CO2-TERMINATED RUBBER BACKGROUND OF THE INVENTION The present invention relates to polymers that can be used as additives in other polymeric systems. More particularly, this invention relates to a process of polymerizing vinyl compounds and, more specifically, a method of terminating the polymerization process to produce a compound having high bulk viscosity, but low solution viscosity.

Certain engineered plastics, such as styrene-maleic anhydride copolymers (SMAs) and high impact polystyrenes (HIPS), are prepared in the presence of a rubber (e. g., a polybutadiene or a styrene-butadiene copolymer) to enhance toughness, impact strength and other properties. A low solution viscosity rubber can disperse more easily in the plastic phase and thus is desirable to users of rubber additives. With respect to HIPS, early in its formation process, phase separation begins because of immiscibility of the rubber within the polystyrene being formed and depletion of the styrene phase; in SMAs, a low solution viscosity may improve the clarity and the gloss of the resultant product.

While low solution viscosity of the additive rubber is highly desirable, it makes commercial handling difficult. Moreover, low solution viscosity typically leads to a liquid or semi-liquid material that is difficult to package and ship. Accordingly, a relatively high bulk viscosity material capable of being baled into a shippable and easy to handle form is desirable to producers of synthetic rubber.

Significant work has been conducted in polymerizing vinyl compounds, particularly conjugated dienes. Use of carbon dioxide, C02 as a terminating agent has been investigated previously. Specifically, C02 reaction termination has provided a reactive product by the immediate protonation or other activation of the polymeric material. For example, U. S.

Patent 3,070,579 teaches reacting a living polymer, i. e., a polymerizable chemo-aromatic hydrocarbon having reactive negatively charged end groups, with a compound such as C02, CS2, 1,2-propylene oxide or ethylene oxide while having the reactants dissolve in a liquid. The patent further states that, because of the reactive end groups, the bifunctional polymeric product is reacted with other groups or compounds.

SUMMARY OF THE INVENTION The present invention provides a method of forming a low solution viscosity, lithium carboxylate polymer including mer units of vinyl compounds. Generally, the process includes polymerizing at least one conjugated diene in the presence of an organolithium initiator substantially to completion and terminating the reaction via the addition of COs. The combination of steps in the present process provides terminating living anionic polymers with CO2, leaving the polymer chain end as a lithium carboxylate (P-COO-Li+). In contrast to prior such processes, the present invention does not protonate nor further react the resultant polymer. Rather, the present invention advantageously seizes on the advantages of using a high bulk viscosity, low solution viscosity rubber as demonstrated by the carboxylate form of the material.

The present invention advantageously provides a low molecular weight carboxylated polymeric material having a bulk viscosity particularly suitable for addition to SMA or HIPS plastics. Solutions of the rubbery material in monomers used to form plastic resin are generally low in solution viscosity, e. g., below about 75 cP, preferably below about 50 cP, most preferably below about 45 cP. The low solution viscosity provides higher gloss and allows greater rubber content to equivalent power levels. Never- theless, the material has a high bulk viscosity (e. g., Mooney Viscosity above about 45, preferably above about 60) and good resistance to cold flow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present low solution viscosity, high bulk viscosity polymers preferably are homopolymers of conjugated dienes terminated with carboxylate groups. To form these polymers, conjugated dienes are polymerized in the presence of an organometallic initiator in a solvent.

Polymerization is of the living anionic type, and the resulting polymers are terminated by C02 groups. Living polymerizations are polymerizations in which propagating centers do not undergo either termination or transfer. After essentially 100% conversion is reached, additional polymerization takes place by adding more monomer to the reaction system. The added monomer is also polymerized quantitatively.

Such polymerizations offer the potential for producing structures with defined end groups and block copolymers.

Although conjugated diene homopolymers are preferred products, conjugated diene copolymers may also be highly desirable where the comonomers impart desirable properties and do not detract from the polymer properties. The comonomers may be vinyl arenes including vinyl aromatic hydrocarbons having alkyl, aralkyl, or cycloalkyl groups attached to the aromatic nucleus and preferably having no more than 20 carbon atoms.

Typical of these aromatic comonomers are styrene, a-methyl styrene, vinyl toluene, ethyl styrene, p-cyclohexyl styrene, vinyl naphthalene, vinyl ethyl naphthalene, vinyl methyl naphthalene, vinyl butyl naphthalene, vinyl diphenyl, vinyl diphenylethane, 4-vinyl-4'-methyidiphenyl, and the like.

