Abstract:
A free radical polymerization process for producing a branched polymer from a vinyl aromatic monomer comprising polymerizing a vinyl aromatic monomer in the presence of a free radical initiator of the formula: ##STR1## wherein R is H, alkyl, or aralkyl, and R&#39; is alkyl or aryl.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for preparing a branched polymer from a vinyl aromatic monomer. 
     Free radical bulk polymerization is a well known process for preparing polymers from vinyl aromatic monomers. However, typical initiators used in these processes produce high viscosity, high molecular weight polymer in the early stages of polymerization, which can lead to gelling. Therefore, these processes are conducted at low temperatures, keeping the polymerization rate and viscosity low such that heat can be removed sufficiently and gelling is prevented. 
     Branched polymers have been produced from aromatic monomers in a variety of ways including the use of a vinyl functional initiator, such as n-butyl-t-butylperoxyfumarate, as described in U.S. Pat. No. 4,376,847. However, in this method, branching occurs in the polymerization reactor, causing gels to form. Gels build up in the polymerization reactor after extended periods of continuous operation and lead to reactor fouling. 
     Peritaconates have been used as chain transfer agents to retard the Mw growth in the production of polystyrene, polymethylmethacrylate and polybutylacrylate as described in WO 94/13705 by Nuhuis et al., of Akzo Nobel. Although it is mentioned that these chain transfer agents can also be used as polymerization initiators, the process in Nuhuis et al. produces polymers of low molecular weight, e.g. exemplified as having a Mn of between about 2,200 and 24,000. 
     Accordingly, it remains highly desirable to provide an efficient method of producing a branched polymer derived from a vinyl aromatic monomer which does not have the foregoing disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is a free radical bulk polymerization process for producing a branched polymer from a vinyl aromatic monomer comprising polymerizing a vinyl aromatic monomer in the presence of a free radical initiator of the formula: ##STR2## wherein R is H, alkyl, aryl, or aralkyl, wherein aralkyl is defined as an aryl group attached to an alkyl group and the alkyl group is attached to the oxygen, any alkyl group contains 1-6 carbon atoms, and R&#39; is alkyl or aryl wherein aryl is an aromatic group containing 1-3 rings. 
     This process produces branched polymers, which can have improved properties over linear polymers in extensional rheology, melt strength, and viscosity. These improved properties can lead to processing advantages when compared to linear polymers. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Vinyl aromatic monomers suitable for use according to the present invention include, but are not limited to, those vinyl aromatic monomers previously known for use in polymerization processes, such as those described in U.S. Pat. Nos. 4,666,987, 4,572,819 and 4,585,825. Preferably, the monomer is of the formula: ##STR3## wherein R&#34; is hydrogen or methyl, Ar is an aromatic ring structure having from 1 to 3 aromatic rings with or without alkyl, halo, or haloalkyl substitution, wherein any alkyl group contains 1 to 6 carbon atoms and haloalkyl refers to a halo substituted alkyl group. Preferably, Ar is phenyl or alkylphenyl with phenyl being most preferred. Typical vinyl aromatic monomers which can be used include: styrene, alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, propyl styrene, vinyl biphenyl, vinyl naphthalene, vinyl anthracene and the like, and mixtures thereof. The vinyl aromatic monomers may also be combined with other copolymerizable monomers. Examples of such monomers include, but are not limited to acrylic monomers such as acrylonitrile, methacrylonitrile, methacrylic acid, methyl methacrylate, acrylic acid, and methyl acrylate; maleimide, phenylmaleimide, and maleic anhydride. In addition, the polymerization may be conducted in the presence of predissolved elastomer to prepare impact modified, or grafted rubber containing products, examples of which are described in U.S. Pat. Nos. 3,123,655, 3,346,520, 3,639,522, and 4,409,369. 
     The initiator used in the process of the present invention is a peritaconate of the formula: ##STR4## wherein R is H, alkyl, or aralkyl, wherein aralkyl is defined as an aryl group attached to an alkyl group which is also attached to the oxygen, any alkyl group contains 1-6 carbon atoms, and R&#39; is alkyl or aryl wherein aryl is an aromatic group containing 1-3 rings. Preferably, R is H and R&#39; is t-butyl. 
     The peritaconates used in the process of the present invention are known compounds and can be made by several known methods, including processes taught in WO 94/13705. 
     The amount of initiator used in the process of the present invention will depend upon the desired Mw of the polymer to be produced. Higher levels of initiator produce lower molecular weight polymers. The initiator is typically present in amounts of from about 10 to about 1500 ppm based on the total weight of starting monomer. Preferably, the initiator is present in amounts from about 100 to about 1400 ppm, more preferably from about 200 to about 1200 ppm, and most preferably from about 300 to about 1000 ppm. 
     Other initiators may also be present in the process of the present invention in combination with the t-alkylperitaconates described above. Examples of other initiators which may be present include but are not limited to t-butylhydroperoxide, ditertiary-butylperoxide, cumene hydroperoxide, dicumylperoxide, 1,1-bis(tertiary-butylperoxy)-3,3,5-trimethylcyclohexane, t-butylperoxybenzoate, 1,1-bis(t-butylperoxy-)cyclohexane, benzoylperoxide, succinoylperoxide and t-butyl-peroxypivilate, and azo compounds such as azo bisisobutyro-nitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbo-nitrile, azobismethyl isolactate and azobiscyanovalerate. Typical amounts are well known in the art and may be used in the process of the present invention providing that the total amount of initiator will be such that a polymer having at least a Mw of 75,000 is produced. 
     In addition to initiators, chain transfer agents such as thiols and mercaptans, e.g., n-dodecylmercaptan, may also be used in the process of the present invention. 
     The process of the present invention can be conducted in the presence of solvent which is inert for the polymeric material being formed. Solvents useful in the process of the present invention include but are not limited to ethyl benzene, benzene, toluene, and the like. 
     The peritaconate initiator can be added to the vinyl aromatic monomer at any time during the polymerization process. It is typically added to the starting monomer(s) prior to polymerization or in the early stages of polymerization, e.g. up to 50% conversion. Preferably, the initiator is dissolved in a solvent and combined with starting monomer(s) prior to the polymerization reaction. 
     Polymerization processes and process conditions for the polymerization of vinyl aromatic monomers are well known in the art. Although any polymerization process can be used, typical processes are continuous bulk or solution polymerizations as described in U.S. Pat. No. 2,727,884 and U.S. Pat. No. 3,639,372 which are incorporated herein by reference. The polymerization is typically conducted at temperatures from about 80° to about 170° C., preferably from about 90° to about 160° C., more preferably from about 100° to about 155° C., and most preferably from about 110° to about 150° C. 
     The polymer produced by the process of the present invention can have a broad range of weight average molecular weight (Mw), ranging from about 75,000 to about 800,000 as measured by gel permeation chromatography (GPC); typically from about 90,000, preferably from about 100,000, more preferably from about 150,000, and most preferably from about 200,000 to about 700,000, preferably to about 600,000, more preferably to about 550,000 and most preferably to about 500,000. 
     The polymers produced by the process of the present invention can find use in foam board, foam sheet and injection molded and extruded products. 
     The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in weight parts or weight percentages unless otherwise indicated. The polymer weight average molecular weight (Mw) is determined using gel permeation chromatography (GPC) and refers to the Mw of the solids. Number average molecular weight (Fin) and (Mz) are also determined using (GPC). 
    
