WO1996025442A1

WO1996025442A1 - Step process for tapered monovinylidene aromatic monomer conjugated diene block copolymers

Info

Publication number: WO1996025442A1
Application number: PCT/US1996/001852
Authority: WO
Inventors: Gary E. Woodson; Calvin P. Esneault; Michael O. Myers; Gary R. Marchand
Original assignee: The Dow Chemical Company
Priority date: 1995-02-14
Filing date: 1996-02-12
Publication date: 1996-08-22

Abstract

The invention is a process for the preparation of tapered block copolymers according to one of the formulae: B-t-A or A-B-t-A, wherein A comprises a polymer block derived from one or more monovinylidene aromatic monomers, B comprises a polymer block derived from one or more conjugated dienes, and t is a tapered polymer block derived from one or more monovinylidene aromatic monomers and one or more conjugated dienes wherein the portion of the block closest to block A is rich in monovinylidene aromatic monomer units, the portion of the block closest to block B is rich in conjugated diene units and the relative amount of conjugates diene and monovinylidene aromatic monomer units gradually changes along the backbone of the tapered block, comprising: a) contacting one or more conjugated dienes with a monofunctional lithium alkyl initiator or a polymer block derived from monovinylidene aromatic monomers with a terminal living lithium anion capable of initiating anionic polymerization and a hydrocarbon solvent, in a closed reactor equipped with a reflux condenser under conditions of reflux; b) partially polymerizing the one or more conjugated dienes; and c) after step b), contacting with the reaction mixture one or more monovinylidene aromatic monomers under conditions such that the unreacted conjugated diene and monovinylidene monomers polymerize; wherein the materials condensed during the process are recycled to the reactor.

Description

STEP PROCESS FOR TAPERED MONOVINYLIDENE AROMATIC MONOMER CONJUGATED DIENE BLOCK COPOLYMERS

The invention relates to a process for the preparation of tapered block copolymers of monovinylidene aromatic monomers and conjugated dienes.

Block copolymers of monovinylidene aromatic monomers and conjugated dienes are well known in the art. Such copolymers demonstrate elastomeric properties and are used in a wide variety of uses, for instance, as hot melt adhesives, pressure sensitive adhesives, as compatibilizers in polymer blends, in polymer modified asphalt, in reactor modifications of polymers such as SMA, ABS and HIPS, and in shoe soles. Such block copolymers are prepared by anionic polymerization processes. In one process, the blocks are prepared sequentially by polymerizing separately, in sequence, the monomers which make up each block, see for example, U.S. Patents 3,149,182; 3,030,346; and 3,390,207. In such processes, the monomers are polymerized in alternating sequence until the final product is prepared and the living anionic end is deactivated by well-known procedures. Alternatively, the block copolymers can be formed by sequential polymerization and then the diblocks with active anionic ends are contacted with a coupling agent to form the final product. Useful coupling agents are multifunctional compounds which have two or more functionalities reactive with the living anionic end of the diblocks, for instance di- or polyhalogenated compounds. The use of coupling allows the preparation of multiarmed block copolymers.

Pure block copolymers demonstrate some disadvantages, for instance, the block copolymers can present processability problems. To improve the processability of block copolymers, tapered block copolymers have been developed. A tapered block copolymer contains a portion which contains both of the monomers used to prepare the blocks with a gradual change in the relative proportions of the monomers such that in the region adjacent to one block, for example, the conjugated diene block, the tapered portion is rich in the monomer which makes up that block and the proportion of the other monomer increases along the backbone toward the other block, for instance, the monovinylidene block. Processes for preparing tapered block copolymers are disclosed in U.S. Patent 3,265,765. Generally, the preparation of tapered block copolymers is performed under near adiabatic conditions without significant heat removal from the reaction mixture, and tapering is accomplished by contacting the conjugated diene and monovinylidene aromatic monomer under reaction conditions. Because of the relative difference in reactivities, a conjugated diene block is formed first, then the tapered portion forms which is initially rich in conjugated diene. As the reaction proceeds, the amount of monovinylidene aromatic monomer incorporated into the backbone increases until the conjugated diene is completely polymerized and the monovinylidene block is formed. Such processes demonstrate some limitations in that only low polymer solids levels may be used during the reaction without either the reaction time becoming long or the temperature of the reaction mixture becoming so high that side reactions such as termination or coupling occur. Twelve percent polymer solids is the upper limit. This results in lower productivity than under the high solids conditions more easily accomplished during synthesis of pure block copolymers. Additionally, the standard diblock process allows only a narrow range of taper of 65 to 87 percent. Further, it is important to control temperature as, at high temperatures, above 100°C, side reactions, such as coupling of the growing polymer chains, can take place which prevent formation of a uniform product and causes reduction in properties. Polymerization of the conjugated diene generally results in high exotherms, thus, there are limits on the amount of conj gated diene which may be used without coupling due to the reaction reaching undesirably high temperatures.

