End-functional polymers synthesis

End-functional polymers, including telechelic and other di-end functional polymers, can be produced by conventional radical polymerization with the aid of functional initiators (Section 7,5.1), chain transfer agents (Section 7.5.2), monomers (Section 7.5.4) or inhibitors (Section 7.5.5). Recent advances in our understanding of radical polymerization offer greater control of these reactions and hence of the polymer functionality. Reviews on the synthesis of end-functional polymers include those by Colombani,188 Tezuka,1 9 Ebdon,190 Boutevin,191 Heitz,180 Nguyen and Marechal,192 Brosse et al.rm and French.194... 

Living polymerization processes lend themselves to the synthesis of end functional polymers their use in this context is described in Chapter 9. In this section we limit discussion to processes based on conventional radical polymerization,... 

Ebdon and coworkers22 "232 have reported telechelic synthesis by a process that involves copolymerizing butadiene or acetylene derivatives to form polymers with internal unsaturation. Ozonolysis of these polymers yields di-end functional polymers. The a,o>dicarboxy1ic acid telechelic was prepared from poly(S-s tot-B) (Scheme 7.19). Precautions were necessary to stop degradation of the PS chains during ozonolysis. 28 The presence of pendant carboxylic acid groups, formed by ozonolysis of 1,2-diene units, was not reported. 

Most reviews on living radical polymerization mention the application of these methods in the synthesis of end-lunctional polymers. In that ideally all chain ends are retained, and no new chains are formed (Section 9.1.2), living polymerization processes are particularly suited to the synthesis of end-functional polymers. Living radical processes are no exception in this regard. We distinguish two main processes for the synthesis of end-functional polymers. 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

The literature on synthesis of end-functional polymers by ATRP through 2000 is discussed in a review by Cocssens and MatyjaszewskiT92 The topic also has coverage in more general reviews on ATRP.268 269... 

The synthesis of end functional polymers by NMP, ATRP and RAFT has already been discussed in Section 9.7. The "grafting to approach involves the covalent attachment of an end-funetionalized polymer with reactive surface groups on the substrate. The approach is inherently limited by the crowding of chains at the surface and the limit this places on the final graft density. 

RAFT polymerization, synthesis of end-functional polymers 563 CiPt distributions 241-2... 

The synthesis of block copolymers is obviously one of the polymer synthesis areas where living polymerization is most effective and convenient [78,79]. In general, block copolymers may be synthesized either (A) via sequential living polymerization or (B) via reactions of end-functionalized polymers that include living polymers both of the two methods are also available in cationic polymerizations ... 

Scheme 4 Synthesis of end-functionalized polymers by living cationic polymerization Methodologies and typical architectures.

Before the development of living cationic polymerization in the 1980s, Kennedy and his co-workers devised another way to synthesize end-functionalized polymers, which uses special reagents called inifer, or initiator-chain transfer agents [129]. The method is primarily for the synthesis of polyisobutene with a tertiary chlorine terminal, which is, however, a synthon for a variety of other functional groups. These developments have been reviewed extensively [1,3,130] and fall outside the scope of this chapter. 

A class of end-functionalized polymers with polymerizable terminal groups are generally called macromonomers. By both functional initiator and terminator methods, a variety of macromonomers have been synthesized in living cationic polymerization of vinyl ethers, styrenes, and isobutene, as summarized in Table 3 [16,31,147,149-151,155,158-171]. Some of these macromonomers are used in the synthesis of graft polymers (Section VI.C). 

A similar, physically bound CuBr catalyst on a silica gel support (L-29) was also employed for MMA polymerization.149 The polymers had narrow MWDs (MJMn 1.3), but the molecular weights were higher than the calculated values. The recycled catalysts have a lower activity but lead to better control of molecular weights i.e., the Mn agreed well with the calculated values, and the MWDs were narrower MJ Mn 1.2). The physically supported catalysts were further employed for the synthesis of end-functionalized polymers.150 When physically supported silica gel catalysts are packed into a continuous column reactor, a controlled polymerization is possible.151... 

One of the most distinguishable characteristics of the metal-catalyzed living radical polymerization is that it affords polymers with controlled molecular weights and narrow MWDs from a wide variety of monomers under mild conditions even in the presence of a protic compound such as water. This permits the synthesis of a vast number of polymers with controlled structures such as end-functionalized polymers, block copolymers, star polymers, etc., where they are widely varied in comparison with those obtained by other living polymerizations. This is primarily due to the tolerance to various functional groups and the polymerizability/controllability of various vinyl monomers as mentioned above. 

The introduction of functional groups at the end(s) or along the polymer chain can produce new materials that can be used as models to study and manipulate fundamental phenomena in polymer science, such as association, adsorption, chain dynamics, and block copolymer morphology.61 65 The synthesis of end-functionalized polymers remains a challenging problem in polymer chemistry. Among the different... 

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