Aromatic vinyl/isoprene block copolymer, process for the production thereof, and hardenable pressure-sensitive adhesive composition containing the same

An aromatic vinyl compound-isoprene block copolymer composition comprising (i) 5-50 wt. % a branched copolymer of the formula: (A-B).sub.n X wherein A is a polymer block of an aromatic vinyl monomer, B is a polymer block of isoprene, and X is a residue of a polyfunctional coupling agent, and (ii) 50-95 wt. % of a diblock copolymer of the formula: A-B wherein A and B are as defined above. This copolymer composition is produced by allowing an aromatic vinyl monomer to contact with an organic lithium initiator to prepare a polymer block A; incorporating isoprene thereto to prepare a diblock copolymer A-B; and then, adding a polyfunctional coupling agent to convert a part of the diblock copolymer A-B to the branched copolymer (A-B).sub.n X. This block copolymer composition is useful for an adhesive or pressure sensitive adhesive composition.

This application is the national phase number 35 U.S.C. .sctn.371 of PCT
 International Application No. PCT/JP97/03967 which has an International
 filing date of Oct. 30, 1997 which designated the United States of
 America.
 1. Technical Field
 This invention relates to an aromatic vinyl compound-isoprene block
 copolymer composition which is useful as a hot-melt adhesive and other
 adhesives, a process for producing the copolymer composition, and an
 adhesive or pressure sensitive adhesive composition-comprising the
 copolymer composition.
 2. Background Art
 A polystyrene-polyisoprene block copolymer and other block copolymers have
 widely been used as a base polymer for an adhesive or pressure sensitive
 adhesive composition. Especially a styrene-isoprene-styrene block
 copolymer is known as a base polymer for a hot-melt adhesive.
 For the production of disposable diapers, sanitary napkins and the like, a
 hot-melt adhesive is applied to a thermoplastic sheet or a nonwoven fabric
 made of a thermoplastic fiber by spiral spraying or other means. As the
 hot-melt adhesive, those which are applicable at a low melt-viscosity and
 a low temperature are desired with a view to preventing discoloration of
 adherends such as polyethylene sheet and deterioration of the hot-melt
 adhesive. The hot-melt adhesives are generally desired to have a melt
 viscosity of not larger than 5,000 cps at a temperature of 150.degree. C.
 However, a hot-melt adhesive composed of a styrene-isoprene block
 copolymer, which has such a low melt-viscosity upon fusion and exhibits
 satisfactory peel adhesion and shear adhesion failure temperature (SAFT),
 has not been proposed yet.
 In U.S. Pat. No. 5,399,627, a styrene-isoprene block copolymer composition
 is disclosed, which is produced by a process wherein a
 polystyrene-polyisoprene diblock copolymer having a minor proportion of a
 poybutadiene block bound to a terminal thereof is coupled with a coupling
 agent having four functionalities, and which is composed of a predominant
 proportion of the thus-produced copolymer with four branches and a minor
 proportion of an unreacted diblock copolymer. This block copolymer
 composition contains only not larger than 29% by weight of the unreacted
 diblock copolymer, and exhibits a poor peel adhesion when it is applied to
 a hot-melt adhesive.
 In Japanese Unexamined Patent Publication No. H1-266156, a styrene-isoprene
 block copolymer composition is disclosed, which is produced by a process
 wherein a polystyrene-polyisoprene diblock copolymer is reacted with a
 tetrafunctional coupling agent in an amount of at least equivalent to an
 initiator, and which is composed of a branched copolymer with three
 branches and an unreacted diblock copolymer. An adhesive made of this
 block copolymer composition has a poor peel adhesion and a low shear
 adhesion failure temperature.
 Further, in the above-mentioned Japanese patent publication, another
 styrene-isoprene block copolymer composition is disclosed which is
 composed of a large amount of a polystyrene-polyisoprene diblock copolymer
 and a minor amount of a branched copolymer with three branches. An
 adhesive of this block copolymer composition has a low shear adhesion
 failure temperature.
 DISCLOSURE OF INVENTION
 An object of the present invention is to provide a block copolymer
 composition which is useful as a hot-melt adhesive or other adhesives,
 which has satisfactory properties required for a hot-melt adhesive,
 especially has a low melt-viscosity and a low application temperature, and
 exhibits well balanced and high peel adhesion and shear adhesion failure
 temperature; and a process for producing the block copolymer composition.
 Another object of the present invention is to provide an adhesive or
 pressure sensitive adhesive composition useful as a hot-melt adhesive or
 other adhesives, which has a low melt-viscosity and a low application
 temperature, and exhibits well balanced and high peel adhesion and shear
 adhesion failure temperature.
 The inventors conducted extensive research for achieving the
 above-mentioned objects, and, found that an adhesive or pressure sensitive
 adhesive composition comprising a specific styrene-isoprene block
 copolymer composition and a tackifier is highly effective as an adhesive
 attaining the objects, which is easily produced by a process wherein
 styrene is polymerized by using an organic lithium initiator to give a
 polymer block A of styrene; isoprene is polymerized in the presence of the
 polymer block A to prepare a polymer block B of isoprene and further bind
 the two polymer blocks A and B together to give an A-B block copolymer;
 and then the A-B block copolymer is allowed to react with a predetermined
 amount of a coupling agent having at least four functionalities, and which
 comprises 5 to 43.9% by weight of a branched copolymer having at least
 four branches and 56.1 to 95% by weight of an unreacted diblock copolymer.