What is needed is a process for the preparation of tapered block copolymers which avoids undesirably high temperatures, allows the use of high solids levels in the reactor and allows the preparation of block copolymers with a wider range of conjugated diene fractions and a wider range of tapering. What is also needed is a process which avoids the formation of high molecular weight products formed by side reactions, such as coupled diblock, and allows production of products with high uniformity.

The invention is a process for the preparation of tapered block copolymers according to one of the formulae B-t-A or A-B-t-A wherein A comprises a polymer block derived from one or more monovinylidene aromatic monomers, B comprises a polymer block derived from one or more conjugated dienes, and t is a tapered polymer block derived from one or more monovinylidene aromatic monomers and one or more conjugated dienes wherein the portion of the block closest to block A is rich in monovinylidene aromatic monomer units, the portion of the block closest to block B is rich in conjugated diene units and the relative amount of conjugated diene and monovinylidene aromatic monomer units gradually changes along the backbone of the tapered block, comprising: a) contacting one or more conjugated dienes with a monof unctional lithium alkyl initiator or a polymer block derived from monovinylidene aromatic monomers with a terminal living lithium anion capable of initiating anionic polymerization and a hydrocarbon solvent, in a closed reactor equipped with a reflux condenser under conditions of reflux; b) partially polymerizing the one or more conjugated dienes; and c) after step b), contacting with the reaction mixture one or more monovinylidene aromatic monomers under conditions such that the unreacted conjugated diene and monovinylidene monomers polymerize; wherein the materials condensed during the process are recycled to the reactor.

The process of the invention allows the preparation of tapered block copolymers at temperatures below those at which excessive amounts of side reactions occur, allows the use of high solids levels in the reactor, and allows the preparation of block copolymers having a wider range of conjugated diene fractions and a wider range of tapering. The process prevents the formation of high molecular weight products due to side reactions and allows production of products with high uniformity and very narrow molecular weight distributions.

The process can be used to prepare either tapered diblock copolymers or half- tapered triblock copolymers. A tapered diblock comprises three regions: a conjugated diene block, a monovinylidene aromatic monomer block and a tapered region disposed between the two blocks containing both monomers in polymerized form with a gradual change from a conjugated diene-rich portion adjacent to the conjugated diene block, to a monovinylidene aromatic monomer-rich portion adjacent to the monovinylidene aromatic monomer block. Such polymers can be represented by the formula B-t-A wherein B represents the conjugated diene block, A represents the monovinylidene aromatic monomer block and t represents the tapered region. A half-tapered triblock comprises a first monovinylidene aromatic monomer block with a conjugated diene block adjacent to the first monovinylidene aromatic monomer block. Also adjacent to the conjugated diene block is a tapered portion of the block copolymer as described above. The tapered portion of the block copolymer is further bound to a second monovinylidene aromatic monomer block. Such half-tapered triblock copolymer may be represented by the formula A-B-t-A wherein A, B and t are as previously described.

The block copolymers of the invention preferably demonstrate a number average molecular weight of 50,000 Daltons or greater. Preferably, the block copolymers of the invention demonstrate a number average molecular weight of 300,000 Daltons or less. If the molecular weight is too high, the block copolymer would not be processable as the viscosity would be too high. The block copolymers of the invention preferably contain 10 percent by weight or greater of monovinylidene aromatic monomer and more preferably 15 percent by weight or greater. The block copolymers of the invention preferably contain 50 percent by weight or less of monovinylidene aromatic monomer and more preferably 40 percent by weight or less. The block copolymers preferably contain a monovinylidene aromatic monomer block fraction of 50 percent by weight or greater, more preferably 55 percent by weight or greater and even more preferably 60 percent by weight or greater. The block copolymers preferably contain a monovinylidene aromatic monomer block fraction of 95 percent by weight or less and more preferably 90 percent by weight or less. Monovinylidene aromatic monomer block fraction is the total amount by weight of monovinylidene aromatic monomer in the pure block compared to the total amount of monovinylidene aromatic monomer in the entire polymer.

