Thermoplastic elastomers anionic polymerization

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

Sequential addition of different monomer charges to a living anionic polymerization system is useful for producing well-defined block copolymers. Thermoplastic elastomers of the triblock type are the most important commercial application. For example, a styrene-isoprene-styrene triblock copolymer is synthesized by the sequence... 

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... 

The annual worlwide production of triblock thermoplastic elastomers, clear impact-resistant polystyrene, and other styrene-diene products produced by anionic polymerization exceeds a couple of billion pounds. (Commercial utilization of anionic polymerization also includes the polymerization of 1,3-butadiene alone.)... 

The development of anionic chemistry over the past 30 years has led to the emergence of new processes and products of Industrial Importance, the most significant being a family of thermoplastic elastomers. These unique elastomers are presently commercialized by Shell Chemical Company as Kratons and by Phillips Chemical Company as Solprenes. Their uniqueness is the result of deliberate design of the polymeric structure and composition. 

A thermoplastic elastomer similar to the above structures was made by utilizing conjugated 1,3 diolefins that can be polymerized anionically. The work of Halasa and co-workers(2 ) illustrate the point. These workers polymerized 1,3-butadiene and isoprene to produce a diblock copolymer of poly(butadiene)-poly(isoprene)... 

The nonterminating nature of living anionic polymerization allows the synthesis of block copolymers,480,481 which are useful thermoplastic elastomers. They have many properties of rubber (softness, flexibility, resilience) but in contrast to rubber can be processed as thermoplastics 482,483 Block copolymers can be manufactured by polymerizing a mixture of two monomers or by using sequential polymerization. 

In the 1960s, anionic polymerized solutron SBR (SSBR) began to challenge emulsion SBR in the automotive tire market. Organolithium compounds allow control of the butadiene microstructure, not possible with ESBR. Because this type of chain polymerization takes place without a termination step, an easy synthesis of block polymers is available, whereby glassy (polystyrene) and rubbery (polybutadicnc) segments can be combined in the same molecule. These thermoplastic elastomers (TPE) have found use ill nontire applications. 

Since soluble multifunctional initiators are more readily available in cationic polymerization than in the anionic counterpart, ABA type linear triblock copolymers have been almost exclusively prepared using difunctional initiation followed by sequential monomer addition. The preparation and properties of ABA type block copolymer thermoplastic elastomers (TPEs), where the middle segment is PIB, have been reviewed recently [47]. 

Block copolymers were developed rapidly in the 1960s when living anionic polymerization was first utilized to synthesize triblock thermoplastic elastomers or elastoplastics. At the same time, step or condensation polymerization to produce thermoplastic polyurethanes, urea-urethane spandex fibers, and later more specialized materials, such as the semicrystalline polyester-polyether copolymers were developed [10]. Imide block or segmented copolymers utilizing... 

It is important to appreciate that polymer produced by an anionic chain-growth mechanism can have drastically different properties from one made by a normal free radical reaction. Block copolymers can be synthesized in which each block has different properties. We mentioned in Chapter 4 that Michael Szwdrc of Syracuse University developed this chemistry in the 1950s. Since that time, block copolymers produced by anionic polymerization have been commercialized, such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers (e.g., Kraton from Shell Chemical Company). They find use as thermoplastic elastomers (TPE), polymers that act as elastomers at normal temperatures but which can be molded like thermoplastics when heated. We will discuss TPEs further in Chapter 7. 

Before BASF investigated this product, Quirk and Hsieh [1], Yuki and coworkers [2,3] and Fischer [4] carried out investigations with this monomer. The first two used the anionic polymerization mechanism and Fischer tried to copolymerize this monomer using free radical polymerization. In the latter case the yields were very low. The use of S/DPE blocks in thermoplastic elastomers [5] has also been briefly described. Some of the work carried out at BASF has been published in a recent review article [6], Owing to the enhanced thermal properties of this copolymer in comparison with atactic polystyrene - the glass transition temperature increases up to 180 °C, depending... 

