WO2001016195A1

WO2001016195A1 - Polymerization process for producing bimodal monovinylidene aromatic polymers

Info

Publication number: WO2001016195A1
Application number: PCT/US2000/022300
Authority: WO
Inventors: William C. Pike; Abigail Baker; Duane B. Priddy
Original assignee: The Dow Chemical Company
Priority date: 1999-09-01
Filing date: 2000-08-15
Publication date: 2001-03-08
Also published as: CO5160366A1; AU6772600A; TW572910B

Abstract

The present invention relates to a process for producing a monovinylidene aromatic polymer having a distinct bimodal Mw distribution using both cationic and free radical polymerization processes.

Description

POLYMERIZATION PROCESS FOR PRODUCING BIMODAL MONOVINYLIDENE AROMATIC POLYMERS

The process of the present invention relates to a polymerization process for producing monovinylidene aromatic polymers.

Generally, monovinylidene aromatic polymers have good toughness and flow properties. However, there has long been a desire to alter these properties, by increasing the flow of the polymer while maintaining or increasing toughness. Lower viscosity is normally attained by lowering the molecular weight of the polymer. However, in some applications, such as blown foams, adequate toughness cannot be attained at lower molecular weights. Plasticizers have also been used to increase the flow properties of monovinylidene aromatic polymers. However, plasticizers typically decrease the heat distortion temperature. Another method of increasing flow involves the addition of a very low molecular weight fraction to the monovinylidene aromatic polymer. However, this requires an additional compounding step and also adds cost to the polymer.

Therefore, there still remains a need for a process of producing a monovinylidene aromatic polymer having good toughness and improved flowability which is cost effective.

The present invention relates to a process for producing a monovinylidene aromatic polymer having a distinct bimodal molecular weight distribution comprising:

a) cationically polymerizing a vinyl aromatic monomer in the presence of a carbocation generator, to produce a first polymer composition comprising

i) a low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight (Mw) of from 1 ,000 to 5,000; and

b) polymerizing a vinyl aromatic monomer under free radical polymerization conditions in the presence of the low molecular weight monovinylidene aromatic polymer fraction, to a level of completion of from 70 to 95 percent, to produce a second polymer composition comprising

ii) a high molecular weight monovinylidene aromatic polymer fraction having a

Mw of from 135,000 to 400,000, and iii) the low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight of from 1 ,000 to 5,000.

This process can be utilized under batch or continuous polymerization conditions. Preferably the process is a continuous process for producing a monovinylidene aromatic polymer having a distinct bimodal molecular weight distribution comprising:

a) cationically polymerizing a vinyl aromatic monomer in the presence of a carbocation generator to a level of completion of from 1 to 10 percent, to produce a first polymer composition comprising:

1 ) a low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight (Mw) of from 1 ,000 to 5,000, and

2) unreacted vinyl aromatic monomer; and

b) polymerizing the first polymer composition under free radical polymerization conditions to a level of completion of from 70 to 95 percent, to produce a second polymer composition comprising:

3) a high molecular weight monovinylidene aromatic polymer fraction having a

Mw of from 135,000 to 400,000, and

4) the low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight of from 1 ,000 to 5,000.

This process utilizes both cationic and free-radical polymerization processes to produce a monovinylidene aromatic polymer having excellent toughness and flowability properties without incorporating additional additives. In the continuous process, the low molecular weight fraction, which acts as a plasticizer, is produced in situ during the polymerization.

In one embodiment, the present invention is a process for producing a monovinylidene aromatic polymer having a distinct bimodal Mw distribution. A bimodal Mw distribution refers to a weight average molecular weight distribution of a polymer which has two distinct peaks or maxima. Monovinylidene aromatic polymers are produced by polymerizing a vinyl aromatic monomer. Vinyl aromatic monomers include, but are not limited to those described in U.S. Patents 4,666,987, 4,572,819 and 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 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, wherein alkylphenyl refers to an alkyl substituted phenyl group, 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 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 of the vinyl aromatic monomer 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. Patent No's 3,123,655, 3,346,520, 3,639,522, and 4,409,369.

Cationic polymerization of a vinyl aromatic monomer is well known in the art and specifically disclosed in U.S. Patent 4,1 12,209, and in JP 02180907, JP 01121305, JP 63068629, and JP 55104219. These references disclose a process wherein low molecular weight monovinylidene aromatic polymer is produced by contacting a solution of vinyl aromatic monomer with a cation generator, under substantially isothermal conditions, at a temperature between 0°C and 180°C.