Preferable monovinylidene aromatic monomers for use herein include styrene and alkyl-substituted derivatives of styrene. Examples include styrene, α-methylstyrene, and vinyl toluene. A more preferred monoaromatic monomer is styrene.

Conjugated dienes preferably employed in the present invention include 1 ,3-butadiene, isoprene or mixtures thereof. Preferably, the conjugated diene is butadiene. The starting materials used for the process of the invention depend on the product desired. In the embodiment wherein the product is a tapered diblock, the starting materials are conjugated dienes, monovinylidene aromatic monomers and alkyl lithium initiators. The conjugated dienes and monovinylidene aromatic monomers are described previously. The amounts of each of the monomers useful in this process are dependent upon the desired molecular weights of the final polymer, the desired molecular weights of the pure blocks, the monovinylidene aromatic monomer block fraction and the amount of taper desired. If the process is properly performed, all of the monomer added will polymerize. Once these parameters are determined, one skilled in the art can readily calculate the amount of the o monomers needed. The monomers are preferably purified prior to use to remove impurities which may deactivate the initiators. Such impurities include water, oxygen and active hydrogen-containing compounds. The monomers can be purified well in advance of introduction into the process or as they are being introduced into the process. Purification may be accomplished by means well known in the art. In one preferred embodiment, the 5 monomers are passed through 4 Angstrom molecular sieves and/or alumina beds.

The alkyl lithium initiators useful in the invention are preferably Ci-iβstraight- and branched-chain alkyl monolithium compounds. Among preferred alkyl lithium initiators are methyl lithium, ethyl lithium, propyl lithium, butyl lithium, amyl lithium, hexyl lithium, α-ethylhexyl lithium and hexadecyl lithium. Even more preferred alkyl lithium initiators are 0 n-butyl lithium, sec-butyl lithium and tert- butyl lithium because they are commercially available and have high solubility in the preferred solvent systems. The amount of alkyl lithium initiator used is dependent upon the desired molecular weight of the final block copolymer. Generally, one initiator molecule is required for each polymer macromolecule. The amount of initiator used is controlled by the size of the reactor and the desired molecular weight of the product and can be calculated by the skilled artisan.

The reaction is performed in a solvent which comprises one or more hydrocarbons, linear ethers or cyclic ethers. Preferable solvents include straight- and branched- chain hydrocarbons, such as pentane, hexane, heptane and octane, as well as alkyl = substituted derivatives thereof ; cycloaliphatic hydrocarbons such as cyclopentane, 0 cyclohexane and cycloheptane, and alkyl derivatives thereof; aromatic and alkyl-substituted aromatic hydrocarbons such as benzene, naphthalene, toluene and xylene; and linear and cyclic ethers such as methyl ether, methyl ethyl ether and tetrahydrof uran. Mixtures of solvents are particularly useful in this invention. The choice of solvent or solvents is based on the target temperature of the reaction as the solvent or solvent mixture is chosen so as to reflux at the 5 target temperature of the reaction. More preferred solvents are straight- or branched-chain aliphatic hydrocarbons, cycloaliphatic hydrocarbons and mixtures thereof. Even more preferred solvents are cyclohexane, isopentane and mixtures thereof. The amount of solvent is chosen to give the desired solids level in the reactor. The highest possible solids level at which the reaction mixture is processable is desired. Preferable solids levels are 10 percent by weight or greater, more preferably 15 percent by weight or greater, even more preferably 18 percent by weight or greater and most preferably 20 percent by weight or greater. Preferably, the solids level is 35 percent by weight or less and even more preferably 30 percent by weight or less. Above 35 percent by weight, the reaction mixture is difficult to process as it becomes too viscous.

In the embodiment wherein the product is a half-tapered triblock copolymer, the starting materials are the conjugated diene, the monovinylidene aromatic monomer, and a monovinylidene aromatic monomer-based polymer chain having a terminal lithium which is capable of initiating anionic polymerization. The polymer chain is prepared by anionic polymerization of one or more monovinylidene aromatic monomers initiated by one of the initiators described herein. The process used to prepare such materials is the same as described herein. In a preferred embodiment, the polymer chain can be prepared in the reaction vessel prior to addition of the conjugated diene. It is preferable to complete the monovinylidene aromatic monomer polymerization prior to addition of the conjugated diene to minimize the inclusion of monovinylidene aromatic monomer units into the conjugated diene block.