    
     EXAMPLE 1 
     Stock solutions are prepared by dissolving 0.0966 g of t-butylperoxybenzoate (tBPB) and 0.0815 g of benzoyl peroxide (BPO) in 210 g of styrene (Sol. 1), 0.0179 g of t-butylperitaconate (tBPIT) in 79.5 g of Sol. 1 (Sol. 2), 0.0355 g of tBPIT in 75.7 g of Sol. 1 (Sol. 3), and 0.0788 g of tBPIT in 101.9 g of styrene (Sol. 4) Approximately 2-3 mls of each solution are placed in two ampoules (12×0.4 in. OD) (0.11 in. wall thickness). The ampoules are sealed under vacuum using the freeze-thaw technique and heated in an oil bath at 90° C. for 1 hour, followed by raping to 140° C. at a rate of 1° C./minute and holding at 140° C. for 1 hour. The polymer is removed from the ampoule at the appropriate time during the heating treatment as detailed in Table 1 and evaluated. The time listed in Table 1 refers to the time from the beginning of the initial heating at 90° C. Results are listed in Table 1. 
     
                                           TABLE 1__________________________________________________________________________    BPO tBPB       tBPIT           % Solids/Sample    (ppm)   (ppm)       (ppm)           Time (h)                Mw/1000                     Mn/1000                          Mz/1000                               Mw/Mn__________________________________________________________________________Sol. 1*    388 460  0  16.4/1.5                188   75  314  2.5Sol. 1*    388 460  0  75.6/2.88                227  112  360  2.0Sol. 2    388 460 225 16.3/1.5                193   76  330  2.5Sol. 2    388 460 225 80.4/2.88                243  115  401  2.1Sol. 3    388 460 469 19.9/1.5                187   69  331  2.7Sol. 3    388 460 469 86.3/2.88                253  114  428  2.2Sol. 40   0  773 12.8/1.5                297   94  511  3.2Sol. 40   0  773 88.6/2.88                346  146  587  2.4__________________________________________________________________________ *Comparative Example 
    