 Based on this finding, the present invention has been completed.
 Thus, in accordance with the present invention, there is provided an
 aromatic vinyl compound-isoprene block copolymer composition comprising:
 (i) 5 to 43.9% by weight of a branched copolymer having at least four
 branches, represented by the following general formula (I):
EQU (A-B).sub.n X (I)
 wherein A is a polymer block of an aromatic vinyl monomer, B is a polymer
 block of isoprene, X is a residue of a polyfunctional coupling agent
 having at least four functionalities, and n is an integer of at least 4,
 and
 (ii) 56.1 to 95% by weight of a diblock copolymer represented by the
 following general formula (II):
EQU A'-B (II)
 wherein A' is a polymer block of an aromatic vinyl monomer, which may be
 the same as or different from the polymer block A, and B is a polymer
 block of isoprene; said block copolymer composition having a weight
 average molecular weight of 10,000 to 500,000.
 In accordance with the present invention, there is further provided a
 process for producing the above-mentioned aromatic vinyl compound-isoprene
 block copolymer which comprises the steps of:
 allowing an aromatic vinyl monomer to contact with an organic lithium
 initiator to prepare a polymer block A composed of aromatic vinyl monomer
 units and having an active terminal exhibiting a polymerization activity;
 incorporating isoprene to the polymer block A-containing polymerization
 system whereby an A-B block copolymer is prepared, which has a structure
 such that the polymer block A of aromatic vinyl monomer is directly bonded
 to a polymer block B composed of isoprene units and having an active
 terminal exhibiting a polymerization activity; and then,
 adding a coupling agent having at least four functionalities to the A-B
 block copolymer-containing polymerization system to convert 5 to 43.9% by
 weight of the A-B block copolymer to a branched copolymer having at least
 four branches represented by formula (I).
 In accordance with the present invention, there is further provided an
 adhesive or pressure sensitive adhesive composition comprising the
 above-mentioned aromatic vinyl compound-isoprene block copolymer
 composition and a tackifier.
 Best Mode for Carrying Out the Invention
 Aromatic vinyl compound-isoprene block copolymer composition
 The aromatic vinyl compound-isoprene block copolymer composition of the
 present invention is characterized as comprising the branched copolymer of
 formula (I) and the diblock copolymer of formula (II).
 (1) Branched copolymer
 The first ingredient, i.e., the branched copolymer of formula (I) contained
 in the block copolymer composition of the present invention is a block
 copolymer having at least four branches which is prepared by coupling an
 A-B block copolymer composed of a polymer block A of aromatic vinyl
 monomer and a polymer block B of isoprene having an active terminal
 exhibiting a polymerization activity, with a polyfunctional coupling agent
 having at least four functionalities.
 The aromatic vinyl monomer used for the preparation of the branched
 copolymer is not particularly limited, and includes, for example, styrene,
 .alpha.-methylstyrene, vinyltoluene and vinylnaphthalene. Of these,
 styrene is preferable. These aromatic vinyl monomers may be used either
 alone or as a combination of at least two thereof.
 The proportion of the aromatic vinyl monomer units in the branched
 copolymer is not particularly limited, but, the content of the polymer
 block of aromatic vinyl monomer units in the entire aromatic vinyl
 compound-isoprene copolymer composition of the present invention is
 preferably in the range of 10 to 50% by weight, more preferably 25 to 50%
 by weight and most preferably 35 to 45% by weight, based on the total
 weight of the block copolymer composition. For satisfying this content in
 the entire copolymer composition, the content of polymer block A contained
 in the branched copolymer is also preferably in the range of about 10 to
 50% by weight, more preferably about 25 to 50% by weight and most
 preferably about 35 to 45% by weight. If the relative proportion of the
 polymer block A of aromatic vinyl monomer is too small, the shear adhesion
 failure temperature is lowered. In contrast, if the relative proportion of
 the polymer block A is too large, the peel adhesion is reduced.
 The molecular weight of the branched copolymer is not particularly limited,
 but, it should appropriately be chosen so that the block copolymer
 composition of the present invention has a weight average molecular weight
 (Mw) of 10,000 to 500,000, preferably 50,000 to 250,000 and more
 preferably 80,000 to 150,000, as expressed in terms of the weight average
 molecular weight converted by polystyrene standard measured by the gel
 permeation chromatography (GPC).
 The content of the branched copolymer having at least four branches in the
 aromatic vinyl compound-isoprene block copolymer composition of the
 present invention is in the range of 5 to 43.9% by weight based on the
 total weight of the block copolymer composition. If the content of the
 branched copolymer with at least four branches is too small, the shear
 adhesion failure temperature is not sufficiently high. If the content of
 the branched copolymer is too large, the aromatic vinyl compound-isoprene
 block copolymer composition has an undesirably high viscosity and its
 proccessability is reduced. The content of the branched copolymer with at
 least four branches is preferably in the range of 15 to 43.9% by weight
 and more preferably 25 to 43.9% by weight.
