GB2368069A

GB2368069A - Dialkylsiloxane-diene-dialkylsiloxane triblock rubber

Info

Publication number: GB2368069A
Application number: GB0117615A
Authority: GB
Inventors: Abderrahim Khadir; Adel Farhan Halasa; Wen-Liang Hsu
Original assignee: Goodyear Tire and Rubber Co
Current assignee: Goodyear Tire and Rubber Co
Priority date: 2000-07-20
Filing date: 2001-07-19
Publication date: 2002-04-24
Also published as: GB0117615D0

Abstract

Dialkylsiloxane-diene-dialkylsiloxane triblock rubbers are made by (1) polymerizing at least one diene monomer in a first polymerization step to produce a diene polymer having two living lithium ends; and (2) adding a hexaalkylcyclotrisiloxane to the diene polymer to produce the dialkysiloxane-diene-dialkylsiloxane triblock rubber. For instance, dimethylsiloxane-polybutadiene-dimethylsiloxane triblock rubber, dimethylsiloxane-polyisoprene-dimethylsiloxane triblock rubber, and dimethylsiloxane-SBR-dimethylsiloxane triblock rubber can be synthesized. There are valuable benefits associated with utilizing dialkylsiloxane-diene-dialkylsiloxane triblock rubber in tire tread compounds. Since the dialkylsiloxane-diene-dialkylsiloxane triblock rubber has improved interaction with silica fillers, it is particularly valuable when used in tire tread compounds that are compounded with high levels of silica. The dialkylsiloxane-diene-dialkylsiloxane triblock rubber of this invention can also be employed in tire sidewall compounds to improve the appearance of tire sidewalls.

Description

DIALKYLSILOXANE-DIENE-DIALKYLSILOXANE TRIBLOCK RUBBER Background of the Invention Lithium compounds are commonly used as initiators for anionic polymerizations.

Such organolithium initiators can be employed in synthesizing a wide variety of rubbery polymers. For instance, organolithium initiators can be used to initiate the anionic polymerization of diolefin monomers, such as 1,3-butadiene and isoprene, into rubbery polymers. Vinyl aromatic monomers can, of course, also be copolymerized into such polymers. Some specific examples of rubbery polymers that can be synthesized using organolithium compounds as initiators include polybutadiene, polyisoprene, styrenebutadiene rubber (SBR), styrene-isoprene rubber, and styrene-isoprene-butadiene rubber (SIBR).

The organolithium compounds that can be used to initiate such anionic polymerizations can be either a specific organomonolithium compound or it can be a multifunctional types of initiator. In commercial applications monolithium compounds are normally used because they are available as pure compounds that are soluble in organic solvents. Multifunctional organolithium compounds are not necessarily specific compounds but rather represent reproducible compositions of regulable functionality.

Many of such multifunctional organolithium compounds must be stored under refrigeration before being used.

United States Patent 5,981, 639 explains that multifunctional initiators used to initiate anionic polymerizations include those prepared by reacting an organomonolithium compounded with a multivinylphosphine or with a multivinylsilane, such a reaction preferably being conducted in an inert diluent such as a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound. The reaction between the multivinylsilane or multivinylphosphine and the organomonolithium compound can result in a precipitate which can be solubilized if desired, by adding a solubilizing monomer such as a conjugated diene or monovinyl aromatic compound, after reaction of the primary components. Alternatively, the reaction can be conducted in the presence of a minor amount of the solubilizing monomer. The relative amounts of the organomonolithium compound and the multivinylsilane or the multivinylphosphine preferably should be in the range of about 0.33 to 4 moles of organomonolithium compound per mole of vinyl groups present in the multivinylsilane or

multivinylphosphine employed.

United States Patent 5, 981, 639 further notes such multifunctional initiators are commonly used as mixtures of compounds rather than as specific individual compounds.

Other multifunctional polymerization initiators can be prepared by utilizing an organomonolithium compound, further together with a multivinylaromatic compound and either a conjugated diene or monovinylaromatic compound or both. These ingredients can be charged initially, usually in the presence of a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound as a diluent. Alternatively, a multifunctional polymerization initiator can be prepared in a two-step process by reacting the organomonolithium compound with a conjugated diene or monovinyl aromatic compound additive and then adding the multivinyl aromatic compound. Any of the conjugated dienes or monovinyl aromatic compounds described can be employed.

