Method of preparing ethylene polymers by controlled high pressure polymerization

Abstract

The present invention relates to a method for the radical polymerization or copolymerization of ethylene at high pressures using a hydroxylamine ester as radical initiator. The hydroxylamine esters according to the invention are suitable initiators for the high pressure polymerization of ethylene leading to high molecular weight polyethylenes with narrow molecular weight distributions (Poyldispersity Index PD=1.2-4.5).

1 . A method for the polymerization or copolymerization of ethylene, which method comprises
 polymerizing or copolymerizing ethylene at an operating pressure of from 500 to 3500 bar, at a polymerization temperature between 100° and 400° C. in a suitable high pressure reactor, operating continuously or batch wise and   in the presence of a radical polymerization initiator,   wherein the polydispersity, PD, of the resulting polyethylene is between 1.2 and 4.5, as measured by gel permeation chromatography, and where the weight average molecular weight is from 284,000 to 547,000,   characterized in that the radical polymerization initiator is a hydroxylamine ester containing a structural element of formula (I) or (I′)   
     
       
         
         
             
             
         
       
       wherein 
       X is hydrogen, C 1 -C 36 alkyl, C 1 -C 36 alkyl which is substituted by halogen, C 6 -C 12 cycloalkyl, C 7 -C 12 bicyclo- or tricycloalkyl, C 2 -C 36 alkenyl, C 2 -C 18 alkynyl, C 6 -C 10 aryl, —O—C 1 -C 18 alkyl, —O—C 6 -C 10 aryl, —NH—C 1 -C 18 alkyl, —NH—C 6 -C 10 aryl or —N(C 1 -C 6 alkyl) 2 ; and 
       X′ is a direct bond or is C 1 -C 36 alkylene, C 2 -C 36 alkenylene, C 2 -C 36 alkynylene, —(C 1 -C 6 alkylene)-phenyl-(C 1 -C 6 alkylene) or a group 
     
     
       
         
         
             
             
         
       
       where 
       * indicates the bond to which the carbonyl groups are attached. 
     
   
   
       2 . A method according to  claim 1  wherein the operating pressure is of from 1000 to 3000 bar. 
   
   
       3 . A method according to  claim 1  wherein the polymerization temperature is of from 140° to 300° C. 
   
   
       4 . A method according to  claim 1  wherein the hydroxylamine ester is present in an amount of from 5 to 500 parts per million based on the weight of the total reaction mixture. 
   
   
       5 . A method according to  claim 1 , wherein the hydroxylamine ester is a compound of formula (Ia) or (Ia′) 
     
       
         
         
             
             
         
       
       wherein 
       X is hydrogen, C 1 -C 36 alkyl, C 1 -C 36 alkyl which is substituted by halogen, C 6 -C 12 cycloalkyl, C 7 -C 12 bicyclo- or tricycloalkyl, C 2 -C 36 alkenyl, C 2 -C 18 alkynyl, C 6 -C 10 aryl, —O—C 1 -C 18 alkyl, —O—C 6 -C 10 aryl, —NH—C 1 -C 18 alkyl, —NH—C 6 -C 10 aryl or —N(C 1 -C 6 alkyl) 2 ; 
       X′ is a direct bond or C 1 -C 36 alkylene, C 2 -C 36 alkenylene, C 2 -C 36 alkynylene, phenylene, —(C 1 -C 6 alkylene)-phenyl-(C 1 -C 6 alkylene) or a group 
     
     
       
         
         
             
             
         
       
       R 20 , R′ 20 , R 30  and R′ 30  are each independently of the others unsubstituted, halo-, CN—, NO 2 — or —COOR 40 -substituted or O— or NR 40 — interrupted C 1 -C 18 alkyl, C 2 -C 18 alkenyl or C 2 -C 18 alkynyl; 
       R 40  is hydrogen, phenyl or C 1 -C 18 alkyl; or 
       R 20  and R 30  and/or R′ 20  and R′ 30 , together with the nitrogen atom to which they are bonded, form a 5- or 6-membered ring which may be interrupted by a nitrogen or oxygen atom and which may be substituted one or more times by C 1 -C 6 alkyl groups and carboxyl groups. 
     
   
   
       6 . A method according to  claim 5 , where in the compound of formula (Ia), R 20  and R 30 , together with the nitrogen atom to which they are bonded, form a piperidine ring which is substituted in the 2,2- and 6,6-positions by C 1 -C 4 alkyl groups and in the 4-position has an ether, amine, amide, urethane, ester or ketal group. 
   
   
       7 . A method according to  claim 5 , where the hydroxylamine ester is a compound of formula (A), (B), (C) or (O) 
     
       
         
         
             