 (2) Diblock copolymer
 The second ingredient, i.e., the diblock copolymer of formula (II)
 contained in the block copolymer composition of the present invention is a
 straight chain diblock copolymer composed of the polymer block A' of
 aromatic vinyl monomer and the polymer block B of isoprene.
 The aromatic vinyl monomer used for the preparation of the diblock
 copolymer is not particularly limited, and includes, for example, styrene,
 .alpha.-methylstyrene, vinyltoluene and vinylnaphthalene. Of these,
 styrene is preferable. These aromatic vinyl monomers may be used either
 alone or as a combination of at least two thereof. As the aromatic vinyl
 monomer, the same monomer as that used for the preparation of the polymer
 block A of the branched copolymer is usually used, but a different monomer
 may be used.
 The proportion of the polymer block A' of aromatic vinyl monomer in the
 diblock copolymer is not particularly limited, but, the content of the
 polymer block of aromatic vinyl monomer in the aromatic vinyl
 compound-isoprene copolymer composition of the present invention is in the
 range of 10 to 50% by weight, preferably 25 to 50% by weight and more
 preferably 35 to 45% by weight, based on the total weight of the block
 copolymer composition. For satisfying this content in the entire copolymer
 composition, the content of polymer block A' contained in the diblock
 copolymer is also preferably in the range of about 10 to 50% by weight,
 more preferably about 25 to 50% by weight and most preferably about 35 to
 45% by weight. If the relative proportion of polymer block A' of aromatic
 vinyl monomer is too small, the shear adhesion failure temperature is
 reduced. In contrast, if the relative proportion of the polymer block A'
 is too large, the peel adhesion is reduced.
 The molecular weight of the diblock copolymer is not particularly limited,
 but, it should appropriately be chosen so that the block copolymer
 composition of the present invention usually has a weight average
 molecular weight (Mw) of 3,000 to 250,000, preferably 15,000 to 200,000
 and more preferably 20,000 to 150,000, as expressed in terms of the weight
 average molecular weight converted by polystyrene standard measured by
 GPC.
 The content of the diblock copolymer in the aromatic vinyl
 compound-isoprene block copolymer composition of the present invention is
 in the range of 56.1 to 95% by weight based on the total weight of the
 block copolymer composition. If the content of the diblock copolymer is
 too small, the peel adhesion is reduced. In contrast, if the content of
 the diblock copolymer is too large, the shear adhesion failure temperature
 is not sufficiently high. The content of the diblock copolymer is
 preferably in the range of 56.1 to 85% by weight and more preferably 56.1
 to 75% by weight.
 (3) Other polymer
 The aromatic vinyl compound-isoprene block copolymer of the present
 invention may comprise, in addition to the above-mentioned branched
 copolymer having at least four branches and the above-mentioned diblock
 copolymer, a branched copolymer having two branches and a branched
 copolymer having three branches. That is, the block copolymer composition
 of the present invention may comprise at least one optional branched
 copolymer selected from branched copolymers having two or three branches
 which are represented by the following formula (III):
EQU (A"-B).sub.m X (III)
 wherein A" is a polymer block of an aromatic vinyl monomer, B is a polymer
 block of isoprene, X is a residue of a polyfunctional coupling agent, and
 m is an integer of 2 or 3. As specific examples of the aromatic vinyl
 monomer used for the polymer block A", there can be mentioned styrene,
 .alpha.-methylstyrene, vinyltoluene and vinylnaphthalene. Of these,
 styrene is preferable. These aromatic vinyl monomers may be used either
 alone or in combination.
 However, the amount of the branched copolymer of formula (III) is
 preferably minor. That is, in order to attain the desired high shear
 adhesion failure temperature, the sum of a branched copolymer having two
 branches and a branched copolymer having three branches is preferably
 smaller than the amount of the branched copolymer having at least four
 branches. Usually the sum of a branched copolymer having two branches and
 a branched copolymer having three branches is not larger than 20% by
 weight, preferably not larger than 15% by weight, based on the weight of
 the aromatic vinyl compound-isoprene block copolymer.
 (4) Physical Properties
 The content of aromatic vinyl monomer units in the aromatic vinyl
 compound-isoprene copolymer is not particularly limited, and, for example,
 is in the range of 10 to 50% by weight, preferably 25 to 50% by weight and
 more preferably 35 to 45% by weight, to obtain highly balanced shear
 adhesion failure temperature and peel adhesion.
 The molecular weight of the aromatic vinyl compound-isoprene copolymer
 composition of the present invention is in the range of 10,000 to 500,000,
 preferably 50,000 to 250,000 and more preferably 80,000 to 150,000, as
 expressed in terms of the weight average molecular weight converted by
 polystyrene standard measured by GPC. If the molecular weight of the
 copolymer composition is too low, the shear adhesion failure temperature
 is lowered. In contrast, if the molecular weight of the copolymer
 composition is too high, the melt viscosity becomes undesirably high and
 the proccessability is reduced.
 The molecular weight distribution of the aromatic vinyl compound-isoprene
 copolymer composition of the present invention is not particularly
 limited, but, the ratio (Mw/Mn) of the weight average molecular weight
 (Mw) to the number average molecular weight (Mn), as expressed in terms of
 the molecular weight converted by polystyrene standard measured by GPC, is
 usually not larger than 4, preferably in the range of 1.1 to 3 and more
 preferably 1.2 to 2.0 to obtain highly balanced shear adhesion failure
 temperature and peel adhesion.
