Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1997-12-11
2001-06-05
Mullis, Jeffrey (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S313000, C525S314000, C525S315000, C525S316000, C525S250000, C525S267000
Reexamination Certificate
active
06242536
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to dispersion polymerization of block copolymers and to the control of the composition of the dispersing agent to effect the properties of the block copolymers.
BACKGROUND OF THE INVENTION
Block copolymers of the S-B-S type and processes for their preparation are well known in art for such uses as self-curing elastomers and as modifiers for diene elastomers. These block copolymers have a central core of a polybutadiene and have a block of polystyrene on each end. However, in view of the greater insolubility or non-dispersibility of the initial polystyrene, a solvent such as an aromatic hydrocarbon or cyclohexane has been used in its preparation.
This problem is recognized in British Patent No. 1,130,770 which states on page 2, lines 60-65, “In each of these processes, however, a vinyl aromatic hydrocarbon is required to be first polymerized and therefore the polymerization solvent used must be wholly or mainly a naphthenic or aromatic hydrocarbon solvent”.
Similar recognition of this solubility problem is indicated in U.S. Pat. Nos. 3,231,635; 3,265,765 and 3,427,364 wherein statements are made regarding the necessity to use aromatic or naphthenic solvents or mixtures of these with other solvents.
British Patent No. 1,412,584 has modified the process of preparing block copolymers so that heptane may be used as the polymerization medium, by making first a block of t-butyl-styrene and describes this first block as a thermoplastic polymer of 10,000 to 100,000 molecular weight to give solubility to the growing block copolymer. The second or middle block of polybutadiene is then added and finally a block of polystyrene. The resulting block copolymer is poly(t-Bu-styrene)-polybutadiene-polystyrene.
U.S. Pat. No. 4,291,139 effects polymerization of block copolymers of the S-B-S type wherein S represents a block of polystyrene and B represents a block of polybutadiene using hexane as diluent by first preparing a “foot” or small block of homopolydiene to which the initial polystyrene block is thereafter attached. This small block of homopolydiene represents no more than 12 parts, preferably no more than 10 parts by weight of homopolydiene per 100 parts by weight of the block of polystyrene to which it is attached. Generally, therefore, this block has a molecular weight of 200-5000 depending on the size of the attached polystyrene block and is not large enough to alter the properties of the resultant S-B-S block copolymers except to give the initial polystyrene block greater dispersibility in hexane. This process is entirely conducted in a hexane solvent.
Because of its lower boiling point and resultant energy savings in the use of hexane as compared to benzene and cyclohexane, it is desirable to have a process for use in preparing S-B-S block copolymers wherein the final block copolymer is recoverable from a dispersing medium primarily containing low boiling alkanes such as hexane. Moreover, the use of a process primarily conducted in alkanes such as hexane makes the process adaptable for use in equipment designed for the preparation of polybutadiene in hexane.
SUMMARY OF THE INVENTION
In accordance with the present invention, the dispersibility of the initial polystyrene block in forming a S-B-S type of copolymer is improved by first preparing a small block of homopolydiene such as homopolybutadiene as a dispersing agent, representing 2-12 parts, preferably 5-10 parts, by weight per 100 parts by weight of the adjacent polystyrene block, and generally having a molecular weight of 200-5,000, depending on the molecular weight of the adjacent polystyrene block. Onto this small block of homopolybutadiene, the first block of polystyrene is formed and affixed and thereby made dispersible in a mixture of alkanes and cycloalkanes, preferably hexane and cyclohexane. This small block of polydiene is not large enough to alter the desired properties of the ultimate S-B-S block copolymer. This modification involves the same or similar techniques as previously used in the preparation of S-B-S polymers except that a combination of alkanes, preferably hexane or technical hexane, and 15 to 60% by weight of cycloalkanes, preferably cyclohexane, is used as the dispersing medium and a small amount of a conjugated diene is added prior to the addition of the amount of styrene used for the formation of the first polystyrene block. After the preparation of the dispersing agent and the first styrene block, the remaining monomer charges to the reactor are fed in aliphatic hydrocarbon diluents, preferably hexane or technical hexane, thereby greatly reducing the final percentage of cycloalkanes including cyclohexane in the reactor to less than 25% of total dispersing medium. The block copolymers produced according to the present process display a narrow molecular weight distribution and improved tensile strength over block copolymers produced in the absence of the additional cycloalkanes used in the solvent system.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a dispersion polymerization process for preparing block polymers having high tensile strength through the subsequent steps of:
(1) forming a polymerization mixture by polymerizing vinyl aromatic monomer in the presence of a polydiene containing dispersing agent (polyBd* or b*) and an organolithium initiator in a dispersing medium containing alkanes or aliphatic hydrocarbons and 15 to 60% by weight of cycloalkanes, preferably cyclohexane, to form a stable dispersion until substantially complete conversion to form a first vinyl aromatic block S; (2) adding conjugated diene monomer in an alkane containing diluent having less than 9% by weight of cycloalkane to the polymerization mixture and polymerizing the conjugated diene monomer until substantially complete conversion to form a diblock polymer S-B; (3) and adding to the polymerization mixture a coupling agent (X) having multiple functionality and coupling the diblock polymer formed in step (2) to form a final block polymer having the structural formula S-B-X-(B-S)
n-1
wherein n is 1, 2, 3 or more and recovering the final block polymer.
Alternatively the dispersion polymerization process for preparing block polymers having high tensile strength can be performed by the subsequent steps of: (1) forming a polymerization mixture by polymerizing a first charge of vinyl aromatic monomer in the presence of a dispersing agent and an organolithium initiator in a dispersing medium containing alkanes or aliphatic hydrocarbons and 15 to 60% by weight of a cycloalkane, preferably cyclohexane, to form a stable dispersion until substantially complete conversion to form a first vinyl aromatic block S; (2) adding conjugated diene monomer in an alkane containing diluent having less than 9% by weight of cycloalkane to the polymerization mixture and polymerizing the conjugated diene monomer until substantially complete conversion to form a diblock polymer S-B; and (3) adding to the polymerization mixture a second charge of vinyl aromatic monomer in an alkane containing diluent having less than 9% by weight of cycloalkane and polymerizing the vinyl aromatic monomer to form a terminal block S
1
on the S-B diblock polymer to form a block polymer S-B-S, in an alkane containing diluent having less than 25% by weight of cycloalkane.
As in previous practice, a hydrocarbon lithium is used as the initiator, preferably either a secondary or tertiary alkyl lithium, or a primary alkyl lithium such as n-butyl lithium with or without a small amount of a suitable modifier to accelerate the initiation step.
To the polymerization system containing an appropriate amount of initiator is added the small amount of a diene required to yield the small soluble block of polydiene resulting in the formation represented by the designation polyBd* or b*. When this small block formation is completed, the appropriate amount of styrene is added to give the desired molecular weight in the first polystyrene of S block. Polymerization is continued until conversion of the styrene monomer t
Graves Daniel F.
Hall James E.
Takeichi Hideo
Bridgestone Corporation
Burleson David G.
McCollister Scott A.
Mullis Jeffrey
LandOfFree
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