A PROCESS FOR PREPARING A BRANCHED POLYMER FROM A VINYL AROMATIC MONOMER

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 formula (I), wherein R is H, alkyl, or aralkyl, and R' is alkyl or aryl.

Description: A PROCESS FOR PREPARING A BRANCHED POLYMER FROM A VINYL AROMATIC MONOMER 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-butyiperoxyfumarate, as described in US-A- 4,376,847. However, in this method, branching occurs in the polymerization reactor, causing gels to form Gels build up in the polymenzation reactor after extended penods of continuous operation and lead to reactor fouling. Pentaconates have been used as chain transfer agents to retard the Mw growth in the production of polystyrene, poiymethylmethacrylate 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, for example exemplified as having a Mn of between 2,200 and 24,000. Accordingly, it remains highly desirable to provide an efficient method of producing a branched polymer deπved from a vinyl aromatic monomer which does not have the foregoing disadvantages of the pπor art. The present invention is a free radical bulk polymerization process for producing a branched polymer from a vinyl aromatic monomer comprising polymeπzing a vinyl aromatic monomer in the presence of a free radical initiator of the formula * 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' 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 extensioπal rheology, melt strength, and viscosity. These improved properties can lead to processing advantages when compared to linear polymers. 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 US-A-4,666,987, US-A-4,572,819 and US-A-4,585,825. Preferably, the monomer is of the formula: R I Ar-C=CH2 wherein R is hydrogen or methyl, Ar is an aromatic πng 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 USP's 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: 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' is alkyl or aryl wherein aryl is an aromatic group containing 1 -3 rings. Preferably, R is H and R' is t-butyl. The peπtaconates 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 10 to 1500 ppm based on the total weight of starting monomer. Preferably, the initiator is present in amounts from 100 to 1400 ppm, more preferably from 200 to 1200 ppm, and most preferably from 300 to 1000 ppm. Other initiators may also be present in the process of the present invention in combination with the t-aikylperitaconates 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-peroxypiviiate, and azo compounds such as azo bisisobutyro-πitrile, 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, for example, 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, for example up to 50 percent 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 US-A-2,727,884 and US-A- 3,639,372. The polymerization is typically conducted at temperatures from 80 to 170 °C, preferably from 90 to 160° C, more preferably from 100 to 155° C, and most preferably from 110 to 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 75,000 to 800,000 as measured by gel permeation chromatography (GPC); typically from 90,000, preferably from 100,000, more preferably from 150,000, and most preferably from 200,000 to 700,000, preferably to 600,000, more preferably to 550,000 and most preferably to 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 inteφreted. 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 (Mn) 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 ), 0.01 9 g of t-butylpeπtaconate (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 mis of each solution are placed in two ampoules (12 x 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 ramping 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 appropπate 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/M n Sol.1 * 388 460 0 16.4/1.5 188 75 314 2.5 sol. r 388 460 0 75.6/2.88 227 112 360 2.0 Sol.2 388 460 225 16.3/1.5 193 76 330 2.5 Sol.2 388 460 225 80.4/2.88 243 115 401 2.1 Sol.3 388 460 469 19.9/1.5 187 69 331 2.7 Sol.3 388 460 469 86.3/2.88 253 114 428 2.2 Sol.4 0 0 773 12.8/1.5 297 94 511 3.2 Sol.4 0 0 773 88.6/2.88 346 146 587 2.4 * Comparative Example Mw, Mz and polydispersity all increase by the addition of BPIT 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 BPIT 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 2.1 2 BPBF/200 2 10.3 634 256 1219 2.5 3 BPBF/200 3 23.3 612 237 1215 2.6 4 BPBF/200 4 40.7 551 212 1114 2.6 5 BPBF/200 5 59.6 490 187 992 2.6 6 BPBF/400 1 4.1 564 250 1019 2.3 7 BPBF/400 2 13.8 689 236 1470 2.9 8 BPBF/400 3 31.3 700 220 1506 3.2 9 BPBF/400 4 47.6 653 220 1460 3.0 10 BPBF/400 5 67 567 208 1234 2.7 11 BPBF/800 1 5.6 454 190 868 2.4 12 BPBF/800 2 20.5 711 196 1671 3.6 13 BPBF/800 3 42.8 843 206 2060 4.1 14 BPBF/800 4 62.2 799 218 1910 3.7 15 BPBF/800 5 87.6 731 207 1790 3.5 16 BPIT/166 1 5.5 484 263 746 1.8 17 BPIT/166 2 13.5 437 228 687 1.9 18 BPIT/166 3 24.9 435 218 710 2.0 19 BPIT/166 4 38.9 452 217 758 2.1 20 BPIT/166 5 59.8 442 209 764 2.2 21 BPIT/331 1 7.3 397 212 625 1.9 22 BPIT/331 2 17.1 377 198 601 1.9 23 BPIT/331 3 30 397 197 663 2.0 24 BPIT/331 4 47.2 462 21 1 803 2.2 25 BPIT/331 5 69.1 469 206 840 2.3 26 BPIT/662 1 9.6 314 169 498 1.9 27 BPIT/662 2 21.6 323 167 523 1.9 28 BPIT/662 3 38.3 388 181 668 2.1 29 BPIT/662 4 59.8 487 209 892 2.3 30 BPIT/662 5 91.7 529 209 995 2.5 Mw continues to build when using BPIT whereas Mw drops off after 40 percent conversion when using BPBF. EXAMPLE 3 Styrene (1350 g), ethylbenzene (150 g) and an initiator are placed in a 1500 mil liter (mL) reactor which is described in Fig. 1 of US-A-4,239,863. The initiators are 1 ,1-bιs(t- butylperoxy)cyclohexane (BBPC)(300 ppm), t-butyl peroxybenzoate (tBPB)(500 ppm), 2,2-bιs-(4,4- di(t-butylperoxy)cyclohexyl)propane (PK12)(300 ppm), and t-butyipeπtaconate (tBPIT)(500 ppm). The mixture is heated to 100 °C over 50 mm and then to 150 °C at a rate of 10 °C/hour. Samples are drawn periodically from the polymenzation 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 15 204 2 20 242 22 284 22 297 26 256 3 33 236 35 256 36 298 40 313 4 59 251 60 230 58 317 60 327 5 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 polymenzation 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.