 (5) Production process
 The process for producing the aromatic vinyl compound-isoprene copolymer
 composition of the present invention is not particularly limited, but, the
 aromatic vinyl compound-isoprene copolymer composition is preferably
 produced by preparing an A-B diblock copolymer having a structure such
 that a polymer block B composed of isoprene units and having a terminal
 exhibiting a polymerization activity is directly bonded to a polymer block
 of aromatic vinyl monomer units; and then, coupling a part of the A-B
 diblock copolymer to give a block copolymer having at least four branches,
 whereby a composition comprising the block copolymer having at least four
 branches and the diblock copolymer is obtained at one time. More
 specifically, the preferable process for producing the aromatic vinyl
 compound-isoprene block copolymer composition comprises the steps of
 allowing an aromatic vinyl monomer to contact with an organic lithium
 initiator to prepare a polymer block A composed of aromatic vinyl monomer
 units and having an active terminal exhibiting a polymerization activity;
 incorporating isoprene to the polymer block A-containing polymerization
 system to give an A-B block copolymer having a structure such that a
 polymer block B composed of isoprene units and having an active terminal
 exhibiting a polymerization activity is directly bonded to the polymer
 block A of aromatic vinyl monomer units; and then, adding a coupling agent
 having at least four functionalities to the A-B block copolymer-containing
 polymerization system to convert 5 to 50% by weight of the A-B block
 copolymer to the branched polymer having at least four branches
 represented by the formula (I).
 Alternatively, the aromatic vinyl compound-isoprene copolymer composition
 can be produced by a process comprising the steps of separately preparing
 the branched block copolymer having at least four branches and the diblock
 copolymer, and then, mixing together the thus-prepared two block
 copolymers at a predetermined mixing ratio.
 The steps in the preferable process for producing the aromatic vinyl
 compound-isoprene block copolymer composition will be described in detail.
 (i) In the first step, an aromatic vinyl monomer is polymerized in a
 polymerization medium by using an organic lithium initiator. As the
 organic lithium initiator, conventional initiators capable of polymerizing
 an aromatic vinyl monomer and isoprene are used. As examples of the
 organic lithium initiator, there can be mentioned organic monolithium
 initiators such as methyllithium, n-propyllithium, n-butyllithium and
 sec-butyllithium. Of these, n-butyllithium is preferable. The amount of
 the organic lithium initiator used can be calculated depending upon the
 intended molecular weight of the copolymer by the method well known to a
 person skilled in the art.
 The polymerization medium used is not particularly limited provided that it
 is inactive for the organic lithium initiator, and it includes, for
 example, open chain hydrocarbon solvents, cyclic hydrocarbon solvents and
 mixtures thereof. As specific examples of the open chain hydrocarbon
 solvents, there can be mentioned open chain alkanes and alkenes having 4
 to 6 carbon atoms such as n-butane, isobutane, n-hexane and their
 mixtures; 1-butene, isobutylene, trans-2-butene, cis-2-butene and their
 mixtures; 1-pentene, trans-2-pentene, cis-2-pentene and their mixtures;
 and n-pentane, isopentane, neopentane and their mixtures. As specific
 examples of the cyclic hydrocarbon solvents, there can be mentioned
 aromatic hydrocarbons such as benzene, toluene and xylene, and alicyclic
 hydrocarbons such as cyclohexane. With a view to controlling the
 polymerization temperature, and the molecular weight distribution of the
 polymer block of aromatic monomer and the aromatic vinyl compound-isoprene
 block copolymer composition, it is preferable to use a mixed solvent
 composed of open chain hydrocarbon solvent having 4 to 6 carbon atoms and
 a cyclic hydrocarbon solvent preferably at a ratio of 5:95 to 40:60 by
 weight, more preferably 10:90 to 40:60 by weight.
 In the present invention, the polymerization of an aromatic vinyl monomer
 can be conducted in the presence of a polar compound. By using a polar
 compound, the rate of polymerization and the molecular weight distribution
 in the polymer block of aromatic vinyl monomer can be controlled. The
 polar compound used preferably includes aromatic and aliphatic ethers and
 tertiary amines having a relative dielectric constant of 2.5 to 5.0 at
 25.degree. C. As specific examples of such polar compounds, there can be
 mentioned aromatic ethers such as diphenyl ether and anisole; aliphatic
 ethers such as diethyl ether and dibutyl ether; tertiary monoamines such
 as trimethylamine, triethylamine and tripropylamine; and tertiary
 polyamines such as tetramethylethylenediamine and
 tetraethylethylenediamine. These polar compounds may be used either alone
 or as a combination of at least two thereof. The amount of the polar
 compound used is preferably in the range of 0.001 to 50 moles, more
 preferably 0.005 to 10 moles, per mole of the organic lithium initiator.