Carbocation generators are protonic acids such as Bronsted or Lewis acids, which react with olefinic double bonds of an organic compound to generate a carbocation and can be characterized as both homogeneous and heterogeneous. The term 'homogeneous carbocation generator', as used herein, refers to any homogeneous catalyst for cationic polymerization of a vinyl aromatic monomer, including any necessary co-catalyst , as defined hereafter. For example, homogeneous carbocation generators include the protonic acids such as sulfuric, hydrochloric, phosphoric, perchloric, dichloro- and trichloroacetic acids, and preferably Freidel-Crafts catalysts, such as boron trichloride, boron trifluoride, etherates thereof, stannic chloride, titanium tetrachloride, aluminum halides and alkyl aluminum halides, in conjunction with any necessary co-catalyst. A small quantity of water is a common co-catalyst for most of the Freidel-Crafts catalyst and also for protonic acids, but other co-catalysts are also known and are often dependent on the solvent employed. Without such a co-catalyst, the polymerization proceeds very slowly, if at all. Preferred homogeneous carbocation generators or catalysts are BF₃ or AICI₃ and a water co-catalyst, with BF₃ being most preferred. The term 'heterogeneous carbocation generator', as used herein, refers to any heterogeneous catalyst for cationic polymerization of vinyl aromatic monomers. For example, heterogeneous carbocation generators include sulfonated ion exchange resins, heteropolyacids, perfluorinated resins such as perfluorosulfonic acid (NAFION™ resins), and acidic or activated clays. These carbocation generators are essentially insoluble in vinyl aromatic monomer. Preferably, the carbocation generator is heterogeneous.

If a homogeneous carbocation generator is employed, it is generally added to the vinyl aromatic monomer prior to or during polymerization and is employed in an amount that is, in total, 1 to 10 percent of the amount of catalyst needed for 99.9 percent polymer conversion to occur. For a BF₃/water carbocation generator, this amount is typically from approximately 5 to 250 ppm, based on the weight of the vinyl aromatic monomer.

A heterogeneous carbocation generator is typically contained in a fixed or fluidized reactor through which the vinyl aromatic monomer is passed at a temperature and feed rate such that 2 to 15 percent of the monomer is converted to polymer. This reactor may be a separate vessel or part of the vessel used for free radical polymerization. The catalyst may also be used as a slurry from which the product can be decanted.

The carbocation generator may be added to the vinyl aromatic monomer stream prior to polymerization, in stages, or, in the case of the heterogeneous catalysts, can be contained within a fixed bed through which the monomer stream is fed.

Typically, the vinyl aromatic monomer is first dissolved in an inert organic solvent which is a solvent for the vinyl aromatic monomer, prior to cationic polymerization. The solvent is not reactive with the vinyl aromatic monomer or appreciably with the carbocation generator. The solvent may generally be described as an aromatic or aliphatic hydrocarbon or halohydrocarbon and include compounds such as 1 ,2-dichloroethane, ethylbenzene, toluene, benzene, carbon tetrachloride, ethyl chloride, ethylene dichloride, nitrobenzene, chlorobenzene, ispropyl chloride, t-butyl chloride, hexane, cyclohexane, sulfur dioxide, and DMF.

The cationic polymerization is conducted until the desired level of low molecular weight polymer is produced. Generally, molecular weights of no more than 20,000 or 30,000 are achieved by cationic polymerization. Typically the weight average molecular weight (Mw) of the low molecular weight polymer fraction is from 1 ,000 to 5,000, preferably from 1 ,000 to 4,000, more preferably from 1 ,000 to 3,000 and most preferably from 1 ,000 to 2,000. Mw, as referred to throughout this application, is measured according to gel permeation chromatography (GPC).

The cationic polymerization can be accomplished by a batch process or as part of a continuous process. In a batch process, the low molecular weight monovinylidene aromatic polymer is produced and then combined with additional vinyl aromatic monomer, wherein the vinyl aromatic monomer is polymerized under free radical polymerization conditions.

In a continuous process, the cationic polymerization produces a first polymer composition comprising vinyl aromatic monomer, low molecular weight monovinylidene aromatic polymer, solvent (if used) and catalyst if a homogeneous catalyst has been used. Typically, a small addition of water, alcohol, for example, methanol, or ammonia is added to the stream in order to neutralize any residual homogeneous catalyst, if used, prior to free radical polymerization. This composition is then exposed to free radical polymerization conditions. Additional vinyl aromatic monomer can also be added prior to the free radical polymerization. Free radical polymerization conditions include thermal initiation, as well as initiation using a free radical polymerization catalyst. Free radical polymerization catalysts are well known in the art, any of which can be used in the process of the present invention. Typical initiators include azo compounds and peroxides such as tert-butylperoxybenzoate, tert-butylperoxyacetate, di-tert-butylperoxide, dibenzoylperoxide, dilauroylperoxide, 1 ,1-bis- tert-butylperoxycyclohexane, 1 ,1 ,-bis-tert-butylperoxy-3,3,5-trimethylcyclohexane and dicumylperoxide.

The amount of free radical initiator, if used, is dependent upon the Mw desired for the high molecular weight fraction. Typically, the initiator is present in an amount of from 10 to 2000 ppm based on the weight of unreacted vinyl aromatic monomer. Suitable free radical polymerization conditions are well known in the art and described in U.S. patents such as U.S. 5,191 ,040, U.S. 5,087,738 and U.S. 4,275,182. The free radical polymerization may be a bulk, solution, emulsion or suspension process. Preferably the process of the present invention is a continuous bulk or solution polymerization.

The free radical polymerization is typically conducted at temperatures from 80°C to

170°C, preferably from 110°C to 160°C, with 115°C to 150°C being most preferred.

The free radical polymerization is conducted until the desired level of high molecular weight polymer is produced. Typically the Mw of the high molecular weight polymer fraction is from 135,000 to 400,000 preferably from 150,000 to 375,000, more preferably from 150,000 to 350,000 and most preferably from 150,000 to 330,000.

The amounts of low molecular weight and high molecular weight polymer fractions in the bimodal Mw polymer produced will depend upon the desired flow properties of the bimodal polymer and the molecular weight of the high molecular weight fraction. Higher molecular weight polymer components will require more low molecular weight fraction in order to obtain adequate flow properties. The amount of low molecular weight polymer fraction in the bimodal polymer is typically from 1 to 15 weight percent, preferably from 1 to 10 percent, more preferably from 1 to 8 percent and most preferably from 1 to 6 percent, based on the total weight of the bimodal polymer produced. The amount of high molecular weight polymer fraction in the bimodal polymer is typically from 85 to 99 weight percent, preferably from 90 to 99 percent, more preferably from 92 to 99 percent and most preferably from 94 to 99 percent, based on the total weight of the bimodal polymer produced.

The composition containing both high and low molecular weight vinyl aromatic polymers may be any combination of two vinyl aromatic polymers but is preferably a blend of polymers having the same composition (that is, homopolymers of the same monomeric units or copolymers having the same comonomeric units in the similar rations). More preferably, both polymers are polystyrene.

After the free radical polymerization, the polymer composition is removed from the reaction zone and devolatilized to remove unreacted monomer, and solvent.

Additionally, other additives may be present in the process of the present invention including antioxidants, plasticizers, flame retarding agents, and chain transfer agents. In one embodiment of the present invention, a monovinylidene aromatic polymer having a distinct bimodal molecular weight distribution is produced using a continuous process comprising:

a1 ) cationically polymerizing a vinyl aromatic monomer in the presence of a homogeneous carbocation generator to a level of completion of from 1 to 10 percent, to produce a first polymer composition comprising:

1) a low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight (Mw) of from 1 ,000 to 5,000, and

II) unreacted vinyl aromatic monomer; and

III) residual homogeneous carbocation generator;

a2) neutralizing the residual homogeneous catalyst, and

b) further polymerizing the first polymer composition under free radical polymerization conditions to a level of completion of from 70 to 95 percent, to produce a second polymer composition comprising:

IV) a high molecular weight monovinylidene aromatic polymer fraction having a Mw of from 135,000 to 400,000, and

V) the low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight of from 1 ,000 to 5,000.

In another embodiment of the present invention, a monovinylidene aromatic polymer having a distinct bimodal molecular weight distribution is produced using a continuous process comprising:

a1 ) cationically polymerizing a vinyl aromatic monomer in the presence of a heterogeneous carbocation generator to a level of completion of from 1 to 10 percent, to produce a first polymer composition comprising:

2) unreacted vinyl aromatic monomer; and b) further polymerizing the first polymer composition under free radical polymerization conditions to a level of completion of from 70 to 95 percent, to produce a second polymer composition comprising:

3) a high molecular weight monovinylidene aromatic polymer fraction having a Mw of from 135,000 to 400,000, and

4) the low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight of from 1 ,000 to 5,000 .

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.