Preferably, such comonomers have no more than 20 carbon atoms. Where such comonomers are desired, generally at least 1%, preferably at least 5%, by weight is used and as much as 60%, but preferably no more than 30%, by weight is used.

A high vinyl content in the carboxylate terminated conjugated diene homo-or co-polymers can be desired. Suitable 1,2-vinyl modifiers may be added to the polymerization mixture to increase the vinyl content to as high as 90% of the conjugated diene-derived mer units. Exemplary 1,2-vinyl modifiers include one or more of hexamethyl phosphoric acid triamide, N, N, N', N'-tetramethylethylene diamine, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethyl- ene glycol dimethyl ether, tetrahydrofuran, 1,4-diazabicyclo [2. 2.2] octane, diethyl ether, triethylamine, tri-n-butylamine, tri-n-butylphosphine, p-dioxane, 1,2-dimethoxy ethane, dimethyl ether, methyl ethyl ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether, anisole, dibenzyl ether, diphenyl ether, dimethylethylamine, bis-oxalanyl propane, tri-n-propyl amine, trimethyl amine, triethyl amine, N, N-dimethyl aniline, N-ethylpiperidene, N-methyl-N- ethyl aniline, and N-methylmorpholine.

One effective initiator for living anionic polymerizations is hydrocarbyl lithium, represented by the formula R'Li where R1 is a C1-C20 hydrocarbyl radical, advantageously a C1-C20 aliphatic radical, preferably a C3-C6 aliphatic radical, but may also be C6-C20 cycloaliphatic or aromatic radical, preferably Ce-Ci2. Preferred R'Li compounds are n-butyl and sec-butyl lithium. Other suitable R'Li compounds include but are not restricted to those in which the R'groups are ethyl, n-propyl, isopropyl, n-amyl, sec- amyl, sec-hexyl, n-hexyl, octyl, nonyl, decyl, dodecyl, octadecyl, phenyl, tolyl, dimethyl-phenyl, ethylphenyl, naphthyl, cyclohexyl, methylcyclohexyl, ethylcyclohexyl, cycloheptyl, allyl, 2-butenyl, 2-methyl butenyl, cyclopentyl- methyl, methylcyclopentylethyl, phenylethyl, cyclopentadieneyl, naphthyl, phenylcyclohexyl, etc. Generally, the catalyst is used in a proportion of about 0.15-20 mmol initiator per 100 g monomer.

The polymerization is advantageously conducted in a solvent such as, for example, C3-C20 alkanes, preferably C5-C10 alkanes, such as butane, pentane, hexane, heptane, octane, nonane, decane, etc. Non-aromatic hydrocarbon solvents such as cycloalkanes, e. g., cyclohexane, methylcyclo- hexane, cycloheptane, etc., may also be used. C6-C20 cycloalkanes are preferred, more preferably C5-C10 cycloalkanes. Toluene and other aromatics may act as telomerizing agents and thereby reduce the average molecular weight of the product. However, where this is not critical, aromatic solvents may be used. Advantageously, a butadiene concentration of about 15-50% is desirable, preferably about 20-25%.

Standard precautions against contamination of an organometallic system with impurities such as water, air, etc., which deactivate or reduce the efficiency of the system should be taken. Consequently, the solvent, reagents, reactor and atmosphere in the reactor area are treated accordingly to avoid such contaminants. Advantageously, less than 25 ppm, preferably less than 5 ppm, of water is present during polymerization.

The polymerizations can be conducted in autoclaves, pressurized reactors or bottles capable of withstanding the pressures generated at the temperature used. Preferably, the pressures will be in a range of about 34- 760 kPa, more preferably between about 200 and 700 kPa. Temperatures are preferably between about room temperature to about 120°C, more preferably between about 30° and 100°C.

While a substantial amount of polymerization is effected within one hour, additional polymerization can be effected at longer residence times, e. g., 3 hours. However, generally 6 hours or more are desired for greater yields, and while polymerization is generally substantially completed within 16 hours, depending on the temperature, there is no harm or adverse result in allowing polymerization to continue 70 hours or more.