 The procedure by which an aromatic vinyl monomer is polymerized is not
 particularly limited, and any of the conventional procedures can be
 employed. For example, a batch polymerization procedure wherein the entire
 amount of the aromatic vinyl monomer and the entire amount of the
 initiator are charged at one time in the polymerization system to effect
 polymerization; a continuous polymerization procedure wherein the aromatic
 vinyl monomer and the initiator are continuously incorporated in the
 polymerization system; and a polymerization procedure wherein a part of
 the aromatic vinyl monomer and a part of the initiator are charged to
 conduct polymerization until a predetermined conversion is reached, and
 then, the remainders of the monomer and initiator are added to continue
 polymerization can be employed. The polymerization temperature is usually
 in the range of 0.degree. C. to 90.degree. C., preferably 20.degree. C. to
 80.degree. C. In the case where the reaction temperature is difficult to
 control, reflux cooling is preferably conducted by using a reactor
 equipped with a reflux condenser.
 (ii) In the second step, isoprene is incorporated into the polymerization
 system comprising the thus-produced polymer block A of aromatic vinyl
 monomer having an active terminal exhibiting a polymerization activity to
 effect polymerization of isoprene to give an A-B block copolymer having a
 structure such that a polymer block B composed of isoprene units and
 having an active terminal exhibiting a polymerization activity is directly
 bonded to the polymer block A of aromatic vinyl monomer units. The
 incorporation of isoprene can be carried out continuously to control the
 heat of reaction.
 (iii) After the completion of polymerization of isoprene, a polyfunctional
 coupling agent having at least four functionalities is incorporated in the
 polymerization system whereby the A-B block copolymer having an active
 terminal is bonded to each other to give the block copolymer having at
 least four branches represented by formula (I). A compound capable of
 accelerating the coupling reaction can be added, if desired.
 To obtain the block copolymer having at least four branches, a coupling
 agent capable of providing at least four sites capable of reacting with
 the active terminals exhibiting a polymerization activity must be used. As
 examples of the coupling agent, there can be mentioned silane compounds
 such as halogenated silanes and alkoxysilanes; tin compounds such as tin
 halides; polycarboxylic acid esters; epoxy compounds such as epoxidized
 soybean oil; acrylic acid esters such as pentaerythritol tetraacrylate;
 epoxysilanes; and divinyl compounds such as divinylbenzene. Of these,
 tetrafunctional coupling agents are preferable, and, as specific examples
 thereof, there can be mentioned tetrachlorosilane, tetrabromosiane,
 tetramethoxysilane, tetraethoxysilane, tetrachlorotin and diethyl adipate.
 Provided that the object of the invention is achieved, these coupling
 agents can be used either alone or as a combination of at least thereof.
 These coupling agents may be used in combination with bi- or
 tri-functional coupling agents.
 The amount of the coupling agent used in the present invention is
 important, and should appropriately be chosen so that the branched
 copolymer having at least four branches and the diblock copolymer are
 produced at the predetermined ratio, although the optimum ratio varies
 depending upon the particular kind of coupling agent. The amount of the
 coupling agent is usually in the range of 0.001 to 0.2 mole, preferably
 0.01 to 0.15 mole and more preferably 0.02 to 0.12 mole per mole of the
 organic lithium initiator. By using this amount of the coupling agent, the
 branched copolymer having at least four branches is produced in an amount
 of 5 to 50% by weight, preferably 15 to 50% by weight and more preferably
 25 to 45% by weight, based on the total weight of the block copolymer
 composition. If the amount of the coupling agent is too large, a block
 copolymer having two branches and represented by the formula (A-B).sub.2
 X, a block copolymer having three branches and represented by the formula
 (A-B).sub.3 X and a straight chain block copolymer having a residue of the
 coupling agent at a terminal and represented by the formula A-B-X are
 produced in large amounts (in these formulae, A, B and X are as defined
 above), and only a minor amount of the branched copolymer having at least
 four branches is produced.
 An appropriate amount of the coupling agent can be determined by
 calculation depending upon the compositions of the branched copolymer and
 the diblock copolymer in the intended block copolymer composition.
 Actually, deactivation of the organic lithium initiator and the coupling
 agent, and other factors must be considered, and thus, the optimum amount
 thereof is preferably determined by conducting preliminary experiments.
 According to the need, a reaction stopper such as methanol can be used to
 control the coupling percentage.
 (iv) After the completion of coupling, a reaction stopper such as water, an
 alcohol or an acid is added to deactivate the polymerization active sites.
 An antioxidant is added to the polymerization product, and then, a polymer
 is separated by a conventional procedure, for example, a steam stripping
 procedure, followed by drying, whereby the aromatic vinyl
 compound-isoprene block copolymer composition of the present invention
 comprising the branched copolymer having at least four branches and an
 uncoupled copolymer, i.e., the diblock copolymer, is obtained.
 As mentioned above, it is possible that the diblock copolymer of formula
 (A'-B) is prepared separately from the branched copolymer having at least
 four branches, and then, the diblock copolymer is mixed together with the
 branched copolymer having at least four branches. The diblock copolymer
 can be prepared by a process wherein a straight chain (A'-B) diblock
 copolymer having a structure such that a polymer block B of isoprene
 having an active terminal exhibiting a polymerization activity is directly
 bonded to a polymer block A' of aromatic vinyl monomer is prepared in a
 manner similar to the above-mentioned steps (i) and (ii); and the
 polymerization active sites are deactivated, an antioxidant is added, and
 then, the (A'-B) diblock copolymer is separated and dried without
 subjected to a coupling reaction in a manner similar to the
 above-mentioned step (iv).