Claims: CLAIMS: 1. A free radical polymerization process for producing a branched polymer having a Mw of at least 75,000 from a vinyl aromatic monomer, comprising polymerizing a vinyl aromatic monomer 5 in the presence of a free radical initiator of the formula: wherein R is H, alkyl, aryl, or aralkyl; and R' is alkyl or aryl. 2. The process of Claim 1 wherein the free radical initiator is t-butylperitaconate. o 3. The process of Claim 2 wherein the t-butylperitaconate is present in an amount of from 10 to 1500 ppm based on the total weight of the starting monomer. 4. The process of Claim 1 wherein the polymerization is conducted at a temperature from 80 to 170 °C. 5. The process of Claim 1 wherein the branched polymer produced has a Mw of at least 5 90,000. 6. The process of Claim 1 wherein at least one other free radical initiator is present in addition to the peritaconate initiator. 7. The process of Claim 6 wherein the other free radical initiator is selected from the group consisting of t-butylhydroperoxide, ditertiary-butylperoxide, cumene hydroperoxide, o dicumylperoxide, 1 ,1 -bis(tertiary-butylperoxy)-3,3,5-trimethyl-cyclohexane, t-butylperoxybenzoate, 1,1-bis(t-butylperoxy)-cyclohexane, benzoylperoxide, succinoylperoxide and t-butyl-peroxypivilate, and azo compounds such as azo bisisobutyro-nitriie, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbo-nitrite, azobismethyl isolactate and azobiscyanovalerate or mixtures thereof. 8. The process of Claim 7 wherein the other free radical initiator is selected from the 5 group consisting of t-butylperoxy-benzoate, benzoyl peroxide, or a mixture thereof. 9. The process of Claim 1 wherein the vinyl aromatic monomer is polymerized in the presence of a chain transfer agent. 10. The process of Claim 9 wherein the chain transfer agent is a mercaptan or a thiol compound. 0 11. The process of Claim 10 wherein the chain transfer agent in n-dodecylmercaptaπ.

Inventors: PIKE WILLIAM C, PRIDDY DUANE B, VOLLENBERG PETER H T, PIKE, WILLIAM, C., PRIDDY, DUANE, B., VOLLENBERG, PETER, H., T.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1996
Main classification: C08F
IPC codes: C08F2, C08F4