The process of the invention is commonly known as sequential in that the various blocks in the block copolymer are added in sequence through anionic polymerization initiated through the lithium ion at the terminal portion of the polymer. The process is performed using a boiling bed process technique. A boiling bed process is characterized by the reaction mixture boiling under process conditions. The solvent(s) are chosen to facilitate boiling of the reaction mixture at the target temperature. The anionic polymerization is exothermic and use of a boiling bed improves temperature control. As the anionic polymerization of the conjugated diene results in a higher exotherm than does anionic polymerization of the monovinylidene aromatic monomer, the use of a process which provides improved temperature control allows the use of higher amounts and concentrations of conjugated diene. The reactor is equipped with a reflux condenser which condenses the vapors generated during the boiling of the reaction mixture. The condensed materials are recycled to the reactor. Preferably, the recycled materials are introduced below the liquid surface of the reaction mixture. The condensate comprises unreacted monomer and solvent. Any volatiles which do not condense are vented from the reactor and directed to appropriate disposal or recycle processes. A preferable reactor useful in performing this process is described in U.S. Patent 5,047,484.

Preferably, the process is performed under an inert atmosphere and in the absence of moisture and oxygen. In order to control the quality of the polymerization, it is preferable to perform a blanking process on the solvent prior to addition of the initiator. The blanking process involves adding small amounts of an organolithium compound to remove impurities such as oxygen, water or acidic materials which may be in the solvent. Blanking processes are well known in the art and one such process is described in U.S. Patent 5,089,572,. In performing the irst step of the reaction, the solvent, initiator or monovinylidene aromatic monomer-based polymer, with a lithium end moiety, and a portion of the conjugated diene are contacted and the polymerization of the conjugated diene is allowed to proceed until the desired conjugated diene pure block molecular weight is achieved. During this step, the reaction temperature is controlled at the desired levels. The concentration of conjugated diene used in this step is controlled by the desired molecular weight of the conjugated diene block and the ability to control the reaction temperature. The concentration of conjugated diene loaded into the reactor is based on the reactor size, target conjugated diene weight percentage and molecular weight and can be readily calculated. Preferably, the conjugated diene is added over an extended time period during which the conjugated diene is undergoing polymerization. This facilitates improved temperature control. Once the desired conjugated diene molecular weight is achieved, the monovinylidene aromatic monomer is introduced into the reaction vessel. In a preferred embodiment, the conjugated diene is added as a continuous stream and, when the desired conjugated diene pure block molecular weight is achieved, the monovinylidene aromatic monomer addition is started. When the desired amount of conjugated diene has been added to the reactor, the feed of conjugated diene is discontinued. Preferably, the monovinylidene aromatic monomer addition is continued beyond the termination of conjugated diene addition. The monovinylidene aromatic monomer addition is continued until the desired amount is added. The time period between the start of the conjugated diene addition and the start of the monovinylidene aromatic monomer addition depends on the desired amount of taper, as indicated by the monovinylidene aromatic monomer block fraction, the target molecular weight of the conjugated diene block, the reactor design and the temperature of the reaction. Preferably, this time period or delay is 5 minutes or greater and more preferably 10 minutes or greater. Preferably, the delay is 32 minutes or less, more preferably 30 minutes or less and even more preferably 20 minutes or less.

Preferably, the reaction mixture is heated prior to exotherm as this improves control of the reaction and prevents an extreme exotherm, which results in loss of control of the reaction or a product which is not uniform. The temperature of the reaction should be chosen such that the reaction is reasonably efficient and such that undesired side reactions are minimized. Side reactions can occur if the temperature is too high. Preferably, the reaction is performed at a temperature of 50°C or greater and more preferably 65°C or greater. Preferably, the reaction is performed at a temperature of 95°C or less and more preferably 85°C or less. The condenser should be cooled to maintain a temperature at which the volatile monomer and solvent components are condensed for recycle. Using the process of this invention, the recycle of the monomers does not adversely affect the structure of the backbone in the polymer, in that the desired block molecular weights and amount of taper can be achieved. It is generally preferred to apply positive pressure in the head space of the reactor in which the reaction takes place. Such positive pressure is useful in keeping volatile components under control during the polymerization reaction. The process is preferably conducted under agitation, such that the reaction mixture is well mixed, that is, the mixing rate exceeds the reaction rate.

After completion of the reaction, the living lithium moiety is deactivated by contacting it with a terminating agent. Such a terminating agent is generally an acidic hydrogen-containing organic compound which functions by replacing the lithium atom at the living end of the polymer with a hydrogen atom, thereby terminating the reaction. Preferable o terminating agents are lower alkanois, with C1.5 lower alkanols particularly preferred. The polymer can be recovered by processes well known in the art.