The ratio of conjugated diene or monovinyl aromatic compound additive employed preferably should be in the range of about 2 to 15 moles of polymerizable compound per mole of organolithium compound. The amount of multivinylaromatic compound employed preferably should be in the range of about 0.05 to 2 moles per mole of organomonolithium compound. Exemplary multivinyl aromatic compounds include 1,2divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene, 1,2, 4-trivinylbenzene, 1,3 divinylnaphthalene, 1,8-divinylnaphthalene, 1,3, 5-trivinylnaphthalene, 2,4divinylbiphenyl, 3,5, 4'-trivinylbiphenyl, m-diisopropenyl benzene, p-diisopropenyl benzene, 1, 3-divinyl-4, 5, 8-tributylnaphthalene and the like. Divinyl aromatic hydrocarbons containing up to 18 carbon atoms per molecule are preferred, particularly divinylbenzene as either the ortho, meta or para isomer and commercial divinylbenzene, which is a mixture of the three isomers, and other compounds, such as the ethylstyrenes, also is quite satisfactory.

United States Patent 4,196, 154 discloses organic liquid soluble multifunctional lithium containing initiators are prepared by reacting an organo lithium compound with an organic compound containing at least one group of the configuration 1, 3-bis (lphenylethenyl) benzene. United States Patent 4,196, 154 reports that such initiators can be prepared in the absence of polar solvents and are very desirable for the polymerization of dienes such as butadiene to a desirable 1,4 configuration.

Summary of the Invention This invention discloses dialkylsiloxane-diene-dialkylsiloxane triblock rubbers and a process for making dialkylsiloxane-diene-dialkylsiloxane triblock rubbers. For instance, dimethylsiloxane-polybutadiene-dimethylsiloxane triblock rubber, dimethylsiloxane-polyisoprene-dimethylsiloxane triblock rubber, and dimethylsiloxane SBR-dimethylsiloxane triblock rubber can be synthesized by employing the technique of this invention.

The present invention more specifically discloses a process for synthesizing a dialkylsiloxane-diene-dialkylsiloxane triblock rubber which comprised the steps of (1) polymerizing at least one diene monomer in a first polymerization step to produce a diene polymer having two living lithium ends, wherein the polymerization is initiated with a dilithium initiator; and (2) adding a hexaalkylcyclotrisiloxane to the diene polymer to produce the dialkylsiloxane-diene-dialkylsiloxane triblock rubber.

Detailed Description of the Invention Preferably the alkylsiloxane blocks of the alkylsiloxane-diene-alkylsiloxane triblock rubber are dimethylsiloxane blocks.

Preferably the diene block of the alkylsiloxane-diene-alkylsiloxane triblock rubber is selected from a polybutadiene block, a polyisoprene block and a stryenebutadiene block.

Virtually any type of dilithium initiator can be used in the process of this invention. However, it is highly preferred for the dilithium initiator to be synthesized by reacting a tertiary-alkyl lithium compound with m-diisopropenylbenzene in an aromatic solvent. The aromatic solvent will typically be an alkyl benzene. The alkyl group in the alkyl benzene will typically contain from 1 to 8 carbon atoms. It is preferred for the alkyl group in the alkyl benzene solvent to contain from 1 to about 4 carbon atoms.

Some preferred aromatic solvents include toluene, ethyl benzene, and propyl benzene.

Ethyl benzene is the most highly preferred aromatic solvent.

It is critical for a tertiary-alkyl lithium compound to be reacted with the mdiisopropenylbenzene. The tertiary-alkyl lithium compound will typically contain from 4 to about 8 carbon atoms. It is preferred for the tertiary-alkyl lithium compound to be tertiary-butyl lithium.

The reaction will typically be conducted at a temperature that is within the range

of about 0 C to about 100 C. It is normally preferred for the reaction between the tertiary-alkyl lithium and the m-diisopropenylbenzene to be carried out at a temperature that is within the range of about 10 C to about 70oC. It is typically more preferred for the reaction temperature to be within the range of about 20 C to about 40 C.

The polymerizations of this invention used in synthesizing the dialkylsiloxanediene-dialkylsiloxane triblock rubber are normally carried out as solution polymerizations in an inert organic medium utilizing a dilithium catalyst. However, it is contemplated that the technique of this invention can also be used to synthesize dialkylsiloxane-diene-dialkylsiloxane triblock rubbers by bulk polymerization or vapor phase polymerization.

