PROCESS FOR DISPERSION POLYMERIZING BUTADIENE

Abstract: Butadiene is polymerized alone or with a monomer such as styrene in a dispersion in propane, butane or a mixture thereof with minor amounts of other C₄ to C₆ alkanes, e.g., n-pentane, a dispensing medium, and a dispensing agent which is preferably a styrene/hydrogenated isoprene block copolymer to achieve stable dispersions at higher solids and butadiene levels than is possible with similar alkane dispersants.

Description: This invention relates to processes for dispersion polymerising conjugated dienes. Polymers and copolymers containing polymerised conjugated dienes such as butadiene are typically prepared in polymerisation systems which include a solvent inert to the polymerisation in which the final polymer or copolymer is at least partially soluble. This creates a problem of complete solvent removal. This invention seeks to remedy this defect. US 3948846 discloses a dispersion of block copolymer particles in a saturated aliphatic hydrocarbon diluent, the block copolymer consists of a thermoplastic tertiary butyl styrene polymer block A of molecular weight 10,000-100,000 which is soluble in the diluent, an elastomeric conjugated diene polymer block B of molecular weight 1,000-100,000 which is terminally attached to block A, and a styrene homopolymer or copolymer block C of molecular weight 20,000-1,000,000 which is terminally attached to block B, is insoluble in the diluent and forms at least part of the core of the particles. The particles may be separated from the diluent while retaining their previous molecular organization, i.e. with the insoluble block C present in the core of the particles and the thermoplastic block A remote from the core, and may be converted by hot pressing into shaped articles have impact resistance. A process is also described for produce such a particle dispersion by anionic polymerization of monomers in the hydrocarbon diluent to form successively the polymer blocks A,B and C. US 4098980 discloses a process for the non-aqueous dispersion polymerization of a conjugated diolefin monomer, for instance, butadiene or isoprene, in a liquid hydrocarbon dispersion medium, for instance, n-butane or n-pentane with a Ziegler Natta catalyst, for instance, triisobutylaluminum/titanium tetrachloride while said conjugated diolefin is in the presence of a block copolymer dispersion stabilizer. The block copolymer dispersion stabilizer is a copolymer which contains at least two blocks of polymer linked by chemical valences, at least one block (A block) is soluble in liquid organic dispersion medium and at least another block (B block) is insoluble in the dispersion medium and the stabilizer acts to disperse polymers of conjugated diolefins which are formed in the stabilizer's presence. The A block is exemplified by polyisoprene, poly(t-butyl styrene) and poly(vinyl toluene). The B block is exemplified by polystyrene, poly-(α-methyl styrene), a copolymer of styrene and isoprene, poly(methyl methacrylate) and the like. According, therefore, to the present invention there is provided a process for dispersion polymerisation of at least two monomers, wherein the first monomer comprises styrene or an alkyl styrene and the second monomer comprises a conjugated diene, to a block copolymer containing at least 25 weight percent of the diene, which process comprises: conducting the polymerisation under superatmospheric pressure with an anionic polymerisation catalyst in a dispersing medium comprising C₁ to C₆ aliphatic hydrocarbon containing a major proportion of propane, n-butane or isobutane and a hydrogenated styrene or an alkyl styrene/conjugated diene block copolymeric dispersing agent effective to provide a stable dispersion, and recovering the block copolymer product. The process is applicable to the preparation of block copolymers comprising one or more A and B blocks in which the A blocks are polymerised styrene or alkyl styrene units and the B blocks are polymerised conjugated diene units. Suitable alkyl styrenes include methyl-substituted styrenes such as para-methylstyrene. Styrenes with polar groups such as chlorine substituents cannot be used in anionic polymerisation to which this invention is particularly applicable. One or more appropriate styrene monomers can be used. The B block comprising polymerised conjugated diene units can be formed from any anionically polymerisable conjugated diene, particularly butadiene, isoprene and dimethyl-butadiene, especially butadiene. Suitable anionic catalysts include butyl-lithium, sec-butyl-lithium and t-butyl-lithium. The concentration of the catalyst is generally from 10⁻¹ to 10⁻⁴ moles/100 g of monomer. The reaction