Anionic polymerization chain modification

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

Copolymerizing VBT with either cationic or anionic substituted styrenes allowed us to obtain a fully water-processable photoresist [36]. Extension of polymeric structures is possible through backbone and side-chain modifications, especially terpolymers. Three-component systems containing the photoreactive monomer, the solubilizing monomer, and a functional monomer open the door to a virtually infinite set of physical and chemical parameters to be exploited and optimized. 

Polymer properties are dependent on many factors, including chain end interactions with substrates such as carbon black or silica fillers, as well as clay and calcium carbonate. At this point, there is a large volume of work that has been done on chain end modification, particularly those made by anionic polymerization with group 1 or group II metals (Bielinsk et al., 1995 Yamato and Oahu, 1996 Schulz et al., 1974), as seen later. 

EPR is currently replaced by EPDM, a modification with a diene monomer, due to its improved workability. A novel type of elastomer (called a thermoplastic elastomer) exhibits quite revolutionary behavior. Here cross-linking is temporary (at room temperature) while it can flow at higher temperatures, like thermoplastics. The typical one (SBS) is a strictly ordered block copolymer of styrene and butadiene, made by an anionic polymerization. The butadiene chains (at a controlled MW of 70,000) are embedded in a rigid phase of polystyrene spheres (MW of 15,000) thus providing temporary cross-linking at ambient conditions, while being processible at high temperatures like thermoplastics. 

The grafting onto, although offering unlimited chance for various architeaures, suffers from stria reaction conditions, which are almost similar to those used in the anionic polymerization process. The highly aaive polymer anion has less tolerance to functional groups for further chemical reaaion or modification. Click chemistry may overcome this shortcoming, but there is still no efficient method to attach long side chains to the backbone. 

Effects of ultrasounds in polymer chemistry are relatively less explored. Sonochemistry of polymers consists of three main fields the degradation and modification of polymers, the ultrasonically assisted synthesis of polymers and the determination of the polymer structure. Special attention has been devoted to ultrasound induced chain degradation [3, 4] and to the ultrasonically influenced preparation of anionic initiators and anionic polymerization [17]. 

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