Iniferter

Polymeric azo-compounds and multifunctional initiators with azo-linkages are discussed elsewhere (see 3.3.3 and 7.6.1) as are azo compounds, which find use as iniferters (see 9.3.4). 

The C-S bond of the sulfide end groups can be relatively weak and susceptible to thermal and photo- or radical-induced homolysis. This means that certain disulfides [for example 7-9] may act as iniferters in living radical polymerization and they can be used as precursors to block copolymers (Sections 7.5.1 and 9.3.2). 

In each of the sections below, we will consider the initiation process separately. For each system, various initiation methods have been applied. In some cases the initiator is a low molecular weight analog of the propagating species, in other cases it is a method oT generating such a species. The initiators first used in this form of living radical polymerization were called iniferters (initiator - transfer agent - chain terminator) or initers (initiator - chain terminator). 

The initiator or iniferter determines the number of growing chains. Several methods of initiation are used. Only three will be considered here. The first involves direct use of a species 1-X (e.g. a dilhiocarbamale ester - Section 9.3.2 or an alkoxyamine - Section 9.3.6) as shown in Scheme 9.4. Ideally, the degree of polymerization is given by eq. I and the molecular weight by eq. 2. 

A further problem with these iniferters is loss of living ends through primary radical termination by disproportionation. The ratio of reported for the cross... 

Otsu, T. and Matsumoto, A.-. Controlled Synthesis of Polymers Using the Iniferter Technique Developments in Living Radical Polymerization. Vol. 136, pp. 75-138. 

Controlled Synthesis of Polymers Using the Iniferter Technique Developments in Living Radical Polymerization... 

Keywords Controlled Polymerization Living Radical Polymerization Iniferter Chain-End Structure Molecular Weight Control Block Copolymer Dithiocarbamate Disulfide Nitroxide Transition Metal Complex... 

Control of the Chain-End Structure of Polymers with the Iniferter... 

In this article these concepts are described first, and the results of the controlled synthesis of polymers using the iniferter technique are discussed. In the last section, the excellent progress of living radical polymerization during recent years, from 1993 up to 1997, is summarized. 

These radical polymerizations may simply be considered as an insertion of monomer molecules into the R-R bond of the initiator leading to the polymer with two initiator fragments. Thus, the end groups of the polymer are controlled by the initiator used. Otsu proposed the name iniferter (im tiator-trans/er agent-terminator) for the initiators with such functions [64]. Many radical initiators, such as peroxides, azo compounds, tetraphenylethane derivatives, and organic sulfur compounds, may be expected to serve as an iniferter, if monomers and polymerization conditions are selected. Some peroxides show relatively high... 

There are two types of A-B and B-B type iniferters. A-B type iniferters thermally or photochemically dissociate into different radicals (Eq. 8) ... 

These iniferters thermally or photochemically dissociate at the weak bonds, and then monomer molecules are inserted by propagation, followed by primary radical termination and/or chain transfer to give polymers (9-11) (Eqs. 9-11), which also contain iniferter bonds at the chain ends. These polymers may further act as the polymeric iniferters. 

On the other hand, the B-B type iniferters dissociate into two identical radicals as follows (Eq. 12) ... 

Polymers 14 and 15 seem to serve as bifunctional iniferters from their structures, but they eventually act as the monofunctional iniferters because of the difference in the chain-end structure (see Sect. 4 for details). 

Table 1. Classification and Applications of Iniferters with the Dithiocarbamate Group...

Iniferter Structure and Reaction Application to Polymer Synthesis... 

Monofunctional Iniferter R-X > R-tMJj-X End-functional polymer, AB-type block copolymer... 

Difunctional Iniferter X-X > X-QA X Telechelic polymer, AB- or ABA-type block copolymer... 

Tetrafunctional Iniferter Xs, X X X > x m ta m tx R x-m/ Vm x Star polymer, Star block copolymer, Cross-linked polymer... 

Monomer Iniferter C=C-R-X > -C M) R-fM X Comb-like polymer, Graft copolymer... 

The A-B type iniferters are more useful than the B-B type for the more efficient synthesis of polymers with controlled structure The functionality of the iniferters can be controlled by changing the number of the A-B bond introduced into an iniferter molecule, for example, B-A-B as the bifunctional iniferter. Detailed classification and application of the iniferters having DC groups are summarized in Table 1. In Eqs. (9)—(11), 6 and 7 serve as the monofunctional iniferters, 9 and 10 as the monofunctional polymeric iniferters, and 8 and 11 as the bifunctional iniferters. Tetrafunctional and polyfunctional iniferters and gel-iniferters are used for the synthesis of star polymers, graft copolymers, and multiblock copolymers, respectively (see Sect. 5). When a polymer implying DC moieties in the main chain is used, a multifunctional polymeric iniferter can be prepared (Eqs. 15 and 16), which is further applied to the synthesis of multiblock copolymers. 

The resulting polymers always have the same functional group X at both chain ends. Therefore, telechelic polymers can be readily synthesized by the two-component iniferter system. An example is the polymerization of several monomers with 4,4J-azobiscyanovaleric acid (16) and dithiodiglycolic acid (17) as the initiator and the chain transfer agent, respectively, to synthesize the polymers having carboxyl groups at both chain ends [69]. 

Such a two-component iniferter technique is also applied to the living radical polymerization of several DC photoiniferters for the design of block and graft copolymer synthesis (Sect. 5). 

When the end groups of the polymers obtained by radical polymerization using certain iniferters still have an iniferter function, such radical polymerization is expected to proceed via a living radical mechanism even in a homogeneous system, i.e.,both the yield and the molecular weight of the polymers produced increase with reaction time. The generalized model is shown in Eq. (18) [16] ... 

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