Syndiotactic 1,2-polybutadiene (SPBD) is a thermoplastic resin which can be utilized in making films, fibers and molded articles. For example, U.S. Pat. No. 4,394,473 and U.S. Pat. No. 4,957,970 disclose the use of SPBD in making bags and packaging. It can also be blended into elastomers, such as polydiene rubbers. Because SPBD contains double bonds which are attached in an alternating fashion to its backbone, it can be cocured with the rubbers in such blends. In fact, SPBD/rubber blends provide a unique combination of properties which make them useful in various tire compounds.
U.S. Pat. No. 4,790,365 discloses that incorporation of SPBD into rubber compositions which are utilized in the supporting carcass or innerliner of tires greatly improves the green strength of those compositions. Electron beam precure (microwave precure) is a technique which has gained wide commercial acceptance as a means of improving the green strength of synthetic elastomers which are used in building tires. However, electron beam precure techniques are costly. The incorporation of SPBD into blends of such synthetic elastomers can often improve green strength to the degree that electron beam precure is not required. The incorporation of SPBD into halogenated butyl rubbers which are utilized as the innerliner compositions for tires also greatly improves the scorch safety of such compositions. U.S. Pat. No. 4,274,462 disclosed that pneumatic tires having improved resistance against heat build-up can be prepared by utilizing SPBD fibers in their tread base rubber.
According to U.S. Pat. No. 4,790,365, the SPBD utilized in making the supporting carcass for tires has a melting point which is within the range of 120.degree. C. to 190.degree. C. and that it is preferred for the SPBD utilized in making the supporting carcass to have a melting point which is within the range of 150.degree. C. to 165.degree. C. The SPBD utilized in making tire innerliners has a melting point which is within the range of 120.degree. C. to 160.degree. C. and preferably has a melting point which is within the range of 125.degree. C. to 150.degree. C. The melting points referred to herein are minimum endotherm values determined from DSC (differential scanning calorimetry) curves.
Techniques for preparing SPBD by polymerizing 1,3-butadiene monomer are well known in the art. These techniques include solution polymerization, suspension polymerization and emulsion polymerization. The SPBD made utilizing these techniques typically have a melting point within the range of about 195.degree. C. to about 215.degree. C. It is accordingly necessary to reduce the melting point of the SPBD to render it suitable for utilization in some applications.
A process is disclosed in U.S. Pat. No. 3,778,424 for the preparation of syndiotactic 1,2-polybutadiene which comprises polymerizing 1,3-butadiene in an organic solvent in the presence of a catalyst composition composed of:
(a) a cobalt compound,
(b) an organoaluminum compound of the formula AlR.sub.3, in which R is a hydrocarbon radical of 1-6 carbons, and
(c) carbon disulfide.
U.S. Pat. No. 3,901,868 reveals a process for producing a butadiene polymer consisting essentially of syndiotactic 1,2-polybutadiene by the successive steps of:
(a) preparing a catalyst component solution by dissolving, in an inert organic solvent containing 1,3-butadiene, a cobalt compound, soluble in the organic solvent, such as (i) cobalt-.beta.-diketone complex, (ii) cobalt-.beta.-keto acid ester complex, (iii) cobalt salt of organic carboxylic acid, and (iv) halogenated cobalt-ligand compound complex, and an organoaluminum compound,
(b) preparing a catalyst composition by mixing the catalyst component solution (prepared in step a) with an alcohol, ketone or aldehyde compound and carbon disulfide,
(c) providing a polymerization mixture containing desired amounts of 1,3-butadiene, the catalyst composition and an inert organic solvent, and
(d) polymerizing 1,3-butadiene at a temperature which is within the range of -20.degree. C. to 90.degree. C.
U.S. Pat. No. 3,901,868 indicates that the melting point of the SPBD produced varies in response to the proportion of alcohol, ketone or aldehyde in the polymerization mixture. U.S. Pat. No. 4,153,767 shows that amide compounds, such as N,N-dimethylformamide, can be used in solution polymerizations to reduce the melting point of SPBD being synthesized.