     Mw, Mz and polydispersity all increase by the addition of tBPIT to the feed, when compared with the results obtained with BPO and tBPB alone. 
     EXAMPLE 2 
     Ampoule samples are Prepared as in Example 1 using tBPIT and t-butylperoxy-n-butylfumarate (BPBF)(Comparative example) as initiators and are subjected to the following heat treatment: placing in an oil bath at 90° C. and ramping to 140° C. at 10° C./hour. The polymer is removed from the ampoule at the appropriate time during the heating treatment as detailed in Table 2 and evaluated for Mw, percent solids etc. The time listed in Table 2 refers to the time from initial heating at 90° C. Results are listed in Table 2. 
     
                                           TABLE 2__________________________________________________________________________Tube #    Init/ppm    Time (h)         % Solids              Mw/1000                   Mn/1000                        Mz/1000                             Mw/Mn__________________________________________________________________________1   BPBF/200    1    3.4  625  302  1067 2.12   BPBF/200    2    10.3 634  256  1219 2.53   BPBF/200    3    23.3 612  237  1215 2.64   BPBF/200    4    40.7 551  212  1114 2.65   BPBF/200    5    59.6 490  187   992 2.66   BPBF/400    1    4.1  564  250  1019 2.37   BPBF/400    2    13.8 689  236  1470 2.98   BPBF/400    3    31.3 700  220  1506 3.29   BPBF/400    4    47.6 653  220  1460 3.010  BPBF/400    5    67   567  208  1234 2.711  BPBF/800    1    5.6  454  190   868 2.412  BPBF/800    2    20.5 711  196  1671 3.613  BPBF/800    3    42.8 843  206  2060 4.114  BPBF/800    4    62.2 799  218  1910 3.715  BPBF/800    5    87.6 731  207  1790 3.516  BPIT/166    1    5.5  484  263   746 1.817  BPIT/166    2    13.5 437  228   687 1.918  BPIT/166    3    24.9 435  218   710 2.019  BPIT/166    4    38.9 452  217   758 2.120  BPIT/166    5    59.8 442  209   764 2.221  BPIT/331    1    7.3  397  212   625 1.922  BPIT/331    2    17.1 377  198   601 1.923  BPIT/331    3    30   397  197   663 2.024  BPIT/331    4    47.2 462  211   803 2.225  BPIT/331    5    69.1 469  206   840 2.326  BPIT/662    1    9.6  314  169   498 1.927  BPIT/662    2    21.6 323  167   523 1.928  BPIT/662    3    38.3 388  181   668 2.129  BPIT/662    4    59.8 487  209   892 2.330  BPIT/662    5    91.7 529  209   995 2.5__________________________________________________________________________ 
    
     Mw continues to build when using BPIT whereas Mw drops off after about 40% conversion when using BPBF. 
     EXAMPLE 3 
     Styrene (1350 g), ethylbenzene (150 g) and an initiator are placed in a 1500 milliliter (mL) reactor which is described in FIG. 1 of U.S. Pat. No. 4,239,863. The initiators are 1,1-bis(t-butylperoxy)cyclohexane (BBPC)(300 ppm), t-butyl peroxybenzoate (tBPB) (500 ppm), 2,2-bis-(4,4-di(t-butylperoxy) cyclohexyl)propane (PK12)(300 ppm), and t-butylperitaconate (tBPIT)(500 ppm). The mixture is heated to 100° C. over 50 min and then to 150° C. at a rate of 10° C./hour. Samples are drawn periodically from the polymerization and analyzed for percent polystyrene and weight average molecular weight using GPC. Results are shown in Table 3. 
     
                                           TABLE 3__________________________________________________________________________Time   BBPC*  BBPC*       tBPB tBPB*                 PK12*                     PK12*                          tBPIT                              tBPIT(hrs)   % conv  Mw/1000       *% conv            Mw/1000                 % conv                     Mw/1000                          % conv                              Mw/1000__________________________________________________________________________1  10  262  13   321  10  318  15  2042  20  242  22   284  22  297  26  2563  33  236  35   256  36  298  40  3134  59  251  60   230  58  317  60  3275  67  247  70   225  69  301  70  319__________________________________________________________________________ % conv = percent conversion *comparative example 
    
     The results show that high Mw polymer is produced in the early stages of polymerization when using BBPC, tBPB or PK12 initiators. Lower Mw polymer is made in the early stages when using tBPIT with Mw building with increasing conversion.