The statistical anionic copolymerization of acrylates and methacrylates is also controlled in the presence of LiOEEM (30), as testified by the copolymerization of MMA and tBuA in THF at —78°C. Block copolymers were also prepared by the sequential polymerization of at least two methacrylates and acrylates. For instance, PMMA- >/c cA -PbBuA and PMMA-fcZocA -PnNonA were synthesized . The addition order of the comonomers is important. Indeed, when living PnBuA is the macroinitiator of the MMA polymerization, the expected block copolymer is contaminated by homo-PnBuA, which is not the case when the polymerization sequence is reversed. A fuUy acrylic-based thermoplastic elastomer, PMMA-fcZocA -P(2EtHA)-fcZocA -PMMA, was prepared by the sequential LiOEEM-ligated polymerization of MMA, 2-EtHA and MMA. ... 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

The possibility of employing block copolymers as materials that might possess desirable properties was originally considered by Mark In the first period the effort in preparing block copolymers was directed to radical polymerization and it was only in 1956 that Szwarc obtained well-defined block copolymers by anionic polymerization . In block copolymers, the incompatibility between polymeric chains becomes an advantage a phase separation of the blocks occurs leading to the formation of microdomains which are responsible for the ecific properties of block copolymers. For instance, the presence in a molecule of an elastomeric block linked by its ends to thermoplastic blocks generates a polymer in which reversible physical multifunctional cross-links allow the behaviour of conventional vulcanized elastomers at room temperature, but the material remains easily moldable at elevated temperature just as normal thermoplastic resins ° ... 

ABA-type block copolymers B-12 with a hard PMMA as the outer segment (A) and a soft poly(nBA) as the inner segment (B) are expected as all-acrylic thermoplastic elastomers. Examples of B-12 have been prepared with copper and nickel catalysts via bifunctional initiation.359-364 Unfortunately, the copolymers by R—Br/Ni-2 via the macroinitiator method were reported to be inferior as thermoplastic elastomers to those by living anionic polymerizations. A possible reason is the presence of short PMMA seg-... 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

I, vinylruthenocene, 66, vinylosmocene, and the T)5-(vinylcyclopentadienyl)metal carbonyl monomers in radical-initiated polymerizations summarized in Scheme 1.1 no longer exists for anionically initiated addition polymerizations. Styrene is readily initiated by such anionic species as BuLi and Na1 Naphth. Living anionic styrene homopolymerizations and block copolymerizations have been extensively commercialized for many years (e.g., Kraton thermoplastic elastomers). However, the exceptionally electron-rich vinyl metal-containing monomers 1, 8-18, 24-30, and 66 were never successfully initiated by anionic systems in our laboratory despite many attempts. In these systems, the a-carbocations are very stable, but the a-carbanions are quite unstable. Thus, the addition of an anion to tbe vinyl function of these monomers is unfavorable. 

To ascertain control of the molecular weight, structure and composition, block copolymers are usually synthesized in anionic polymerization. The block copolymers of commercial interest are specifically prepared from monomers that upon polymerization yield immiscible macromolecular blocks, a smaller one rigid and the other flexible. The rigid blocks form physical crosslinks that upon heating above the transition point make the copolymer to flow. Thus, these materials belong to the growing family of thermoplastic elastomers. 

One of the major areas for potential involves the synthesis of polyolefin block copolymers. A PP-EPR-PP or PE-EPR-PE block copolymer could have large potential as is or in blends with other polyolefins. PE-EPR-PE block copolymers have been synthesized via anionic polymerization of butadiene-isoprene-butadiene ABA block copolymers followed by hydrogenation [Mohajer et al, 1982 Rangarajanout et al., 1993]. These materials would have utility in hot melt adhesive formulations as well as general-purpose thermoplastic elastomer applications. Improvements on the synthesis procedures to offer viable approaches to polyolefin block copolymers could open up a new class of commercial polyolefins. In summary, several opportunities exist for new combinations of commercial blends from the list of commodity polymers. 

The PIB macroinitiators can also initiate living anionic polymerization of a wide variety of functional monomers, such as vinyl pyridine, N,N-dimethylacryl-amide, and a variety of protected monomers, such as silylated 2-hydroxyethyl methacrylate. Polymerization studies with these monomers are in progress. The resulting products are potential new thermoplastic elastomers, dispersing agents, blending compounds, emulsifiers, non-ionic surfactants, biomaterials etc. 

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