Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Mn, Mw and PD = Mw/Mn of polypropylene obtained with the MgCl2/ethyl benzoate/p-cresol/AlEt3/TiCl4-AlEt3/methyl-p-toluate (CW-catalyst) have been obtained as a function of temperature, monomer, and AlEt3 concentrations. The values of Mn after a few minutes of polymerization range from 50% to 100% of the final values. I.I. (percent yield of stereoregular polymer) and P.D. of stereoregular polymer remain constant during a batch polymerization. The MW distribution is very narrow with PD ⋍ 2.0 between 0° and 50° using optimum A/T ratio. It is broader for polymerizations employing low or high A/T ratio or elevated temperature, reaching P.D. = 2.78 at 70°C. The corresponding P.D. values for the total polymer ranged from 3.1 to 4.5 due to contributions from the heptane soluble fraction. The latter gives bimodal GPC chromatogram. The Mn of stereoregular polymer is much smaller than the reciprocal of concentration of metal polymer bonds, [MPB]-1; the ratio of the two quantities is a measure of number of polymer chains not bonded to a metal ion to the number of those which are bonded to either Ti or Al. Depending upon the experimental conditions, this ratio varied from 2.3 to 23.5 for short polymerization time, and 16 to 31 for long polymerization time.Chain transfer with AlEt3 is ineffective for the CW-catalyst system. From the plots of [MPB] versus yield, the rate constants for this process obtained at 50° are: ktr1iA = 2.1 × 10-4 sec-1; ktr1aA = 2 × 10-5 sec-1; ktr2iA = 1.1 × 10-4 sec-1; and ktr2aA = 1.2 × 10-5 sec-1 where the subscripts i, a, 1 and 2 refer to stereospecific and nonstereospecific sites of first and second kind respectively. Thr transfer rates increase with both increasing and decreasing temperatures consistent with the Langmuir-Hirschelwood adsorption model.The MW of isotactic polymers are dependent upon monomer concentration. The chain transfer with monomer has rate constants at 50° of ktr1iM = 1.34 × 10-2 M-1 sec-1 and ktr2iM = 1.2 × 10-3 M-1 sec-1. In addition there is chain transfer by β-hydride elimination with ktr,iβ = 2.7 × 10-2 sec-1 at 50° which is the chain lifetime limiting process at low monomer concentration. The rate of chain transfer with monomer can exceed that of β-hydride elimination at high propylene pressure.The very narrow PD for the stereoregular polymers suggest there is only one chemically distinct isospecific active site.
Type of Medium: