Tetrablock copolymer and compositions containing same

#The present invention relates to a novel tetrablock copolymer and to compositions containing such tetrablock copolymer. More particularly the tetrablock copolymer is a selectively hydrogenated ABAB block copolymer having a particular molecular weight distribution and microstructure, resulting in a polymer having a unique set of property advantages. Also claimed are blends of the tetrablock copolymer and other constituents including styrene polymers, olefin polymers and tackifying resins.

#The preparation of block copolymers of mono alkenyl arenes and conjugated dienes is well known. One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. No. 3,595,942 and U.S. Pat. No. Re. 27,145. A number of other variations for block copolymer structures have been found since then. One of the types of block copolymers that has found limited utility in the past have been tetrablock copolymers having the structure ABAB or BABA, where the A block is a styrene block and the B block is a conjugated diene block, typically either an isoprene block or a butadiene block. These polymers in turn have sometimes been hydrogenated. Such tetrablock copolymers are disclosed in a variety of patents, including U.S. Pat. Nos. 4,874,821; 5,378,760; 5,492,967; 5,549,964; 5,554,697; 6,106,011; and 6,239,218.

#One of the many end uses for block copolymers and tetrablock copolymers is in fibers and films. See, for example, U.S. Pat. Nos. 5,549,964 and 5,705,556. However, during film and fiber formation, breaks are a common problem for highly elastic rubber compounds. Many of the existing block copolymers and formulations based on such block copolymers continue to have problems with breaks. What is needed is a polymer and compound that possesses enhanced strength to produce tougher films and fibers that are much less likely to break during processing. In addition, highly elastic compounds have a tendency to orient during injection molding in long or complex molds. This orientation leads to warpage and non-uniform shrinkage during de-molding or heating. What is needed then is a material with good elastic properties that can be easily injection molded into a part with isotropic properties. It is also desirable to produce a polymer with a higher modulus thus providing a stiffer rubber. A stiffer, stronger rubber allows the use of less polymer to achieve a desired stretching force, and is therefore, more economical.

#FIG. 1 shows stress-strain curves for various compounds of one of the tetrablock copolymers of the present invention with varying amounts and types of polystyrene.

#FIG. 2 shows stress-strain curves for various compounds of the present invention, including tetrablock copolymer, polystyrene, polyethylene and tackifying resin.

#FIG. 3 shows stress-strain curves comparing compounds of the present invention against a compound based on a selectively hydrogenated styrene-isoprene tetrablock copolymer.

#The tetrablock copolymers of the present invention are linear polymers prepared by contacting the monomers to be polymerized sequentially with an organoalkali metal compound in a suitable solvent at a temperature within the range from about −150° C. to about 300° C., preferably at a temperature within the range from about 0° C. to about 100° C. Particularly effective anionic polymerization initiators are organolithium compounds having the general formula RLin where R is an aliphatic, cycloaliphatic, aromatic, or alkyl-substituted aromatic hydrocarbon radical having from 1 to 20 carbon atoms; and n is an integer of 1 to 4. Preferred initiators include n-butyl lithium and sec-butyl lithium. See generally, U.S. Pat. Nos. 4,039,593 and U.S. Re. Pat. No. 27,145 for typical synthesis.

#The tetrablock is a selectively hydrogenated A1-B1-A2-B2 block copolymer where the A blocks are polymer blocks of mono alkenyl arenes, preferably polymer blocks of styrene. The B blocks prior to hydrogenation are polymer blocks of 1,3-butadiene, where between about 25 and 60 percent of the units have a 1,2-vinyl bond content, preferably between about 30 and about 55 1,2-vinyl bond content. The control of microstructure in the synthesis of the polymer is through the addition of a control agent during polymerization of the butadiene. A typical agent is diethyl ether. See U.S. Pat. No. Re 27,145 and U.S. Pat. No. 5,777,031, the disclosure of which is hereby incorporated by reference.

#The tetrablock copolymer is selectively hydrogenated using any of the several hydrogenation processes know in the art. For example the hydrogenation may be accomplished using methods such as those taught, for example, in U.S. Pat. Nos. 3,494,942; 3,634,594; 3,670,054; 3,700,633; and U.S. Re. Pat. No. 27,145, the disclosures of which are hereby incorporated by reference. The methods known in the prior art and useful in the present invention for hydrogenating polymers containing ethylenic unsaturation and for selectively hydrogenating polymers containing aromatic and ethylenic unsaturation, involve the use of a suitable catalyst, particularly a catalyst or catalyst precursor comprising an iron group metal atom, particularly nickel or cobalt, and a suitable reducing agent such as an aluminum alkyl. Also useful are titanium based catalyst systems. In general, the hydrogenation will be accomplished in a suitable solvent at a temperature within the range from about 20° C. to about 100° C., and at a hydrogen partial pressure within the range from about 100 psig to about 5,000 psig. Catalyst concentrations within the range from about 10 ppm wt to about 500 ppm wt of iron group metal based on total solution are generally used and contacting at hydrogenation conditions is generally continued for a period of time with the range from about 60 to about 240 minutes. After the hydrogenation is completed, the hydrogenation catalyst and catalyst residue will, generally, be separated from the polymer.