Styrene-diene triblock copolymers

Ethylene-Propylene-Diene Terpolymer/Polyacetylene and Styrene-Diene Triblock Copolymer/ Polyacetylene Blends... 

In this paper, we wish to report two different types of elastomer blends, ethylene-propylene-diene terpolymer/polyacety-lene and styrene-diene triblock copolymer/polyacetylene, in the hope that the stability of polyacetylene might be improved by... 

The styrene-diene triblock copolymer consists of individual chains of three blocks, an elastomeric diene block in the center and a thermoplastic styrene block on each end. This polymer is called a thermoplastic elastomer. It exhibits some of the physical properties of elastomers at use temperature and is as pro-cessable as conventional plastics (5). The styrene/diene triblock copolymer has the unique morphology of glassy polystyrene domains in the rubbery diene matrix. Therefore, such an elastomer does not require conventional vulcanization since the glassy polystyrene domains act as physical crosslinks. 

As stated previously, styrene-diene triblock copolymers are the most important category of thermoplastic elastomers. Unlike most other TPEs, they can be blended with large quantities of additives without a drastic effect on properties. In almost all applications, the actual triblock copolymer content is less than 50%. Oils are used as a processing aid and do not result in a significant loss of properties if the polystyrene domains are not plasticized. For this reason, naphthalenic oils are preferred. The use of inert fillers such as clays or chalks reduces the cost of the final material. Unlike conventional rubbers, inert fillers do not have a substantial effect on the mechanical properties of TPEs. Thermoplastics such as polyethylene or polypropylene are also used to improve the solvent resistance and can increase the upper service temperature. Polystyrene homopolymer is used as a processing aid, which also increases the hard phase weight fraction and causes the material to stiffen. 

The control of chain structure and molecular weight afforded by the organolithium polymerization of dienes, has, of course, been of great technological interest [161,162,209]. Such product developments have been mainly in the form of (1) polybutadiene elastomers of various chain structures [162, 198,209] and functional end groups [210], (2) liquid polybutadienes [211], (3) butadiene-styrene copolymers (solution SBR) [69, 161, 162, 209], and (4) styrene-diene triblock copolymers (thermoplastic elastomers) [212]. 

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by... 

Although styrene-diene diblock copolymers are used in some applications, particularly in the area of viscosity index improvement (VII) additives for motor oil, styrenic block copolymers are most often used as thermoplastic elastomers. In these applications the styrene blocks phase separate, crosslinking the rubber blocks in a thermally reversible fashion. The simplest structure capable of exhibiting this behavior is a linear styrene-diene-styrene triblock. The most obvious way to produce such a molecule is by sequential polymeriza-... 

Bifunctional Initiation. The bifunctional initiators like alkali metal complexes of polycyclic aromatic compounds can be used to produce ABA triblock copolymers even when the A anion is not sufficiently basic to initiate polymerization of B monomers. In these cases polymerization would be started with monomer B to produce a polymeric dianion which could initiate polymerization of the A monomer which is added later. These initiators can be prepared only in aliphatic ethers, however. This precludes their use for the synthesis of useful styrene-diene ABA copolymers because polydienes made anionically in such solvents have low 1,4 contents and are not good rubbers. 

As a continuation of this work, various analogs of these triblock copolymers were synthesized, such as a-methylstyrene-b-isoprene-b-a-methylstyrene, a-methylstyrene-b-(propylene sulfide)-b-a-methylstyrene and a-methylstyrene-b-dimethylsiloxane-b-a-methylstyrene. All of these showed similar morphology and structure-property relations as the styrene-diene triblocks, as might have been expected. It was noteworthy, however, that when the polystyrene end blocks were replaced by poly-a-methylstyrene, there was a noticeable increase in modulus and tensile strength, at any given temperature. This was presumably due to the enhanced ability of the poly-a-methylstyrene domains to withstand greater stresses and higher temperatures,... 

Thermoplastic elastomeric behavior requires that the block copolymer develop a microheterogeneous two-phase network morphology. Theory predicts that microphase separation will occur at shorter block lengths as the polarity difference between the A and B blocks increases. This prediction is borne out as the block lengths required for the polyether-polyurethane, polyester-polyurethane, and polyether-polyester multiblock copolymers to exhibit thermoplastic elastomeric behavior are considerably shorter than for the styrene-diene-styrene triblock copolymers. 

ABA triblock copolymers of the styrene-diene type are well known, and owe their unique properties to their heterophase morphology. This arises from the incompatibility between the polystyrene A blocks and the polydiene B blocks, leading to the formation of a dispersion of very small polystyrene domains within the polydiene matrix. This type of elastic network, held together by the polystyrene "junctions", results in thermoplastic elastomer properties. 

Since their commercial introduction by Shell in 1965, the poly(styrene-b-(buta-diene or isoprene)-b-styrene) triblock copolymers (SBS or SIS) have attracted a... 

Difunctional initiators such as sodium naphthalene are useful for producing ABA, BABAB, CAB AC, and other symmetric block copolymers more efficiently by using fewer cycles of monomer additions. Difunctional initiators can also be prepared by reacting a diene such as /n-diisoprope ny I benzene or l,3-bis(l-phenylethenyl)benzene with 2 equiv of butyl-lithium. Monomer B is polymerized by a difunctional initiator followed by monomer A. A polymerizes at both ends of the B block to form an ABA triblock. BABAB or CABAC block copolymers are syntehsized by the addition of monomer B or C to the ABA living polymer. The use of a difunctional initiator is the only way to synthesize a MMA-styrene-MMA triblock polymer since MMA carbanion does not initiate styrene polymerization (except by using a coupling reaction—Sec. 5-4c). 

A second route is termed sequential anionic polymerization. More recently, also controlled radical techniques can be applied successfully for the sequential preparation of block copolymers but still with a less narrow molar mass distribution of the segments and the final product. In both cases, one starts with the polymerization of monomer A. After it is finished, monomer B is added and after this monomer is polymerized completely again monomer A is fed into the reaction mixture. This procedure is applied for the production of styrene/buta-diene/styrene and styrene/isoprene/styrene triblock copolymers on industrial scale. It can also be used for the preparation of multiblock copolymers. 

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