 Adhesive or Pressure Sensitive Adhesive composition
 The adhesive or pressure sensitive adhesive composition of the present
 invention comprises the above-mentioned aromatic vinyl compound-isoprene
 block copolymer composition and a tackifier.
 In the adhesive or pressure sensitive adhesive composition of the present
 invention, rubber ingredients can be incorporated provided that the
 intended adhesive properties are obtained, which ingredients include, for
 example, styrene block copolymers such as a styrene-butadiene-styrene
 block copolymer, a styrene-ethylene-butylene-styrene block copolymer and a
 styrene-ethylene-propylene-styrene block copolymer; diene polymer rubbers
 such as butadiene rubber and isoprene rubber; and natural rubber.
 The tackifier used in the present invention is not particularly limited,
 and includes, for example, natural resin and synthetic resin tackifiers
 which are known and conventionally used in general adhesive compositions.
 The natural resin tackifier includes rosin type resins and terpene type
 resins. As specific examples of the rosin type resins, there can be
 mentioned rosins such as gum rosin, tall rosin and wood rosin; modified
 rosins such as hydrogenated rosin, disproportionated rosin and polymerized
 rosin; and esters such as glycerol ester and pentaerythritol esters, of
 these rosins and modified rosins. As specific examples of the terpene type
 resins, there can be mentioned terpene resins such as .alpha.-pinene,
 .beta.-pinene and dipentene (limonene); aromatic modified terpene resins;
 hydrogenated terpene resins; and terpene-phenol resins. The synthetic
 resin tackifier includes a polymerized type and a condensed type. As
 specific examples of the polymerized type resin, there can be mentioned
 petroleum resins such as aliphatic (C.sub.5)petroleum resins, aromatic
 (C.sub.9)petroleum resins, copolymerized (C.sub.5 /C.sub.9) petroleum
 resins, hydrogenated petroleum resins (alicyclic petroleum resins), DCPD
 and other alicyclic petroleum resins; cumarone-indene resins; and
 pure-monomer petroleum resins such as styrene resins and substituted
 styrene resins. As specific examples of the condensed type resins, there
 can be mentioned phenol resins such as alkyl phenol resins and
 rosin-modified phenol resins, and xylene resins. Of these, petroleum
 resins, especially alicyclic petroleum resins are preferable.
 The tackifier can be used either alone or as a combination of at least two
 thereof. The amount of the tackifier is usually in the range of 10 to 500
 parts by weight, preferably 50 to 350 parts by weight and more preferably
 70 to 250 parts by weight, based on 100 parts by weight of the aromatic
 vinyl compound-isoprene block copolymer composition.
 In the adhesive or pressure sensitive adhesive composition of the present
 invention, if desired, rubber ingredients can be incorporated provided,
 which include, for example, styrene block copolymers such as a
 styrene-butadiene-styrene block copolymer, a
 styrene-ethylene-butylene-styrene block copolymer and a
 styrene-ethylene-propylene-styrene block copolymer; diene polymer rubbers
 such as butadiene rubber and isoprene rubber; and natural rubber.
 Additives can also be incorporated, which include, for example, a
 softening agent (plasticizer), an antioxidant, a heat stabilizer, an
 ultraviolet absorber and a filler.
 The softening agent includes those which are known and conventionally used
 for adhesive or pressure sensitive adhesive compositions, such as
 aromatic, paraffinic and naphthenic extender oils, and liquid polymers,
 for example, polybutene and polyisobutylene. Of these, paraffinic and
 naphthenic extender oils are preferable. The amount of the softening agent
 is usually in the range of 10 to 500 parts by weight, preferably 20 to 300
 parts by weight and more preferably 30 to 150 parts by weight, based on
 100 parts by weight of the aromatic vinyl compound-isoprene block
 copolymer composition because good viscosity characteristics are obtained
 and undesirable bleeding can be minimized.
 As specific examples of the antioxidant, there can be mentioned hindered
 phenol compounds such as 2,6-di-tert.-butyl-p-cresol, pentaerythrityl
 tetrakis[3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate] and
 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-tert.
 -butylanilino)-1,3,5-triazine; thiodicarboxylate esters such as dilauryl
 thiodipropionate; and phosphate salts such as tris(nonylphenyl) phosphate,
 4,4'-butylidene-bis(3-methyl-6-tert.-butylphenyl)-ditridecyl phosphate.
 These antioxidants may be used either alone or in combination.
 The adhesive or pressure sensitive adhesive composition of the present
 invention can be used as a solvent type adhesive or pressure sensitive
 adhesive composition dissolved in a solvent such as n-hexane, cyclohexane,
 benzene and toluene; an emulsion type adhesive or pressure sensitive
 adhesive composition, which is an aqueous emulsion prepared by dispersing
 in water by using an emulsifier; or a hot-melt type adhesive or pressure
 sensitive adhesive composition which contains no solvent. Especially the
 adhesive or pressure sensitive adhesive composition is suitable for a
 hot-melt adhesive.