U.S. Pat. No. 4,429,085 discloses a process for producing syndiotactic 1,2-polybutadiene by suspension polymerization in an aqueous medium. In this aqueous polymerization process polybutadiene which has an essentially syndiotactic 1,2-microstructure is made by the steps of:
(A) preparing a catalyst component solution by dissolving, in an inert organic solvent containing 1,3-butadiene (a) at least one cobalt compound selected from the group consisting of (i) .beta.-diketone complexes of cobalt, (ii) .beta.-keto acid ester complexes of cobalt, (iii) cobalt salts of organic carboxylic acids having 6 to 15 carbon atoms, and (iv) complexes of halogenated cobalt compounds of the formula CoX.sub.n, wherein X represents a halogen atom and n represents 2 or 3, with an organic compound selected from the group consisting of tertiary amine alcohols, tertiary phosphines, ketones and N,N-dialkylamides, and (b) at least one organoaluminum compound of the formula AlR.sub.3, wherein R represents a hydrocarbon radical of 1 to 6 carbon atoms;
(B) preparing a reaction mixture by mixing said catalyst component solution with a 1,3-butadiene/water mixture containing desired amounts of said 1,3-butadiene;
(C) preparing a polymerization mixture by mixing carbon disulfide throughout said reaction mixture, and
(D) polymerizing said 1,3-butadiene in said polymerization mixture into polybutadiene while agitating said polymerization mixture.
U.S. Pat. No. 4,751,275 discloses a process for the preparation of SPBD by the solution polymerization of 1,3-butadiene in a hydrocarbon polymerization medium, such as benzene, toluene, cyclohexane, or n-hexane. The catalyst system used in this solution polymerization contains a chromium-III compound which is soluble in hydrocarbons, a trialkylaluminum compound, and a dialkylphosphite, such as di-neopentylphosphite or di-butylphosphite.
U.S. Pat. No. 4,902,741 and U.S. Pat. No. 5,021,381 discloses a process for preparing a syndiotactic 1,2-polybutadiene latex by emulsion polymerization which comprises polymerizing 1,3-butadiene monomer in an aqueous reaction mixture which is comprised of (1) water, (2) at least one emulsifier, (3) 1,3-butadiene monomer, (4) a catalyst emulsion composition which is prepared by dissolving in an inert organic solvent containing at least one polyene (a) at least one cobalt compound selected from the group consisting of (i) .beta.-ketone complexes of cobalt, (ii) .beta.-keto acid ester complexes of cobalt, (iii) cobalt salts of organic carboxylic acids having 6 to 15 carbon atoms, and (iv) complexes of halogenated cobalt compounds of the formula CoX.sub.n, wherein X represents a halogen atom and n represents 2 or 3, with an organic compound selected from the group consisting of tertiary amine alcohols, tertiary phosphines, ketones and N,N-dialkylamides, and (b) at least one organoaluminum compound of the formula AlR.sub.3 wherein R represents a hydrocarbon radical of 1 to 6 carbon atoms to produce a catalyst component solution, and microfluidizing the catalyst component solution with an oil, a surfactant, and water to an average particle size which is within the range of about 10 nanometers to about 1000 nanometers; and (5) at least one member selected from the group consisting of carbon disulfide and phenyl isothiocyanate.
The synthesis of SPBD in an aqueous medium offers several important advantages over solution polymerizations. Water, as a medium in which to carry out such a polymerization, is less expensive, more easily purified, less sensitive to oxygen, and has a higher heat capacity. Conducting such polymerizations in an aqueous medium also permits for higher monomer and higher solids concentrations because of the lower viscosity of a polymer suspension or emulsion compared with that of a polymer solution. The main drawback associated with aqueous suspension and emulsion polymerizations for producing SPBD is the difficulty associated with reducing the melting point of the SPBD. In other words, it is difficult to control the chemical structure and hence the crystallinity and melting point of SPBD which is synthesized in an aqueous medium. Even though numerous modifiers can be used to reduce the level of crystallinity and resulting melting point of SPBD which is synthesized in solution, there are few efficient modifiers for reducing the crystallinity of SPBD which is synthesized in an aqueous medium.
U.S. Pat. No. 5,011,896 discloses the use of 4-(alkylamino)benzaldehydes, 4-(dialkylamino)benzaldehydes, 2,4-di-(alkoxy)benzaldehydes, 2,6-di-(alkoxy)benzaldehydes, 2,4,6-tri-(alkoxy)benzaldehydes, and 4-(1-azacycloalkyl)benzaldehydes, and 4-(1-azacycloalkyl)benzaldehydes as modifiers for reducing the melting point of SPBD which is synthesized in an aqueous medium. Nevertheless, there is still a need for more highly efficient modifiers which can be used on a commercial basis.