Anionic polymerization triblock polymers

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

The preparation of ABA triblock polymers requires use of a telechelie bisthiol prepared by termination of anionic polymerization initiated by a difunctional initiator. The relative yields of homopolymer, di- and triblock obtained in these experiments depend critically on conversion.273... 

We use polystyrene-Z>-polybutadiene block copolymers as the starting material with preformed polymer architecture. These polymers are comparatively cheap and easily accessible.1 For the present problems a series of narrowly distributed polystyrene-6-polybutadiene block copolymers with rather different molecular weights were synthesized via anionic polymerization (Figure 10.4, Table 10.1). As a test for the modification of technological products, a commercial triblock copolymer was also used. 

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... 

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. 

Block Copolymers. The manufacture of block copolymers became possible in 1956 by M. Szwarc s discovery (24) of "living" polymers prepared in homogeneous anionic polymerization. Diblock, triblock, and multiblock copolymers are produced ionically in the presence of sodium naphthalene, butyllithium, or Ziegler-type catalysts. 

Moreover, PFS block co-polymers can be accessed via transition metal-catalyzed ROP of silicon-bridged [l]ferro-cenophanes (Section 12.06.3.3.4) in the presence of a polymer terminated with a reactive Si-H bond. This technique has been used successfully for the synthesis of both diblock and triblock co-polymers. For example, water-soluble PFS-/ -PEO 106 (PEO = poly(ethylene oxide)) has been prepared from monomer 72 and commercially available poly(ethylene glycol) modified at the end group (Scheme 9). In such cases, the polydispersity of the PFS blocks is higher than that obtained from anionic ROP (typically, PDI = 1.4) and the polydispersity of the co-block is determined by that of the original Si-H functionalized material. Nevertheless, block co-polymer syntheses that use the transition metal-catalyzed approach are very convenient, as the stringent purification and experimental requirements for living anionic polymerizations are unnecessary. 

A new technique was developed recently, by introducing cationic to anionic transformation. A living carbocationic polymerization of isobutylene is carried out first. After it is complete, the ends of the chains are transformed quantitatively to polymerization-active anions. The additional blocks are then built by an anionic polymerization. A triblock polymer of poly(methyl methacrylate)-polyisobutylene-poly(methyl methacrylate) can thus be formed. The transformation involves several steps. In the first, a compound like toluene is Friedel-Craft alkylated by a,6t>"di-rerr-chloro-polyisobutylene. The ditolylpolyisobutylene which forms is lithiated in step two to form a,cu-dibenzyllithium polyisobutylene. It is then reacted with 1,1-diphenylethylene to give the corresponding dianion. After cooling to -78 °C and dilution, methyl methacrylate monomer is introduced for the second polymerization in step three. 

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