The tapered diblock polymers prepared by this invention demonstrate excellent processing characteristics as measured by toluene solution viscosity (TSV) and cyclohexane solution viscosity (CSV). Solution viscosities are determined according to ASTM D 2196 using a 5 Brookfield LVT viscometer and small sample adapter at 25°C. TSV was determined at

5.43 weight percent polymer in solution with a spindle number 18. CSV is determined at a variety of conditions (10 percent solids reference condition) using spindle number 31. The TSV is preferably 10 centipoise or greater and more preferably 15 centipoise or greater. The TSV is preferably 45 centipoise or less and more preferably 30 centipoise or less. The molecular 0 weight distributions (main peak M_w/M_n) are preferably 1.1 or less, more preferably 1.05 or less, even more preferably 1.03 or less and most preferably 1.02 or less.

The diblocks prepared by this process can either be used as a diblock or subjected to a coupling reaction to form triblock or radial polymers. Processes for performing such coupling reactions are well-known in the art. See U.S. Patents 4,845,173 and 4,096,203. 5 If the coupling agent isdifunctional, the resulting product is a fully tapered triblock having a structure B-t-A-t-B wherein block A comprises two monovinylidene aromatic monomer blocks and the residue of the coupling agent, or A-D-A wherein D is the residue of the coupling agent. Where the coupling agent is trif unctional or greater, the resulting product is a radial block copolymer of the structure DfA-t-B)_n wherein n is 3 or greater, and preferably 0 from 3 to 6.

The final polymer may be used in standard uses well known in the art for conjugated diene-monovinylidene aromatic monomer block copolymers, including in adhesive formulations, as elastomers, in asphalt formulations and in shoe soles. The block copolymers of this invention may be used according to well known methods in the art. It is contemplated that 5 the block copolymers prepared and described herein may be contacted with antioxidants, tackifying resins, oils and processing aids as dictated by the needs of the final end-use.

The monovinylidene aromatic monomer block fraction is defined as the fraction of the total monovinylidene aromatic monomer in the copolymers of this invention which is present in one or more monovinylidene aromatic monomer blocks A. The remainder of the monovinylidene aromatic monomer in the copolymers is present in the tapered block, t, and is variously known as random, isolated or non-block monovinylidene aromatic monomer. Monovinylidene aromatic = MVAM(A) monomer block fraction MVAM(A) + MVAM(t) where:

MVAM(A) = units monovinylidene aromatic monomer in block(s) A per unit of polymer;

MVAM(t) = units monovinylidene aromatic monomer in block t per unit of polymer; and A and t are as previously defined.

The amounts of block and random monovinylidene aromatic monomer in the copolymers of this invention can be determined from solution H nuclear magnetic resonance spectra of the copolymers. The techniques for this analysis, for copolymers in which the conjugated diene is butadiene or isoprene, and the monovinylidene aromatic monomer is styrene, are described by Mochel, V. D., Rubber Chemistry and Technology, 1967, 40, pp. 1200-1211, and by Sardelis, K., et al., Polymer, 1984, 25, pp. 1011-1019.

The molecular weights for the polymers of this invention were determined by size-exclusion chromatography. Commercially available polystyrene standards were used for calibration and the molecular weights of copolymers corrected according to Runyon et al., Applied Polymer Science, 1969, 13, p. 2359, and Tung, L H„ J. Applied Polymer Science. 1979, 24, p. 953.

The styrene content of styrenic block copolymers is determined based on the refractive index values determined from thin films of the polymer. Optically homogeneous films of less than 0.5 mm thickness are prepared in a hydraulic press heated to 175°C. Refractive index values are determined at 25°C using an Abbe refractometer Model #10400, and percent styrene by weight determined by the following equation percent styrene = (1441.24 * Rl) - 2187.95, where Rl is the refractive index of the polymer film.

