar and both models fit published data on polyethylene equally well.