 The hot melt adhesive is especially suitable for making disposable goods
 such as disposable diapers, sanitary napkins, hospital gowns, bed pads and
 surgery drapes; and for use as constructive adhesive, elastic adhesives,
 and other uses. It also is suitable for making pressure sensitive adhesive
 tapes, labels, books and assemblies. The pressure sensitive adhesive tapes
 include a broad range of tapes such as those which are used for packaging
 tapes, office stationery, double coated tapes, masking tapes and
 electrical insulation tapes. The adhesive or pressure sensitive adhesive
 composition exhibits a high peel adhesion. It further exhibits a low
 viscosity no matter how it is applied as a solvent adhesive or a hot melt
 adhesive, and thus, it has good proccessability for coating. When the
 adhesive or pressure sensitive adhesive composition is used for making
 labels, it exhibits low viscosity and high peel adhesion, and thus, the
 proccessability for coating and adhesion characteristics are satisfactory,
 and further, the coated sheet exhibits good die-cuttability.
 The invention will now be described by the following working examples. In
 the examples and comparative examples, parts and % are by weight unless
 otherwise specified.
 In the working examples, molecular weights of polymers were determined by a
 high performance liquid chromatography using tetrahydrofuran as a carrier
 and expressed in terms of a weight average molecular weight (Mw) converted
 by polystyrene standard. The amount of respective ingredients in the
 copolymer composition was calculated from the area of peak as obtained by
 a high performance liquid chromatography.

EXAMPLE 1
 Preparation of Block Copolymer Composition
 A 50 liter pressure-resistant reactor was charged with 18.75 kg of a mixed
 solvent composed of 30% n-butane/70% cyclohexane, 240 milli-mole of
 dibutyl ether (relative dielectric constant: 3.06 at 25.degree. C.) and
 230 milli-mole of n-butyl lithium as an initiator. Then 3.04 kg of styrene
 was incorporated in the reactor to conduct polymerization at 30.degree. C.
 for one hour, and then, 4.96 kg of isoprene was added to conduct
 polymerization for about 1.5 hours while the reaction temperature was
 controlled to fall between 50.degree. C. and 60.degree. C. by reflux
 cooling. Then 25 milli-mole of tetramethoxysilane as a coupling agent was
 added to conduct a coupling reaction for 2 hours. Thereafter, 50 ml of
 methanol as a polymerization stopper and 40 g of 2,6-di-tert.
 -butyl-p-cresol were added and the mixture was thoroughly mixed together.
 Then the mixed solution was gradually added dropwise to hot water
 maintained at a temperature of 85 to 95.degree. C. to evaporate off the
 solvent from the solution. The thus-obtained polymer was subjected to
 pulverization and hot-air drying to give a block copolymer composition.
 The block copolymer composition had a styrene content of 38% by weight, a
 weight average molecular weight (Mw) of 96,000 as expressed in terms of
 that of polystyrene, and a molecular weight distribution (Mw/Mn) of 1.43.
 The copolymer composition was composed of 35.3% of a branched copolymer
 having four branches, 4.7% of a branched copolymer having three branches,
 1.2% of a branched copolymer having two branches and 58.8% of a diblock
 copolymer.
 Adhesive Composition
 A kneader having stirring blades was charged with 30 parts by weight of the
 above-mentioned block copolymer composition, and then, 50 parts of a
 tackifier "Alkon M-100" supplied by Arakawa Chem. Ind. Co.), 20 parts of
 naphthenic process oil ("Shellflex 371" supplied by Shell Chem. Co.) and
 0.2 part of an antioxidant ("Irganox 1010" supplied Ciba-Geigy Co.) were
 added. After the atmosphere in the kneading system was substituted with
 nitrogen, kneading was conducted at a temperature of 160 to 180.degree. C.
 to prepare an adhesive or pressure sensitive adhesive composition. The
 melt viscosity of the adhesive or pressure sensitive adhesive composition
 was measured at 150.degree. C. by using a Brookfield thermogel viscometer.
 A polyester film having a thickness of 30 .mu.m was coated with the
 adhesive or pressure sensitive adhesive composition to a thickness of 30
 .mu.m, and adhesive properties of the adhesive coating were evaluated.
 Evaluation of Adhesive Properties
 The peel adhesion (N/m) was evaluated according to PSTC-1 (Peel adhesion
 test at 180.degree. angle issued by Pressure Sensitive Tape Council,
 U.S.A.) by using stainless steel or polyethylene as an adherend material.
 The shear adhesion failure temperature (.degree. C.) was measured by using
 stainless steel as an adherend material at an adhered area of 10.times.25
 mm, a load of 1 kg and a temperature elevation rate of 0.5.degree. C./min.
 The evaluation results are shown in Table 1.
 EXAMPLES 2 to 5
 Comparative Examples 1 to 7
 Block copolymer compositions were prepared and their characteristics were
 evaluated, and adhesive compositions were prepared from the block
 copolymer compositions and their adhesive properties were evaluated, by
 substantially the same procedures as described in Example 1, wherein
 tetrachlorosilane (TCS) was used instead of tetramethoxysilane (TMS) as a
 coupling agent and the amount of the coupling agent was varied as shown in
 Tables 1 and 2 in Examples 2 and 3, and Comparative Example 1;
 phenyl-trichorsilane (PTCS) was used instead of TMS as a coupling agent
 and the amount of the coupling agent was varied as shown in Table 2 in
 Comparative Example 2; and the amount of tetramethoxysilane (TMS) was
 varied as shown in Tables 1 and 2 in Examples 4 and 5, and Comparative
 Example 3. The evaluation results are shown in Tables 1 and 2.
