Anionic polymerization diblock polymers

Immediately after the discovery of the living character1 of anionic polymerization,2 polymer chemists started synthesizing well-defined linear homopolymers and diblock copolymers with low molecular... 

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. 

Bates [3] discussed the role of molecular architecture in polymer-polymer phase behavior. There are a number of molecular configurations available to a pair of chenucally distinct polymer species. Star polymers with a specific arm number with predeternuned molecular weights and with narrow molecular weight distribution can be synthesized using anionic polymerization. Diblock and multiblock arrangements are possible. Different polymers can be combined into a single material in several different ways that can lead to a variety of phase behaviors. Four factors control the phase behavior of polymer-polymer systems [3]. These are... 

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... 

Polymers and copolymers were laboratory-prepared samples. Samples W4 and W7 of the diblock copolymer AB poly(styrene-fo-tetramethylene oxide) (PS—PT) were synthesized by producing a polystyrene prepolymer whose terminal group was transformed to a macroinitiator for the polymerization of THF. Samples B13 and B16 of the diblock copolymer AB poly[styrene-h-(dimethyl siloxane)] (PS-PDMS) were prepared by sequential anionic polymerization. Samples of statistical copolymers of styrene and n-butyl methacrylate (PSBMA) were produced by radical copolymerization. Details of synthetic and characterization methods have been reported elsewhere (15, 17-19). 

Narrow polydispersity diblock copolymers of PS—PMMA and PS—PEO were produced by anionic polymerization using conventional high-vacuum methods. The average AB copolymer composition was determined by NMR (model EM30, Varian, UK). Narrow dispersity PS and PMMA standards (Polymer Laboratories) were used for both instrument and SEC column calibrations. Samples were prepared as nominally 1-mg/mL solutions in the eluent and spiked with toluene as a flow rate marker before full loop 100-fxh injection. Each copolymer was analyzed three times. 

A new star—block copolymer architecture, the inverse star—block copolymer, was recently reported.87 These polymers are stars having four polystyrene-/risoprene) copolymers as arms. Two of these arms are connected to the star center by the polystyrene block, whereas the other two are connected through the polyisoprene block. The synthetic procedure is given in Scheme 32. The living diblocks (I) were prepared by anionic polymerization and sequential addition of monomers. A small quantity of THF was used to accelerate the initiation of the polymerization of styrene. The living diblock copolymer (I) was slowly added to a solution of SiCL. The reaction was monitored by SEC on samples with-... 

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. 

It is also possible to form block co-polymers via the living anionic ROP of the phosphorus-bridged ferrocenophanes (Section 12.06.3.3.3). " Diblock co-polymers such as PI-/)-PFP 108 can be prepared by the sequential anionic polymerization of isoprene and ferrocenophane 107 (Scheme 10). These materials yield spherical micelles in -hexanes with an amorphous PFP core and a PI corona. The PI corona can be cross-linked via radical reactions to yield a permanently cross-linked shell, which retains its integrity even in good solvents for both blocks. With PFP block co-polymers the possibility of the coordination of various catalytically active transition metal moieties to the phosphorus centers may prove useful for catalysis and for materials science applications. 

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