In the first step of the process of this invention at least one diene monomer is polymerized with a dilithium initiator. This polymerization can be a homopolymerization of a conjugated diolefin monomer, a copolymerization of two conjugated diolefin monomer, a copolymerization of a conjugated diolefin monomer with a vinyl aromatic monomer, or a terpolymerization of two conjugated diolefin monomers with a vinyl aromatic monomer. It is, of course, also possible to make diene block by polymerizing a mixture of conjugated diolefin monomers with one or more ethylenically unsaturated monomers, such as vinyl aromatic monomers. The conjugated diolefin monomers which can be utilized in the synthesis of diene block generally contain from 4 to 12 carbon atoms. Those containing from 4 to 8 carbon atoms are generally preferred for commercial purposes. For similar reasons, 1,3-butadiene and isoprene are the most commonly utilized conjugated diolefin monomers. Admixtures of 1,3-butadiene and isoprene or 1, 3-butadiene and styrene are also suitable. Some additional conjugated diolefin monomers that can be utilized include 2,3-dimethyl-1, 3butadiene, piperylene, 3-butyl-1, 3-octadiene, 2-phenyl-1, 3-butadiene, and the like, alone or in admixture.

Some representative examples of ethylenically unsaturated monomers that can potentially be copolymerized into the diene block using the modifiers of this invention include alkyl acrylates, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate and the like; vinylidene monomers having one or more terminal CH2=CHgroups; vinyl aromatics such as styrene, a-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene and the like; a-olefins such as ethylene, propylene, 1-butene and the like; vinyl halides, such as vinylbromide, chloroethane (vinylchloride), vinylfluoride,

vinyliodide, 1, 2-dibromoethene, 1, 1-dichloroethene (vinylidene chloride), 1, 2dichloroethene and the like ; vinyl esters, such as vinyl acetate ; a, a-olefinically unsaturated nitriles, such as acrylonitrile and methacrylonitrile; a, a-olefinically unsaturated amides, such as acrylamide, N-methyl acrylamide, N, N-dimethylacrylamide, methacrylamid and the like.

Polydiene blocks that are copolymers of one or more diene monomers with one or more other ethylenically unsaturated monomers will normally contain from about 50 weight percent to about 99 weight percent conjugated diolefin monomers and from about 1 weight percent to about 50 weight percent of the other ethylenically unsaturated monomers in addition to the conjugated diolefin monomers. For example, copolymers of conjugated diolefin monomers with vinylaromatic monomers, such as styrene-butadiene rubbers which contain from 50 to 95 weight percent conjugated diolefin monomers and from 5 to 50 weight percent vinylaromatic monomers, are useful in many applications.

Vinyl aromatic monomers are probably the most important group of ethylenically unsaturated monomers that are commonly incorporated into polydienes. Such vinyl aromatic monomers are, of course, selected so as to be copolymerizable with the conjugated diolefin monomers being utilized. Generally, any vinyl aromatic monomer that is known to polymerize with organolithium initiators can be used. Such vinyl aromatic monomers typically contain from 8 to 20 carbon atoms. Usually, the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. The most widely used vinyl aromatic monomer is styrene. Some examples of vinyl aromatic monomers that can be utilized include styrene, I-vinylnaphthalene, 2-vinylnaphthalene, a-methylstyrene, 4phenylstyrene, 3-methylstyrene and the like.

In solution polymerizations the inert organic medium which is utilized as the solvent will typically be a hydrocarbon which is liquid at ambient temperatures which can be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquids under the conditions of the polymerization. It is, of course, important for the solvent selected to be inert. The term"inert"as used herein means that the solvent does not interfere with the polymerization reaction or react with the polymers made thereby. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal hexane, benzene, toluene, xylene, ethylbenzene and the like, alone

or in admixture. Saturated aliphatic solvents, such as cyclohexane and normal hexane, are most preferred.

The amount of dilithium initiator utilized will vary from one organodilithium compound to another and with the molecular weight that is desired for the diene block being synthesized. As a general rule in all anionic polymerizations, the molecular weight (Mooney viscosity) of the polymer produced is inversely proportional to the amount of catalyst utilized. As a general rule, from about 0.01 phm (parts per hundred parts by weight of monomer) to 1 phm of the lithium catalyst will be employed. In most cases, from 0. 01 phm to 0.1 phm of the lithium catalyst will be employed with it being preferred to utilize 0.025 phm to 0.07 phm of the lithium catalyst. The number average molecular weight (Mn) of the diene block will typically be within the range of about 25,000 to about 700,000.