is conducted in an aliphatic hydrocarbon medium comprising a major proportion of propane, isobutane or n-butane, which is a non-solvent for the polymer. The medium can contain alkanes having between about 4 and 6 carbon atoms, in minor amounts in addition to the propane or butane. Examples of solvents useful in conjunction with propane or butane are isobutane, pentane, isopentane, hexane, 2,2-dimethyl-butane, and petroleum ether. These solvents preferably comprise from 0 to 25 weight percent and most preferably from 0 to 15 weight percent of the dispersing medium, the major proportion being propane or butane. The preferred major component in the dispersing medium is n-butane and the preferred minor component is n-pentane. Preferably the n-pentane comprises 10 to 20 weight percent, and the n-butane comprises 80 to 90 weight percent. The process of polymerisation can be carried out at temperatures between 30°C and about 100°C. Because propane or butane, which are necessary components of the dispersing medium, boil well below room temperature, the reaction is conducted under super-atmospheric pressure, suitably 2 to 18 x 10⁵Pa (2 to 18 bars), preferably 2 to 10 x 10⁵Pa (2 to 10 bars) and most preferably 4 to 8 x 10⁵Pa (4 to 8 bars). The highly volatile solvent system is advantageous in separating the polymer particles from the dispersing medium. The polymeric dispersing agent which is used to maintain the polymer in suspension as it is formed is an A-B block copolymer containing blocks which are similar to or the same as the blocks in the copolymer being produced. A diblock polymer comprising hydrogenated styrene and isoprene blocks having a number average molecular weight of 140,000 and a styrene content of about 40% has been found to be particularly advantageous. The dispersing agent is generally present in amounts of about 0.1 to 5.0 weight percent of the monomers. The polymer product may be recovered by flash drying; alternatively, the polymer product containing the dispersing medium may be transferred to an extruder and directly extruded into a foam using the dispersing medium as a foaming agent. Generally linear block copolymers prepared in accordance with this invention have two or more (e.g., 2-10) alternating blocks of (A) units comprising polymerised styrene or an alkyl styrene and (B) units comprising polymerised conjugated diene units. Where the copolymer contains three or more blocks, it is preferred that the terminal blocks are (A) blocks. Thus, a copolymer containing five blocks has the configuration A-B-A-B-A. Block copolymers containing less than 75 weight percent of the A (styrenic) units are preferred. Thus, the B component generally comprises at least 25 weight percent of the block copolymer, and preferably 25 to 80, for example 25 to 40, weight percent. The process of this invention is also suitable for the preparation of "star-block" or radial copolymers having a non-linear configuration such as described in U.S. Patents 3,639,517 and 4,091,053. Because the process used for polymerisation is a dispersion (slurry) process, the high viscosity of high molecular weight polymer imposes no constraint on the molecular weight of the block copolymers. And since high molecular weight polymers have desirable physical properties, the process of this invention is advantageously used in producing polymers having high molecular weight. Accordingly, number average molecular weights in the range of 80,000 to 200,000 are contemplated and suitable copolymers in the molecular weight range of 100,000 to 150,000 have been prepared. Although the block copolymers have high molecular weights, high polymer concentrations relative to the solvent dispersing medium in the range of 20 weight percent to 40 weight percent of polymer are readily achieved. Using polymerisation techniques, it would not be possible to obtain such high molecular weights at the high polymer concentrations readily achievable with the process of this invention because the solution viscosities would be too high. Block copolymers having excellent physical properties, particularly elongation and impact strength, can be obtained. The polymers are useful, as such, in the preparation of containers such as cups, that can be coextruded with high impact polystyrene to achieve a glossy and clear surface, and they can be blended with crystal polystyrene to improve the physical properties of polystyrene. They exhibit excellent compatibility with polystyrene than many presently available materials used for this purpose. In accordance with a further aspect of this invention, there is provided a foamable composition which comprises a block copolymer comprising polymerised styrene or an alkyl