The following examples are included for illustrative purposes only and are not intended to limit the scope of the claims. Unless otherwise stated, all parts and percentages are by weight. Example 1

A 00-gallon (1516-liter) reactor was equipped with a liquid agitator and a condenser for the return of condensed solvent and monomers below the liquid surface. The reactor contents initially comprised 544 kg (1200 pounds) of approximately 85 percent/- 15 percent weight basis mixture of dry deoxygenated cyclohexane and isopentane under an atmosphere of dry nitrogen. For the reaction sequence, the entire reactor was open to the vapor condenser. The temperature of the mixture was adjusted to 68°C using the external reactor jacket heat transfer system and the pressure was reduced to 1.24 bar/atm (18 psia). To the reactor mixture was then added 0.58 kg (1.3 pounds) of a 12 weight percent solution of sec- butyl lithium in cyclohexane. Immediately following this initiation, a flow of 3.6 to 4.1 kg/minute (8 to 9 pounds/minute) of dry, deoxygenated butadiene monomer was started. A total of 106 kg (233 pounds) of butadiene was added in this manner. When the butadiene charge to the reactor reached a certain quantity, in this example, 72 kg (158 pounds), and with the butadiene still flowing and polymerization occurring, a flow of dry, deoxygenate styrene was started to the reactor. At this point, the reactor temperature was approximately 91 °C. The rate of addition of the styrene to the reactor was approximately 4.5 kg/minute 0 (10 pounds/minute). A total of 18.7 kg (41.2 pounds) of styrene was added in this manner. The system was allowed to continue to react for 30 minutes after monomer addition was complete to ensure that virtually all butadiene and styrene were polymerized. After a total of 70 minutes from the time of first butadiene addition, 73 grams of isopropanol were added to the reaction mixture to terminate the polymer chain ends. Samples were collected from the reactor and 5 analyzed for solution viscosity (TSV and CSV), percent styrene weight fraction, percent styrene block fraction and molecular weight using gel permeation chromatography. The results are compiled in Table I. Examples 2 to 4

Examples 2 to 4 were completed using the procedure of Example 1 , except the o percent solids, styrene addition time, relative butadiene addition and quantities of monomers were varied. The amounts and results are compiled in Table I.

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Claims

Claims:

1. A process for the preparation of tapered block copolymers according to one of the formulae B-t-A or A-B-t-A wherein A comprises a polymer block derived from one or more monovinylidene aromatic monomers, B comprises a polymer block derived from one or more conjugated dienes, and t is a tapered polymer block derived from one or more monovinylidene aromatic monomers and one or more conjugated dienes wherein the portion of the block closest to block A is rich in monovinylidene aromatic monomer units, the portion of the block closest to block B is rich in conjugated diene units and the relative amount of conjugated diene and monovinylidene aromatic monomer units gradually changes along the o backbone of the tapered block, comprising: a) contacting one or more conjugated dienes with a monof unctional lithium alkyl initiator or a polymer block derived from monovinylidene aromatic monomers with a terminal living lithium anion capable of initiating anionic polymerization and a hydrocarbon solvent, in a closed reactor equipped with a 5 reflux condenser under conditions of reflux; b) partially polymerizing the one or more conjugated dienes; and c) after step b), contacting with the reaction mixture one or more monovinylidene aromatic monomers under conditions such that the unreacted conjugated diene and monovinylidene monomers polymerize; 0 wherein the materials condensed during the process are recycled to the reactor.

2. The process according to Claim 1 wherein the solids level of the reaction mixture is from 10 to 35 percent.

3. The process according to Claim 1 or 2 wherein the monovinylidene block fraction is from 50 to 95 percent. 5

4. The process according to any one of Claims 1 to 3 wherein the block copolymer prepared has a number average molecular weight of from 50,000 to 300,000 Daltons and a monovinylidene aromatic monomer content of 10 to 50 percent by weight.

5. The process according to any one of Claims 1 to 4 wherein the reaction 0 temperature is from 50°C to 95°C.

6. The process according to any one of Claims 1 to 5 wherein from 5 to

32 minutes elapse between initiation of the conjugated diene polymerization and addition of the monovinylidene aromatic monomer.

7. The process according to any one of Claims 1 to 6 wherein the 5 monovinylidene aromatic monomer is styrene and the conjugated diene is butadiene, isoprene or a mixture thereof.

8. The process according to any one of Claims 1 to 7 wherein the conjugated diene is contacted with an alkyl lithium initiator and the product is a tapered diblock.

9. The process according to any one of Claims 1 to 7 wherein the conjugated diene is contacted with a polymer block derived from monovinylidene aromatic monomers with a terminal living lithium anion capable of initiating anionic polymerization and the product is a half-tapered triblock.

10. The process according to any one of Claims 1 to 7 wherein the one or more conjugated dienes is contacted with the initiator in step a) which process further comprises d) contacting the diblock prepared in step c) with a coupling agent under conditions to prepare a fully tapered triblock of the structure B-t-A-t-B or a radial block copolymer of the structure D A-t-B)_n o wherein D is the residue of the coupling agent and n is an integer of 3 or greater.

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