Hydrogenation of polymers

Details Hydrogenation of polymers Hydrogenation of polymers

1994-11-17

2001-01-23

Yoon, Tae (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S332900, C525S339000, C528S490000, C528S499000, C524S800000

Reexamination Certificate

active

06177521

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved method to prepare hydrogenated polymers.
BACKGROUND TO THE INVENTION
Polymers that contain ethylenic unsaturation can be hydrogenated to improve properties, and in particular to improve stability. Such hydrogenation processes are disclosed in, for example, U.S. Pat. Nos. 3,700,633 and 3,700,748, the disclosures of which are incorporated herein by reference. These processes include combining a solution or suspension of the polymer with Group VIII metal alkoxide or carboxylate and a metal alkyl. Group VIII metals in general will function as the active metal in these systems, and in particular, nickel, iron, cobalt and palladium are known to be effective. This mixture is then contacted with hydrogen at an elevated pressure and temperature, resulting in hydrogenation of ethylenic unsaturation.
A particularly difficult aspect of these hydrogenation processes is removal of the hydrogenation catalyst metals after hydrogenation is complete. After hydrogenation is complete, the metal components are typically removed from the polymer solution by contacting the polymer solution with an aqueous phase of, for example, carboxylic acid, sulfuric acid or phosphoric acid. An oxidizing agent may also be used to react with the metal or metal compound either prior to or simultaneously with the carboxylic acid, sulfuric acid or phosphoric acid. The aqueous phase is then separated from the polymer solution.
Removal of the hydrogenation catalyst metal is important because small amounts of hydrogenation catalyst metal are detrimental to many polymer properties, including stability and color.
U.S. Pat. No. 4,992,529 discloses a method for separating metal contaminants from organic polymer solutions using a high molecular weight monocarboxylic acid in combination with an inorganic acid. Although this method can be effective for many polymer and metal combinations, it has been found to be less effective than what is desired for other combinations. In particular, when anionically polymerized polymers are initiated with lithium, and the lithium is present in the final polymer solution in relatively high concentrations, removal of both the lithium and nickel hydrogenation catalyst metal becomes particularly difficult, and relatively large amounts of the high molecular weight monocarboxylic acid is required.
Removal of metals from a hydrogenated polymer solution is affected by the amount of water remaining in the polymer cement after the metals removal step is complete. This is because the remaining metals are, after contact of the solution with an acidic aqueous solution, often concentrated in the small amount of water entrained within the polymer cement. Effective separation of the water from the polymer cement is necessary for the effective removal of metals.
It is therefore an object of the present invention to provide a method to hydrogenate polymers containing ethylenic unsaturation in a polymer cement wherein the hydrogenated polymer cement contains not more than about 0.8 percent by weight entrained water. In another aspect, it is an object to provide such a method wherein hydrogenation catalyst metals are effectively removed without addition of excessive amounts of a contaminant.
SUMMARY OF THE INVENTION
These and other objects of the invention are achieved by a method to hydrogenate a polymer containing ethylenic unsaturation comprising the steps of: providing a solution or suspension of the polymer containing ethylenic unsaturation with an amount of Group VIII metal alkoxide or carboxylate and metal alkyl hydrogenation catalyst effective to permit hydrogenation of the ethylenic unsaturation upon exposure to hydrogen; exposing the polymer solution or suspension to a hydrogen partial pressure for a time period sufficient to hydrogenate greater than about 90 percent of the ethylenic unsaturation; adding additional Group VIII metal alkoxide or carboxylate and metal alkyl hydrogenation catalyst to the polymer solution or suspension after greater than about 90 percent of the ethylenic unsaturation has been hydrogenated; mixing the hydrogenated polymer solution or suspension with the additional Group VIII metal alkoxide or carboxylate and metal alkyl hydrogenation catalyst with an aqueous solution of an acid; and separating a hydrogenated polymer solution or suspension that contains less than about 0.8 percent by weight water from the aqueous solution.
The present invention utilizes a surprising discovery that addition of nickel carboxylate and aluminum alkyl hydrogenation catalyst above that necessary or desirable for hydrogenation of ethylenic unsaturation results in improved separation of the catalyst metals, and the aqueous phase, from the polymer solution or suspension.
The polymer of the present invention is preferably a polymer derived by anionic polymerization of diolefin components such as butadiene or isoprene. These polymers are often block copolymers that include blocks of polymerized vinyl aromatics such as styrene.
The inclusion of the additional catalyst is particularly useful when the polymer has been prepared by anionic polymerization using an metal alkyl initiator such as a lithium alkyl, and the concentration of the metal initiator in the polymer solution or suspension is relatively great, for example, above about 20 ppm. These relatively high initiator metal contents occur when the polymers are of low molecular weights, as in certain viscosity index improvers and adhesives, and especially when multiple polymer arms are initiated and then coupled to form the final polymer molecules.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
The polymer of the present invention can be any polymer containing ethylenic unsaturation. Polybutadienes, polyisoprenes, or copolymers thereof, and block copolymers containing blocks that are primarily polybutadiene, polyisoprene or copolymers thereof are preferred because properties of these polymers are considerably improved by hydrogenation. The block copolymers are preferably block copolymers with a monovinyl aromatic such as styrene. Aromatic unsaturation is preferably not hydrogenated by the method of the present invention. These block copolymers are preferred because blocks of unhydrogenated monovinyl aromatics provide crystalline domains that have glass transition temperatures above which the polymers can be melt processed, and below which form crosslinks between polymer molecules. Such polymer can therefore be an elastomeric thermoplastic that is reprocessable.
Polymers that are prepared by anionic polymerization and then hydrogenated are typically hydrogenated in the solution or suspension in which the polymers are polymerized. Such solution or suspension therefore contains the metal ions used as initiators for the anionic polymerization. These initiator ions have been found to interfere with removal of hydrogenation catalyst metals when the initiator metals are present in a sufficiently high concentration. Initiator ion concentrations of around 20 ppm are often enough to cause relatively difficult catalyst metals removal.
Polymer hydrogenation catalysts useful in the present invention include nickel alkoxide or carboxylates, including 2-ethyl-1-hexanoates reduced with aluminum alkyl compounds. These hydrogenation catalysts are disclosed in, for example, U.S. Pat. Nos. 3,700,633 and 3,700,748.
Hydrogenation can be carried out with a hydrogen partial pressure of between about 1 atmosphere and 1500 psi. More typically, the hydrogen partial pressure is between about 100 and about 1200 psi. Hydrogenation can be carried out at temperatures of between about 25° C. and about 100° C. The time of hydrogenation under such conditions will typically be between about 0.1 and about 4 hours. Such hydrogenation will typically hydrogenate greater than 90 percent of the initial ethylenic unsaturation of the polymer.
The amount of catalyst used for initial hydrogenation in the present process is less than that which is needed for effective separation of metals from the hydrogenated pol

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