 In Comparative Example 4, a block copolymer composition was prepared by a
 process described in U.S. Pat. No. 5,394,627 wherein, after completion of
 polymerization of isoprene, 1.2% by weight, based on the weight of
 isoprene, of butadiene was added and tetrachlorosilane in an amount shown
 in Table 2 was reacted with all other conditions remaining the same as in
 Example 1. The characteristics of the block copolymer composition were
 evaluated, and an adhesive or pressure sensitive adhesive composition was
 prepared from the block copolymer composition and the adhesive properties
 of the adhesive or pressure sensitive adhesive composition were evaluated,
 by the same procedures as in Example 1. The evaluation results are shown
 in Table 2.
 In Comparative Examples 5, 6 and 7, the following commercially available
 styrene-isoprene-styrene block copolymer compositions A, B and C were
 used, respectively. Their characteristics were evaluated, and adhesive
 compositions were prepared from the block copolymers and their adhesive
 properties were evaluated, by the same procedures as in Example 1. The
 evaluation results are shown in Table 2.
 Commercially available block copolymer composition A: "Kraton D1124P"
 supplied by Shell Co.
 Commercially available block copolymer composition B: "Vector 4211"
 supplied by Dexco Co.
 Commercially available block copolymer composition C: "Vector 4411"
 supplied by Dexco Co.
 The abbreviations for coupling agents occurring in Tables 1 and 2 are as
 follows.
 TMS: tetramethoxysilane
 TCS: tetrachlorosilane
 PTCS: phenyltrichlorosilane
 TABLE 1
 Example
 1 2 3 4
 5
 Coupling agent TMS TCS TCS TMS
 TMS
 Coupling agent/initiator mol ratio 0.11 0.07 0.09 0.05
 0.12
 Block copolymer
 Styrene content (%) 38 41 37 37
 31
 Mw of copolymer composition (.times.1,000) 96 98 116
 74 135
 Molecular weight distribution (Mw/Mn) 1.43 1.36 1.39 1.29
 1.46
 Composition of copolymer composition (%)
 Diblock copolymer 58.8 72.5 64.5 80.6
 56.1
 Copolymer with 2 branches 1.2 0 0 0
 1.7
 Copolymer with 3 branches 4.7 0.8 1.0 1.7
 7.3
 Copolymer with 4 branches 35.3 26.7 34.5 17.7
 34.9
 Melt viscosity of adhesive composition 1,930 3,700 3,800
 1,060 4,500
 (150.degree. C., cps)
 Peel adhesion (N/m)
 to steel 1,170 1,250 1,200 1,150
 1,120
 to polyethylene 530 550 570 560
 550
 Shear adhesion failure temperature (.degree. C.) 71 70 70
 68 68
 TABLE 2
 Comparative
 Example
 1 2 3 4
 5 6 7
 Coupling agent TCS PTCS TMS TCS
 -- -- --
 Coupling agent/initiator mol ratio 0.18 0.12 0.21 0.25
 -- -- --
 Block copolymer
 A B C
 Styrene content (%) 38 36 31 37
 30 29 44
 Mw of copolymer composition (.times.1,000) 107.6 92 151
 125 145 120 90
 Molecular weight distribution (Mw/Mn) 1.55 1.30 1.38 1.39
 1.52 1.02 1.05
 Composition of copolymer composition (%)
 Diblock copolymer 34.2 66.5 23.9 8.0
 27.6 0 0
 Copolymer with 2 branches 2.9 2.4 14.4 0
 4.6 100 100
 Copolymer with 3 branches 28.2 31.4 51.7 22.0
 67.8 0 0
 Copolymer with 4 branches 34.7 0 10.0 70.0
 0 0 0
 Melt viscosity of adhesive composition 1,870 2,050 17,500
 3,800 9,200 6,700 4,200
 (150.degree. C., cps)
 Peel adhesion (N/m)
 to steel 960 1,050 940 920
 950 970 960
 to polyethylene 490 520 480 470
 500 460 470
 Shear adhesion failure temperature (.degree. C.) 70 65 72
 68 71 63 70
 Industrial Applicability
 The aromatic vinyl compound-isoprene block copolymer composition of the
 present invention is useful for a hot-melt adhesive composition, and other
 adhesive or pressure sensitive adhesive compositions. An adhesive or
 pressure sensitive adhesive composition made from the block copolymer
 composition of the present invention has a low-melt-viscosity and
 low-application-temperature and exhibits high and balanced peel adhesion
 and shear adhesion failure temperature.
 A hot-melt adhesive made from the block copolymer composition of the
 present invention satisfy the following characteristics.
 Melt viscosity at 150.degree. C.: not larger than 5,000 cps
 Shear adhesion failure temperature: at least 66.degree. C.
 Peel adhesion to steel: at least 1,000 N/m
 Peel adhesion to polyethylene: at least 500 N/m