styrene and at least 25 weight percent of polymerised conjugated diene and, as foaming agent, a C₄ to C₆ aliphatic hydrocarbon containing a major proportion of propane, butane or isobutane. This invention also provides a directly extruded foam formed from such a composition. The following Examples, in which all parts are by weight unless otherwise stated, illustrate the invention. Example 1 A 1 litre glass high pressure reactor which is controlled by 930.79kPa (135 psi) rupture disk was flushed with N₂ and the following ingredients added: 360 g n-butane 60 g pentane 40 g styrene 1.2 g polymeric dispersant (Shell-Vis 1702 ) 0.1 cc tetrahydrofuran The reactor was pressurised by N₂ up to 413.69 to 689.48kPa (60-100 psi) to keep solvents in liquid form depending on temperature (25°C-65°C) and 0.5 ml of 1.0 M sec-butyl lithium was added. Reaction was carried out for 45 min.- 2 hours. Later 30 g of butadiene was introduced into the pressurised reactor. Temperature was 60°C and upon reaction completion, methanol was added to terminate the polymerisation and 10 ml. of 5% solution of Halby DCTDP (a phenolic antioxidant) was added. The dispersion was stable throughout the reaction. The pressure was released and the A-B block polymer was recovered as a powder form. Comparative Example Using the procedure of sample I but substituting n-hexane for the n-butane/n-pentane system resulted in agglomeration of swollen polymeric particles which exhibit very high viscosity and none of the characteristics of a dispersion. At this stage, the polymerisation of further monomer is unfeasible. Example 2 Following the procedure of Example 1 prior to termination of the reaction, additional styrene is added incrementally to add an A block resulting in an A-B-A block copolymer, and incremental amounts of styrene and butadiene are separately added in stages to produce A-B-A-B-A and A-B-A-B-A-B-A block copolymers.

Claims: A process for the dispersion polymerisation of at least two monomers, wherein the first monomer comprises styrene or an alkyl styrene and the second monomer comprises a conjugated diene, to a block copolymer containing at least 25 weight percent of the diene, which process comprises: conducting the polymerisation under superatmospheric pressure with an anionic polymerisation catalyst in a dispersing medium comprising C₄ to C₆ aliphatic hydrocarbon containing a major proportion of propane, n-butane or isobutane and a hydrogenated styrene or an alkyl styrene/conjugated diene block copolymeric dispersing agent effective to provide a stable dispersion, and recovering the block copolymer product. A process according to claim 1 wherein the dispersing medium comprises n-butane and from 0 to 25 weight percent of pentane. A process according to claim 2 wherein the dispersing medium comprises n-butane and from 10 to 20 weight percent of pentane. A process according to any preceding claim wherein the pentane comprises n -pentane. A process according to any preceding claim wherein the dispersing agent comprises a hydrogenated styrene/isoprene block copolymer. A process according to any preceding claim which is conducted at pressures from 2 to 10 x 10⁵Pa (2 to 10 bars). A process according to any preceding claim wherein the catalyst comprises sec-butyl lithium. A process according to any preceding claim wherein the polymer product containing the dispersing medium is transferred to an extruder and directly extruded into a foam by using the dispersing medium as a foaming agent. A process according to any preceding claim wherein the polymer product is recovered by flash drying. A process according to any of claims 1 to 9 wherein the polymer product comprises a block copolymer containing 3 or more alternating A and B units where (A) is polymerised styrene or alkyl styrene units and (B) is polymerised conjugated diene units. A process according to any of claims 1 to 9 wherein the polymer product comprises a star-block copolymer comprising (A) polymerised styrene or alkyl styrene units and (B) polymerised conjugated diene units. A foamable composition which comprises a block copolymer comprising polymerised styrene or an alkyl styrene and at least 25 weight percent of polymerised conjugated diene and, as foaming agent, a C₄ to C₆ aliphatic hydrocarbon containing a major proportion of propane, butane or isobutane. A foamable composition according to claim 12 wherein the co-polymer is defined in 10 or 11. A directly extruded foam formed from a composition according to claim 12 or 13.

Inventors: GUNESIN, BINNUR ZEYNEP, PINDRIS, PAUL ANDREW
Applicants: MOBIL OIL CORPORATION
Application year: 1988
Main classification: C08F36
IPC codes: C08F2, C08F36, C08J9, C08F6, C08F297