Normally, from about 5 weight percent to about 35 weight percent of the monomer or monomers will be charged into the polymerization medium (based upon the total weight of the polymerization medium including the organic solvent and monomer).

In most cases, it will be preferred for the polymerization medium to contain from about 10 weight percent to about 30 weight percent monomer. It is typically more preferred for the polymerization medium to contain from about 20 weight percent to about 25 weight percent monomer.

The polymerization temperature will normally be within the range of about 5 C to about 100oC. For practical reasons and to attain the desired microstructure the polymerization temperature will preferably be within the range of about 40 C to about 90oc. Polymerization temperatures within the range of about 60 C to about 90 C are most preferred. The microstructure of the rubbery polymer is somewhat dependent upon the polymerization temperature.

The polymerization is allowed to continue until essentially all of the monomer has been exhausted. In other words, the polymerization is allowed to run to completion.

Since a dilithium catalyst is employed to polymerize the monomer, a polymer having two living lithium ends is produced. The living polymer synthesized will have a number average molecular weight which is within the range of about 25,000 to about 700,000.

The rubber synthesized will more typically have a number average molecular weight which is within the range of about 50,000 to about 250,000.

To increase the level of vinyl content the polymerization can be carried out in the

presence of at least one polar modifier. Ethers and tertiary amines that act as Lewis bases are representative examples of polar modifiers that can be utilized. Some specific examples of typical polar modifiers include diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamin, triethylamine, N, N, N', N'- tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and the like.

The modifier can also be a 1,2, 3-trialkoxybenzene or a 1,2, 4-trialkoxybenzene.

Some representative examples of 1, 2, 3-trialkoxybenzenes that can be used include 1, 2, 3trimethoxybenzene, 1, 2, 3-triethoxybenzene, 1, 2, 3-tributoxybenzene, 1, 2, 3trihexoxybenzene, 4, 5, 6-trimethyl-1, 2, 3-trimethoxybenzene, 4, 5, 6-tri-n-pentyl-l, 2, 3triethoxybenzene, 5-methyl-1, 2, 3-trimethoxybenzene, and 5-propyl-l, 2, 3 trimethoxybenzene. Some representative examples of 1,2, 4-trialkoxybenzenes that can be used include 1,2, 4-trimethoxybenzene, 1,2, 4-triethoxybenzene, 1,2, 4tributoxybenzene, 1,2, 4-tripentoxybenzene, 3,5, 6-trimethyl-1, 2,4-trimethoxybenzene, 5 propyl-l, 2,4-trimethoxybenzene, and 3, 5-dimethyl-1, 2,4-trimethoxybenzene.

Dipiperidinoethane, dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol, dimethyl ether and tetrahydrofuran are representative of highly preferred modifiers.

United States Patent 4,022, 959 describes the use of ethers and tertiary amines as polar modifiers in greater detail.

The utilization of 1,2, 3-trialkoxybenzenes and 1,2, 4-trialkoxybenzenes as modifiers is described in greater detail in United States Patent 4,696, 986. The teachings of United States Patent 4,022, 959 and United States Patent 4,696, 986 are incorporated herein by reference in their entirety. The microstructure of the repeat units that are derived from butadiene monomer is a function of the polymerization temperature and the amount of polar modifier present. For example, it is known that higher temperatures result in lower vinyl contents (lower levels of 1, 2-microstructure). Accordingly, the polymerization temperature, quantity of modifier and specific modifier selected will be determined with the ultimate desired microstructure of the diene block being synthesized being kept in mind.

After the polymerization of the diene monomer has been completed to produce the polymer having two living lithium ends (the diene block), hexaalkylcyclotrisiloxane

is added. The hexaalkylcyclotrisiloxane then polymerizes onto both of the ends of the diene block by a ring opening polymerization. This produces the dialkylsiloxane-diene dialkylsiloxane triblock rubber. The hexaalkylcyclotrisiloxanes that can be used are of the general structural formula:

wherein R represents an alkyl group containing from 1 to 8 carbon atoms. It is normally preferred for R to represent an alkyl group containing from 1 to 4 carbon atoms.