             
         
       
       wherein 
       G 1 , G 2 , G 3  and G 4  are each independently of the others alkyl having from 1 to 4 carbon atoms; 
       G 5  and G 6  are each independently of the other hydrogen or C 1 -C 4 alkyl; 
       m is a number 1-2; 
       R, when m is 1, is hydrogen, uninterrupted C 1 -C 18 alkyl or C 2 -C 18 alkyl interrupted by one or more oxygen atoms, or is cyanoethyl, benzoyl, glycidyl, a monovalent radical of an aliphatic carboxylic acid having from 2 to 18 carbon atoms, of a cycloaliphatic carboxylic acid having from 7 to 15 carbon atoms or of an α,β-unsaturated carboxylic acid having from 3 to 5 carbon atoms or of an aromatic carboxylic acid containing from 7 to 15 carbon atoms, it being possible for each carboxylic acid to be substituted in the aliphatic, cycloaliphatic or aromatic unit by from 1 to 3 groups —COOZ 12  wherein Z 12  is hydrogen, C 1 -C 20 alkyl, C 3 -C 12 alkenyl, C 5 -C 7 cycloalkyl, phenyl or benzyl; or R is a monovalent radical of a carbamic acid or phosphorus-containing acid or is a monovalent silyl radical; 
       R, when m is 2, is C 2 -C 12 alkylene, C 4 -C 12 alkenylene, xylylene, a bivalent radical of an aliphatic dicarboxylic acid having from 2 to 36 carbon atoms or of a cycloaliphatic or aromatic dicarboxylic acid having from 8 to 14 carbon atoms or of an aliphatic, cycloaliphatic or aromatic dicarbamic acid having from 8 to 14 carbon atoms, it being possible for each dicarboxylic acid to be substituted in the aliphatic, cycloaliphatic or aromatic unit by one or two groups —COOZ 12 ; or 
       R is a bivalent radical of a phosphorus-containing acid or a bivalent silyl radical; 
       p is 1, 
       R 1  is C 1 -C 12 alkyl, C 5 -C 7 cycloalkyl, C 7 -C 8 aralkyl, C 2 -C 18 alkanoyl, C 3 -C 5 alkenoyl or benzoyl; 
       R 2  is C 1 -C 18 alkyl, C 5 -C 7 cycloalkyl or C 2 -C 8 alkenyl, each unsubstituted or substituted by a cyano, carbonyl or carbamide group, or is glycidyl, a group of formula —CH 2 CH(OH)—Z or of formula —CO—Z or —CONH—Z, wherein Z is hydrogen, methyl or phenyl; 
       n is a number 1 or 2; 
       when n is 1, 
       R 3  is C 2 -C 8 alkylene or hydroxyalkylene or C 4 -C 36 acyloxyalkylene; or, 
       when n is 2, 
       R 3  is (—CH 2 ) 2 C(CH 2 —) 2  and 
       X is as defined in  claim 5 . 
     
   
   
       8 . A method according to  claim 5 , wherein the substituent X is selected from the group consisting of C 1 -C 36 alkyl, C 2 -C 19 alkenyl and C 6 -C 10 aryl. 
   
   
       9 . A method according to  claim 1  wherein the hydroxylamine ester is of formula (C′) 
     
       
         
         
             
             
         
       
       wherein X is hydrogen or C 1 -C 18 alkyl and R 100  is C 4 -C 24 alkyl. 
     
   
   
       10 . A method according to  claim 1 , wherein the hydroxylamine ester is an oligomer or polymer obtained by reacting a dicarboxylic acid with a compound of formula A1 or B1 or by reacting a diisocyanate with a compound of formula A1, 
     
       
         
         
             
             
         
       
       B1 
       wherein 
       G 1 , G 2 , G 3  and G 4  are each independently of the others C 1 -C 4 alkyl, or G 1  and G 2  together and G 3  and G 4  together, or G 1  and G 2  together or G 3  and G 4  together are pentamethylene; 
       G 5  and G 6  are each independently of the other hydrogen or C 1 -C 4 alkyl; and 
       R 1  is C 1 -C 12 alkyl, C 5 -C 7 cycloalkyl, C 7 -C 8 aralkyl, C 2 -C 18 alkanoyl, C 3 -C 5 alkenoyl or benzoyl. 
     
   
   
       11 . A method according to  claim 1  wherein an additional organic radical initiator is added. 
   
   
       12 . A method according to  claim 11  wherein the organic radical initiator is selected from the group consisting of isobutyryl-peroxide, isopropylperoxy-dicarbonate, di-n-butylperoxy-dicarbonate, di-sec-butylperoxy-dicarbonate, dicyclohexylperoxy-dicarbonate, di(2-ethylhexyl)peroxy-dicarbonate, t-butyl-perneodecanoate, t-butyl-perpivalate, bis(3,5,5-trimethyl-hexanoyl)peroxide, didecanoyl-peroxide, dilauroyl-peroxide, t-butyl-perisobutyrate, t-butyl-per2-ethylhexanoate, t-butyl-peracetate, t-butyl-per-3,5,5-trimethylhexanoate, t-butyl-perbenzoate, di-t-butyl-peroxide, t-butyl-hydroperoxide and di-t-amylperoxide. 
   
   
       13 . A method according to  claim 1  wherein additionally a chain transfer agent is added. 
   
   
       14 . A method according to  claim 13  wherein the chain transfer agent is selected from the group consisting of ketones, aldehydes, C 3 -C 20 alkanes, C 3 -C 20 alkenes, mercaptanes and disulfides. 
   
   
       15 . A method according to  claim 1  wherein a comonomer is present which is a monomer containing a vinyl group, an allyl group, a vinylidene group, a diene group or an olefinic group and which is other than ethylene. 
   
   
       16 . A method according to  claim 15  wherein a comonomer is present which is selected from the group consisting of methylacrylate, ethylacrylate, n-butylacrylate, vinylacetate, styrene, α-methylstyrene and methylmethacrylate. 
   
   
       17 . An ethylene polymer or copolymer obtained by the method of  claim 1 .