EXAMPLE 1

GENERAL PROCEDURE FOR FIXED BED REACTOR:

A 1.3 cm. I.D. X 38 cm glass reactor having an outer jacket containing circulated heat transfer fluid and a thermocouple sleeve is partially filled with ethylbenzene. The catalyst is slowly added until the desired amount (5 to 15 g) is loaded into the column, leaving at least 5 ml of free reactor volume above the catalyst. The vinyl aromatic monomer, with or without solvent, is then pumped through the catalyst bed at the desired temperature and feed rate. Feed rates vary from a minimum of 1.0 ml/min. to a maximum of 20 ml/min. Samples are taken periodically and tested for percent solids (conversion) and molecular weight.

PREPARATION OF THE LOW MOLECULAR WEIGHT POLYMER:

The reactor is loaded with 10 g of CWC Montmorillonite Clay H+ as described above. Styrene is passed through the reactor at 50°C and 2 ml/min. to obtain a stream which contains 3 weight percent of low molecular weight polystyrene with a Mw of 1500, an Mn of 630, and a styrene dimer content of less than 5 percent (based on polymer).

PREPARATION OF THE BIMODAL Mw POLYMER:

A solution of styrene monomer containing 6 weight percent low molecular weight polystyrene resin having a Mw of 1600 (Mn of 700) prepared by the method described above, is thermally polymerized at 130°C to obtain a bimodal resin, wherein 90 percent of the resin has a Mw of 340,000 (Mn of 195,000) and 10 percent of the resin has a Mw of 1600 (Mn of 700).

EXAMPLE 2:

A solution containing 3.5 weight percent low molecular weight polystyrene (Mw 1400, Mn 660) prepared as in Example 1 , 8 percent ethylbenzene, 88.5 percent styrene monomer, and 30 ppm sulfoethylmethacrylate is passed through a stirred tube reactor with a temperature gradient from 128 to 160°C. A slurry of zinc stearate in ethylbenzene is added about halfway through the reactor such that the final concentration of zinc stearate in the product is 2400 ppm. The partial polymer is devolatilized at 230°C and 10 mm, followed by pelletization, to give a bimodal polystyrene resin wherein 95 percent of the resin has a Mw of 298,000 (Mn of 127,000) and 5 percent has a Mw of 1400. This resin exhibits an enhanced flow rate (MFR of 5 vs. 2) over monomodal high Mw polystyrene prepared from styrene monomer under identical polymerization conditions, while decreasing the Tg of the resin only slightly (106°C vs. 108°C).

Claims

CLAIMS:

1 . A process for producing a monovinylidene aromatic polymer having a distinct bimodal molecular weight distribution comprising:

a) cationically polymerizing a vinyl aromatic monomer in the presence of a carbocation generator to produce a first polymer composition comprising:

i) a low molecular weight monovinylidene aromatic polymer having a weight average molecular weight (Mw) of from 1 ,000 to 5,000; and

b) polymerizing a vinyl aromatic monomer in the presence of the first polymer composition under free radical polymerization conditions to a level of completion of from 70 to 95 percent, to produce a second polymer composition comprising:

ii) a high molecular weight monovinylidene aromatic polymer fraction having a Mw of from 135,000 to 400,000, and

iii) the low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight of from 1 ,000 to 5,000 .

2. A continuous process for producing a monovinylidene aromatic polymer having a distinct bimodal molecular weight distribution comprising:

2) unreacted vinyl aromatic monomer; and

3) a high molecular weight monovinylidene aromatic polymer fraction having a Mw of from 135,000 to 400,000, and 4) the low molecular weight monovinylidene aromatic polymer fraction having a weight average molecular weight of from 1 ,000 to 5,000.

3. The process of Claim 1 or 2 wherein the vinyl aromatic monomer is styrene.

4. The process of Claim 1 or 2 wherein the low molecular weight monovinylidene aromatic polymer fraction has a Mw of from 1 ,000 to 2,000.

5. The process of Claim 1 or 2 wherein the high molecular weight monovinylidene aromatic polymer fraction has a Mw of from 150,000 to 400,000.

6. The process of Claim 1 or 2 wherein the monovinylidene aromatic polymer fraction of i) is from 1 to 10 weight percent of the bimodal polymer produced.

7. The process of Claim 1 or 2 wherein the carbocation generator is BF₃.

8. The process of Claim 1 or 2 wherein the carbocation generator is a sulfonated ion exchange resin, a heteropolyacid, perfluorinated resin, or an acidic or activated clay.

9. The process of Claim 1 or 2 wherein the free radical polymerization is conducted in the presence of a free radical polymerization catalyst.

10. The process of Claim 9 wherein the free radical polymerization catalyst is 1 ,1- bis(t-butylperoxy)cyclohexane .