Hexamethylcyclotrisiloxane is highly preferred and is of the general structural formula :

Normally from about 1 weight percent to about 25 weight percent of the hexaalkylcyclotrisiloxane will be polymerized onto the ends of the diene block, based upon the total weight of the dialkylsiloxane-diene-dialkylsiloxane triblock rubber.

Preferably, from about 5 weight percent to about 10 weight percent of the hexaalkylcyclotrisiloxane will be polymerized onto the ends of the diene block, based upon the total weight of the dialkylsiloxane-diene-dialkylsiloxane triblock rubber.

The living dialkylsiloxane-diene-dialkylsiloxane triblock rubber can optionally be coupled with a suitable coupling agent, such as a tin tetrahalide or a silicon tetrahalide.

The rubbery polymer is then recovered from the organic solvent. The dialkylsiloxane

diene-dialkylsiloxane triblock rubber can be recovered from the organic solvent and residue by any means, such as decantation, filtration, centrification and the like. It is often desirable to precipitate the rubbery polymer from the organic solvent by the addition of lower alcohols containing from about 1 to about 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the rubbery polymer from the polymer cement include methanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the rubber from the polymer cement also"kills"the living polymer by inactivating lithium end groups. After the rubbery polymer is recovered from the solution, steam stripping can be employed to reduce the level of volatile organic compounds in the polymer. The inert solvent and residual monomer can then be recycled for subsequent polymerization.

There are valuable benefits associated with utilizing dialkylsiloxane-dienedialkylsiloxane triblock rubber in tire tread compounds. Since the dialkylsiloxane-dienedialkylsiloxane triblock rubber has improved interaction with silica fillers, it is particularly valuable when used in tire tread compounds that are compounded with high levels of silica. The dialkylsiloxane-diene-dialkylsiloxane triblock rubber of this invention can also be employed in tire sidewall compounds to improve the appearance of sidewalls of the tire.

This invention is illustrated by the following examples that are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight.

Comparative Example A In this example, a stable and hydrocarbon soluble dilithio initiator was prepared.

Neat m-diisoproprenylbenzene (100 mmoles) was added, under nitrogen, to a dried quart bottle containing 400 ml. of reagent grade ethylbenzene at room temperature. To this was added in the increment of four portions of 50 mmoles of tert-butyllithium (in hexanes) with constant shaking. It was left at room temperature for 2 hours after the addition of t-BuLi was completed. The bottle containing the reaction mixture was then rotated in a polymerization bath at 65 C bath for two hours. After removing it from the bath and left to cool at room temperature, the resulting reddish brown solution containing dilithio initiator was titrated using the Gilman double titration method for active lithium.

The GC-MS analysis of the hydrolyzed (with D20) product indicated that more than 95% dilithio species was formed.

Example 1 In this experiment a dimethylsiloxane-polyisoprene-dimethylsiloxane triblock rubber was synthesized by anionic polymerization. In the first step of the process 95 weight percent 1,3-butadiene and 5 weight percent isoprene in hexane was copolymerized in a one-gallon (3.785 liter) reactor. The copolymerization was initiated with dilithioisopropenylbenzene and was carried out at 65 C in the presence of a mixture of sodium dodecylbenzenesulfonate and TMEDA (as a modifier). The monomer premix was first introduced into the reactor followed by the di-lithium initiator (0.46M). The polymerization was allowed to continue until a full conversion was attained. The dilithium initiator lead to the production of a styrene-butadiene polymer having two living lithium ends. Then hexamethylcyclotrisiloxane monomer was added to the reactor at room temperature (about 20 C) as a 13 percent solution in cyclohexane. To reduce

association in non-polar solvent (hexane) 300 cc of tetrahydrofuran (THE) was added.

The reaction was terminated using chlorotrimethysilane and was subsequently dried.

NMR characterization showed that this dialkylsiloxane-diene-dialkylsiloxane triblock rubber contained 14 weight percent polydimethylsiloxane, 46.3 weight percent vinylpolybutadiene, 29.2 weight percent 1,4-polybutadiene, 5.9 weight percent substituted divinyl cyclohexane, and 4.6 weight percent 1,4-polyisoprene.

