Using 70 mol % norbornene in the monomer mixture before polymerization, the influences of different polymerization conditions were studied. It was found that by increasing the temperature from 10 to 70ЊC, the yield increased and the molecular weight decreased; and when increasing the ethylene pressure from 2 to 6 atm, yield and molecular weight increased. The highest glass transition temperatures, however, were achieved at 30ЊC and 4 atm. Further improvements could be obtained by increasing the amount of catalyst or Al/Zr ratio. Using the conditions of 30ЊC, 4 atm, 4 mg catalyst, 3000 Al/Zr, 250 mL toluene solution, and 30 min, the amount of norbornene was increased from 31.1 to 90 g, resulting in an increase in the glass transition temperature from 98 to 155ЊC.
When characterized with 13 C-NMR, it was found that norbornene-ethylene copolymers had a more complicated microstructure when dimethylsilyl bis(indenyl) zirconium dichloride was used as the catalyst compared to ethylene bis(indenyl) zirconium dichloride. One could see more block sequences but less alternating sequences. For both catalysts the highest amounts of block sequences were obtained for high norbornene concentrations, medium to high Al/Zr ratios, and low polymerization temperatures. There were also more alternating sequences for high norbornene concentrations and high polymerization temperatures. The isolated norbornene units (separated by more than one ethylene unit) were all exoconfiguration. No unsaturation was seen.
SynopsisIn the synthesis of low-density polyethylene there is a direct relationship between the synthesis conditions, molecular structure, and technical properties of the product. The evaluation of the molecular structure of the polymer is therefore important. The most important structure parameters in low-density polyethylene are molecular weight distribution (MWD), degree of long-chain branching (LCB), and short-chain branching distribution (SCBD). Through chain transfer to polymer it is possible to get composite molecules made up of chains with different amounts of short-chain branching. By using temperature rising elution fractionation (TREF) and plotting the methyl contents or the DTA melt temperatures of the fractions as functions of the elution temperature, the SCBD within composite molecules can be evaluated. The deviations from linear relationships are caused by such SCBD within composite molecules. In order to demonstrate the buildup of composite molecules, samples from different parts of a reactor were investigated.
When testing copolymers of ethylene and aryl-substituted norbornenes with the Charpy impact method it was found that under certain polymerization conditions their impact properties increased with increasing polymerization time. It was also found that this improvement on impact properties was due to increasing exo/endo ratios of the incorporated aryl-substituted norbornene comonomers. This effect is important because impact-resistant copolymers can be made using aryl-substituted norbornene comonomers with low exo/endo ratios, which are more easily synthesized and purified.
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