Cocatalysts induction time

The induction period can also be shortened or even eliminated by the addition of reducing agents either to the catalyst or to the reactor. Particularly effective are the alkyls or hydrides of aluminum, boron, zinc, lithium, magnesium, etc. When added in ppm quantities, they can eliminate the induction time of Cr(VI)/silica and also raise the steady-state polymerization rate. Some metal alkyls can remove poisons and redox byproducts. All metal alkyls no doubt help reduce the Cr(VI), perhaps to Cr(IV). And some may even help alkylate the chromium, similar to the chemistry of Ziegler catalysts. Figure 16 shows how triethylaluminum cocatalyst can be used to shorten the induction time [52],... 

The induction time becomes a major problem during the production of ultrahigh-MW polyethylene (UF1MW PE). This type of PE is used when extreme toughness is desired, in applications such as in ski bottoms, artificial limbs, gears, and bullet-proof vests. UF1MW PE is more resistant to abrasion than steel. To minimize chain transfer on Cr/silica, low activation and reaction temperatures are necessary, and lowering these temperatures increases the induction time. To overcome this issue, the catalyst can be reduced to Cr(II) in CO, or metal alkyl cocatalysts can... 

Cocatalysts enhance overall average activity primarily by accelerating the development of polymerization rate and to a lesser degree by raising the maximum rate attained [52,684,686-688]. The induction time is shortened (and usually eliminated), and the polymerization rate then rises to its maximum more quickly. The more rapid development of activity in the presence of cocatalyst suggests that active sites are reduced, and even initiated, more quickly. The higher rate suggests that some sites are created that would not have become active otherwise. 

The Cr-based catalyst was patented by Phillips in 1958 to manufacture HDPE commercially. The Phillips catalyst is typically H CrO impregnated on silica and calcined at a high temperature ( 800°C). The resultant material polymerizes ethylene after an induction time dvuing which much of the SiO -supported Cr + ions are reduced to Cr ". An important point to note is that unlike the H catalyst, the heterogeneous Cr catalyst does not require any cocatalyst. 

The use of ethers as cocatalysts for the cationic polymerisation of alkenyl monomers induced by Lewis acids has received little systematic attention and the mechanism through which these compounds operate is not well understood. The complex diethyl-ether-boron fluoride has been extensively used as a very convenient cationic initiator, but mostly for preparative purposes. As in the case of alcohols and water, ethers are known to act as inhibitors or retarders in the cationic polymerisation of olefins, if used obove cocatalytic levels, because they are more nucleophilic than most rr-donor monomers. Imoto and Aoki showed that diethyl ether, tetrahydrofuran, -chloro-diethyl ether and diethyl thioether are inhibitors for the polymerisation of styrene-by the complex BF3 EtjO in benzene at 30 °C, at a concentration lower than that of the catalyst, but high enough (0.5 x 10 M) to quench the active species formation for a time. Their action was temporary in that the quenching reaction consumed them, and therefore induction periods were observed, but the DP s of the polystyrenes were independent of the presence of such compounds, as expected from a classical temporary inhibition. 

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