Example 2 In this experiment a dimethylsiloxane-polyisoprene-dimethylsiloxane triblock rubber was synthesized by anionic polymerization. In the first step of the process isoprene was homopolymerized in a one-gallon (3.785 liter) reactor. The homopolymerization was initiated with dilithiosiopropenylbenzene was carried out at 65 C in the absence of polar modifiers. The monomer premix was first introduced into

the reactor followed by the di-lithium initiator (0. 46M). The polymerization was allowed to continue until a full conversion was attained. The di-lithium initiator lead to the production of a polyisoprene polymer having two living lithium ends and molecular weight of 100,000 g/mol. Then hexamethylcyclotrisiloxane monomer was added to the reactor at room temperature (about 20 C) as a 13 percent solution in cyclohexane. To

reduce association in non-polar solvent (hexane) 300 cc of tetrahydrofuran (THF) was added. The reaction was terminated using chlorotrimethysilane and was subsequently dried. NMR characterization showed that this dialkylsiloxane-diene-dialkylsiloxane triblock rubber contained 5.6 weight percent polydimethylsiloxane, 86. 7 weight percent 1, 4-polyisoprene, and 7.7 weight percent 3,3-polyisoprene.

While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention.

Claims

CLAIMS : 1. Dialkylsiloxane-diene-dialkylsiloxane triblock rubber.
2. An alkylsiloxane-diene-alkylsiloxane triblock rubber as specified in claim 1 wherein the alkylsiloxane blocks are dimethylsiloxane blocks.
3. An alkylsiloxane-diene-alkylsiloxane triblock rubber as specified in claim 2 wherein the diene block is a polybutadiene block.
4. An alkylsiloxane-diene-alkylsiloxane triblock rubber as specified in claim 2 wherein the diene block is a polyisoprene block.
5. An alkylsiloxane-diene-alkylsiloxane triblock rubber as specified in claim 2 wherein the diene block is a styrene-butadiene block.
6. A process for synthesizing a dialkylsiloxane-diene-dialkylsiloxane triblock rubber which comprised the steps of (1) polymerizing at least one diene monomer in a first polymerization step to produce a diene polymer having two living lithium ends, wherein the polymerization is initiated with a dilithium initiator; and (2) adding a hexaalkylcyclotrisiloxane to the diene polymer to produce the dialkylsiloxanediene-dialkylsiloxane triblock rubber.
7. A process as specified in claim 6 wherein the hexaalkylcyclotrisiloxane is of the general structural formula:

wherein R represents an alkyl group containing from I to 8 carbon atoms.
8. A process as specified in claim 7 wherein R represents an alkyl group containing from 1 to 4 carbon atoms.
9. A process as specified in claim 6 wherein the hexaalkylcyclotrisiloxane is hexamethylcyclotrisiloxane.
10. A process as specified in any one of claims 6-9 wherein the polymerization process is conducted in an organic solvent.
11. A process as specified in any one of claims 1-10 wherein the diene monomer is 1,3-butadiene.
12. A process as specified in any one of claims 1-10 wherein the diene monomer is isoprene.
13. A process as specified in any one of claims 1-10 wherein the diene monomer is a mixture of 1,3-butadiene and isoprene.
14. A process as specified in any one of claims 1-10 wherein the diene monomer is a mixture of 1,3-butadiene and styrene.
15. A process as specified in any one of claims 1-14 wherein the first step is conducted in the presence of a polar modifier.
16. A process according to any one of claims 6-15 wherein the polymerization process is conducted at a temperature within the range of 40oC to 90oC.
17. A process according to any one of claims 6-15 wherein the

polymerization process is conducted at a temperature within the range of 60oC to 90oC.
18. A process according to claim 6 wherein the dilithium initiator is synthesized by reacting a tertiary-alkyl lithium compound with m-diisopropenylbenzene in an aromatic solvent.
19. A process as specified in claim 18 wherein said process is conducted at a temperature that is within the range of about 10C to about 70oC.
20. A process as specified in claim 18 wherein said process to synthesize the

dilithium initiator is conducted at a temperature that is within the range of about 20 C to 40oC.
21. A dimethylsiloxane-polyisoprene-dimethylsiloxane triblock rubber substantially as hereinbefore described in Example 1 or 2.
22. A process of synthesizing a dimethylsiloxane-polyisoprenedimethylsiloxane triblock rubber substantially as hereinbefore described in Example 1 or 2.