Production of polyethylene having a broad molecular weight distribution

Abstract

A process for producing high density polyethylene having a bimodal molecular weight distribution, the process comprising pre-contacting a chromium-based catalyst with an alkyl metal oxane, the chromium-based catalyst comprising chromium oxide on a silica-containing support, to form a catalyst system wherein the atomic ratio of the metal of the oxane to the chromium is less than 1, and polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the presence of the catalyst system. A catalyst system for polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms to produce high density polyethylene, the catalyst system comprising in combination a chromium-based catalyst comprising chromium oxide on a silica-containing support and an alkyl metal oxane with which the chromium-based catalyst has been precontacted whereby the atomic ratio of the metal of the oxane to the chromium is less than 1. A process for producing high density polyethylene having a bimodal molecular weight distribution, the process comprising polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the presence of a catalyst system comprising a chromium-based catalyst, comprising chromium oxide on a silica-containing support, and an alkyl metal oxane in an amount of around 0.45 wt % based on the weight of the chromium-based catalyst.

1 . A process for producing high density polyethylene having a bimodal molecular weight distribution, the process comprising pre-contacting a chromium-based catalyst with an alkyl metal oxane, the chromium-based catalyst comprising chromium oxide on a silica-containing support, to form a catalyst system wherein the atomic ratio of the metal of the oxane to the chromium is less than 1, and polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the presence of the catalyst system.  
     
     
         2 . A process according to  claim 1  wherein the metal/chromium atomic ratio is from 0.1 to less than 1.  
     
     
         3 . A process according to  claim 1  wherein the alkyl metal oxane is methyl aluminum oxane.  
     
     
         4 . A process according to  claim 1  wherein the alkyl metal oxane comprises around 0.45 wt % based on the weight of the chromium-based catalyst.  
     
     
         5 . A process according to  claim 1  wherein the catalyst system is present in an inert diluent and the alkyl metal oxane is present in an amount of from 0.5 to 5 ppm based on the weight of the diluent.  
     
     
         6 . A process according to  claim 1  wherein the chromium-based catalyst comprises around 1 wt % chromium, based on the weight of the chromium-based catalyst.  
     
     
         7 . A process according to  claim 1  wherein the chromium-based catalyst has a specific surface area of from 230 to 750 m 2 /g and a volume porosity of from 1 to 3 cc/g.  
     
     
         8 . A process according to  claim 1  wherein for copolymerization the ethylene and comonomer are dissolved in an inert diluent and the solution comprises from 0.5 to 8 wt % ethylene, from 0.5 to 6 wt % comonomer and the balance inert diluent.  
     
     
         9 . A process according to  claim 1  wherein the polymerization or copolymerization is performed at a temperature of from 85 to 11° C. at a pressure of from 20 to 42 bar.  
     
     
         10 . A catalyst system for polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms to produce high density polyethylene, the catalyst system comprising in combination a chromium-based catalyst comprising chromium oxide on a silica-containing support and an alkyl metal oxane with which the chromium-based catalyst has been precontacted whereby the atomic ratio of the metal of the oxane to the chromium is less than 1.  
     
     
         11 . A catalyst system according to  claim 10  wherein the metal/chromium atomic ratio is from 0.1 to less than 1.  
     
     
         12 . A catalyst system according to  claim 10  wherein the alkyl metal oxane comprises methyl aluminum oxane.  
     
     
         13 . A catalyst system according to  claim 10  wherein the alkyl metal oxane comprises around 0.45 wt % based on the weight of the chromium-based catalyst.  
     
     
         14 . A catalyst system according to  claim 10  wherein the catalyst system is present in an inert diluent and the alkyl metal oxane comprises from 0.5 to 5 ppm based on the weight of the diluent.  
     
     
         15 . A process for producing high density polyethylene having a bimodal molecular weight distribution, the process comprising polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the presence of a catalyst system comprising a chromium-based catalyst, comprising chromium oxide on a silica-containing support, and an alkyl metal oxane in an amount of around 0.45 wt % based on the weight of the chromium-based catalyst.