Direct Diblock Synthesis

4-vinylpyridine (109) often serve as precursors to other structures. However, with the use of other nitroxidcs, and lower reaction temperatures, a mueh wider range of block copolymers is possible including PS- j/ocA -PtBA with niiroxide 89.  

An issue when making the second (and subsequent) blocks from styrenic monomers is that thermal initiation or an added initiator will provide a homopolymer impurity. 

Although, ATRP appears most suited to polymerization of methacrylate monomers, a very wide range of monomers can and have been used as is 

Optimal conditions for ATRP depend strongly on the particular monomer(s) to be polymerized. This is mainly due to the strong dependence of the activation-deactivation equilibrium constant (Al), and hence the rate of initiation, on the type of propagating radical (Section 9.4.1.3). When using monomers of different types, polymer isolation and changes in the catalyst are frequently necessary before making the second block 

Macroinitiator Monomer 2 Catalyst /Ligand Solvent Temp. C 

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Influence on the phase behaviour is observed if even small amounts (0.5 wt %) of hairy silica-like nanoparticles obtained by block-copolymer-directed sol-gel synthesis are added to a block copolymer [53,214]. On the example of a PS-fo-PI diblock copolymer a depression of the ODT of approx. 20 K was observed (Fig. 68), accompanied by a broadening of the transition. The largest depression was observed for rod-like nanoparticles, which is explained by the energy introduced by the defects [215]. 

Consistently, Anderson and coworkers showed that the polymerization of MMA in THF at —78 °C is living when initiated by DPHLi (10), which is nothing but the model of the diphenylalkyl anion (9) of the PS macroinitiator used in the synthesis of PS-fcZock-PMMA (equation 21). It must be noted that DPHLi (10) results from the direct addition of DPE (8) to n-Buli (equation 22)". The molecular weight of PMMA is predetermined by the monomer-to-initiator molar ratio and the MMA conversion. The polydispersity index is low (1.04 < Mw/Mn < 1.16). The livingness of this polymerization was confirmed by the successful resumption of the polymerization of lauryl methacrylate (LMA), and formation of the parent PMMA-fc/ock-PLMA diblocks. The anionic polymerization of MMA in THF at —78°C is thus living , provided that sterically hindered initiators are used. 

CON Convertine, A.J., Lokitz, B.S., Vasileva, Y., Myrick, L.J., Scales, C.W., Lowe, A.B., and McCormick, C.L., Direct synthesis of thermally responsive DMA/NIPAM diblock and DMA/NIPAM/DMA triblock copolymers via aqueous, room temperature RAFT polymerizatiorr, A/ocrowo/ecw/es, 39, 1724, 2006. 

Living polymerization of azo monomers is one of the most effective ways to prepare well-defined azo BCs. Generally, a monodispersed macroinitiator should be prepared first. It is then used as an initiator for the subsequent polymerization of azo monomers. Finkelmann and Bohnert (1994) first reported the synthesis of LC-side chain AB azo BCs by direct anionic polymerization of an azo monomer. As shown in Scheme 12.1, the polymerization of polystyrene (PS)-based diblock copolymers was carried out from a PS-lithium capped with 1,1-diphenylethylene (DPE), whereas the poly(methyl methacrylate) (PMMA)-based diblock copolymers were prepared by addition of methyl methacrylate (MMA) monomers to the living azo polyanion, obtained by reaction of l,l-diphenyl-3-methylpentylithium (DPPL) with the azo monomer in tetrahydrofuran (THF) at lower temperature. By this method, a series of well-defined azo BCs were obtained with controlled molecular weights and narrow polydispersities (Lehmann et al., 2000). 

One of the few reports on the preparation of mesoporous sihca nanofibers with a soft template other than triblock copolymers of the Pluronic type deals with the synthesis of sihca nanofibers with high aspect ratios containing linear arrays of mesopores by a solution-induced self-assembly process (Fig. 23), as previously reported for thin-film configurations [184]. To this end, PS-fo-PEO diblock copolymers were employed as structure-directing agents in sol solutions containing toluene/ethanol mixtures. For a Dp-value of 35 nm, a single line of mesopores formed, for a Dp-value of 60 nm two parallel rows of mesopores were obtained [185]. 

In addition to ATRP and NMRP, RAFT has also been used in this transformation. As mentioned previously, block copolymers with desired properties can be prepared by careful selection of the CTA, initiator, and monomers. A novel methodology employing this transformation for the synthesis of well-defined AB diblock and ABA triblock copolymers of poly (vinyl alcohol) (PVA) and PEO was recently reported. For this purpose, mono- or difimctional PEOs with xanthate end group were synthesized by anionic polymerization and employed as CTAs in the RAFT polymerization of vinyl acetate (VAc) to yield well-defined PEO-b-PVAc and PVAc-l -PEO-l7-PVAc. Eventually, direct hydrolysis of acetate groups by sodium hydroxide in methanol solution and... 

Figure 34 Template-directed synthesis of metal oxide nanowires by using PFSi7-/hP2VPi7o diblock copolymer cylindrical micelles with various functional and structural modifications. Reproduced with permission from Wang, H. Patil, A. J. Liu, K. etal. Adv. Mater. 2009,21,1805-1808. ...

Chains anchored to the surface, by either chemical grafting or an insoluble block, always produce a repulsion in gc solvents. Consequently, copolymers, e.g. diblock, comb, or graft, comprise the most effective stabilizers. Advances in polymer synthesis continue to increase the macromolecules available for this application (e.g.4). Direct grafting to the particle is a feasible alternative but requires chemistry specific to the particle. 

The direct synthesis of a diblock copolymer of styrene and THF has been achieved by coupling polystyryl-lithium with living cationic poly THF. Analogous procedures have led to the preparation of ABA and (AB)n multiblock copolymers. 

The chain-end fidelity inherent to the NMP process was widely applied to the synthesis of various macromolecular architectures such as diblock or triblock copolymers. Not only may these architectures be prepared with a similar degree of control to other CLRP methods, but NMP also provided easier routes to block copolymers, not directly accessible using existing CLRP techniques. A typical example is the dirert polymerization of MAA under SGI control, in contrast to ATRP where tert-butyl (meth)actylate should be polymerized and subsequently deprotected. 

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