When polymerization is completed, the catalyst is deactivated by the addition of a small amount of C02, which is preferably added in an amount of at least about 0.5 up to more than 1 molar equivalent of Li initiator. The C02 terminates the living polymer chains, resulting in carboxlyate end groups on most of the polymer chains. It can be added to the polymeriza- tion mixture by bubbling through in a gaseous form. The C02 then reacts with the reactive end groups of the living polymer chains to effectively end the living polymerization. The resulting polymer has carboxylate end groups which are stabilized by the Li+ initiator residues present in the polymeriza- tion mixture along with some undesired covalently coupled polymer chains.

A small amount of antioxidant, such as di-t-butyl cresol, is preferably added to the polymer product. The antioxidant is preferably added in an amount less than 4 weight percent, more preferably less than 2 weight percent of the total solution. The polymer can be recovered and dried before processing, preferably by drum drying at an appropriate temperature for evaporation of remaining solvent. Alternatively, a steam desolventiza- tion method is used to recover the polymer product.

The number average molecular weight (Mn) of the carboxylate- terminated polymer product is advantageously in the range of about 100,000 to 300,000, preferably about 150,000 to 250,000. Furthermore, narrow molecular weight ranges may be desired for specific properties.

Molecular weights reported herein are determined by Dilute Solution Viscosity (DSV).

Other polar additives, such as maleic anhydride, can be added to decrease solution viscosity if so desired. In addition, additives such as tetramethyl ethylene diamine, 2-ethylhexanoic acid, acetonitrile, and mixtures thereof may be added to the rubbery polymer composition to fur- ther improve the solution viscosity. The precipitated product can be filtered and washed with alcohol and finished by addition of suitable stabilizers and inhibitors followed by drying according to known methods. The product may be extracted with acids, bases, complexing agents, etc., to reduce catalyst residues to a low level prior to addition of stabilizers or inhibitors.

After formation, the rubbery polymer is then advantageously mixed with other polymer systems such as HIPS, SMAs, or acrylonitrile styrene butadiene copolymers (ABS). These polymer systems are reinforced and/or modified by the addition of rubbers such as those of the present invention.

EXAMPLES Example 1: Increased Bulk Viscosities via C02 Termination Mooney viscosities (bulk viscosity measurement) were determined for different living polymers which were split in two and terminated with a proton source (H+ from water or isopropanol) or C02 : Table 1: Mooney Viscosities M, Mooney Viscosity Mooney Viscosity with with H+ Termination C02 Termination Polybutadiene 151 K 23.7 112.2 Polybutadiene 163 K 35.8 94.0 Butadiene/styrene 160 K 27.5 60.5 copolymer Butadiene/styrene 18 K 40.5 75.9 copolymer In each of these examples, C02 was added to the polymerization mixture to terminate the polymerization. Following the COZ acldition, an antioxidant was added to precipitate the polymer product and the product was drum dried. The data of Table 1 show that the C02-terminated rubbers showed significantly increased bulk viscosity.

Example 2: Solution Viscosities of P-C02 Li with additives A living polybutadienyl lithium was treated with C02. This P-C02 Li+ was dissolved in toluene for solution viscosity determinations (in toluene).

Various additives were also added to further reduce solution viscosity.

Solution Viscosity (cP) Additive (amount) * 92.8 none 26.8 maleic anhydride (10%) 27.1 maleic anhydride (5%) 27.5 maleic anhydride (1 %) 89.4 none 93. 5 none 92.0 isopropanol (1.5%) 67.9 tetramethyl ethylene diamine 29.5 2-ethylhexanoic acid (1.5%) 92.5 acetonitrile (1.5%) *°/0 based on weight of polymer As seen from these data, additives such as maleic anhydride or carboxylic acids like 2-ethylhexanoic acid work cooperatively with the COs- terminated polybutadiene to significantly decrease solution viscosities.

Similar results were observed for solutions of these rubbery polymers in a monomer such as styrene.

Example 3: C02 terminated-Continuously Polymerized Polybutadiene A living lithium high vinyl polybutadiene was treated with C02 with the following results : Mooney viscosity if H+-terminated 13 Mooney viscosity if COs-terminated 46 Solution viscosity after C02 treatment 119 cP Solution viscosity after C02 treatment 72 cP and addition of maleic anhydride Comparative, H+-terminated polybutadiene : Mooney viscosity 35 Solution viscosity 97 cP The H+ terminated polybutadiene used in the comparative examples was a medium-vinyl, unmodified polybutadiene with a Mw of about 200,000, and was obtained from Firestone Polymers, LLC (Akron, Ohio).