Macroinitiator

Several macrointermediates to obtain this kind of copolymer were used via free radical, ionic, and/or free radical-ionic coupling polymerization. In this manner, macroinitiators, macromonomers, and macromono-meric initiators will be discussed in this chapter. 

Macroinitiators are macromolecules having peroxygen and/or azo groups that can thermally initiate a vinyl polymerization to obtain block copolymers in one step. They can be classified as macroperoxyinitiators (MPl), macroazoinitiators (MAI), and macroazo-peroxyiniti-ators. 

Vinyl polymerization initiated by macroinitiator yield AB, ABA, or (AB) types block copolymers. Macroinitiators such as macrobis peroxides, polyazoesters, and... 

Block Copolymers Derived from Macroinitiators Recent Advances of Synthesis, Properties, and Applications... 

Block copolymers have been synthesized on an industrial scale mainly by anionic or cationic polymerization, although monomers for block components are limited to ones capable of the process. Intensive academic and technological interest in radical block copolymerization using macroinitiators is growing. This process can be implemented in plants with easier handling of materials, milder conditions of operation, and a variety of materials to give various kinds of block copolymers to develop a wide application area [1-3]. 

The principle of synthesizing block copolymers by macroinitiators is as follows ... 

Monomer A is polymerized initiated with a pair of radicals formed by thermolysis of an active site of macroinitiator. Since growing chain A propagates from the residual segment of the initiator, polymer A thus formed retains unreacted active sites in the chain end. 

In detail, the structure of a macroinitiator with active sites in the main chain is classified into two types that derive different types of block copolymers, as shown in Fig. 1. 

Block copolymerization is carried out by thermolysis of the macroinitiator in bulk, solution, suspension, or emulsion system. Further, it is possible to apply photolysis of azo group. In another case, an ionic active site coupled with an azo group is utilized [3]. 

Utilization of another function of the macroinitiator was tried in emulsion polymerization [30]. An MAI composed of PEG (molecular weight of a segment is 1000) linked with AGP units was confirmed to be usable as a surface active initiator (Inisurf) for preparing PSt-b-PEG [30]. A higher molecular weight block copolymer was obtained in comparison with the case of solution copolymerization. 

In the polymerization of St initiated with type II MAI composed of polyvinylpyrrolidone (PVP), block efficiency was kept to 80% when feed concentration was above 3 mol/L, but it drastically decreased below 3 mol/ L (Fig. 2) [36,37]. AIBN, the typical low-molecular weight azo initiator, shows a drastic decrease in its initiation efficiency below a critical feed monomer concentration, i.e., 0.5 mol/L. In the case of MAI, it seems that a similar decrease in initiation efficiency occurs at much higher critical monomer concentration due to immobility of macroinitiating radicals. 

Quantitative analysis of the peroxy group of macroinitiators is performed by iodometry [38] and that of the azo group is done by ultraviolet (UV) spectrometry. Recently, type II MAI composed of PU was determined of its azo concentration by UV [20]. When the UV absorption spectral peak of the azo group overlaps other peaks, DSC is available by determining the azo group from the exothermal peak area [1IJ. 

The utilization of macroinitiators for producing block copolymers provides the following advantages ... 

Aliphatic disulfides are not thought to be effective as initiators in this context. However, Endo et a . K have described the use of the cyclic 1,2-disulfides 11 and 12 as initiators in a controlled radical polymerization. Polymerization of S at 120 °C gave a linear increase in molecular weight with conversion and the PS formed was used as a macroinitiator to form PS-6/oet-PMMA. The precise mechanism of the process has not been elucidated. 

The first use of sterically hindered hexasubstituted ethanes [e.g. 33] as initiators of polymerization was reported by Bledzki et al.77,78 The use of related initiators based on silylatcd pinacols [e.g. 34, 35] has been reported by Crivcllo et a/.,7l>82 jjan( os et ai i anOther initiators of this class include 36 fi X/ and 37.The rates of decomposition of hexasubstituted ethanes and the derived macroinitiators are known to vary according to the degree of steric... 

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

Otsu and Tazaki90 have reported on the use of triphenylmethylazobenzene (39) as an initiator. In this case, phenyl radical initiates polymerization and the triphenylmethyl radical reacts mainly by primary radical termination to form a macroinitiator. The early report91 that triphenylmethyl radical does not initiate MMA polymerization may only indicate a very low rate of polymerization. The addition of triphenylmethyl radical to MMA has been demonstrated in radical... 

It is of interest to speculate on the precise structure of the macroinitiator species in these polymerizations. The work of Engel et a .94 suggests the likelihood of a quinonoid intermediate (e.g. 45, Scheme 9.13), at least for the polymerizations involving triphenylmethyl radical (44). 

There will be a gradual loss of stable radical with these systems as the di- or triarylmethyl radicals produced from the macroinitiator can add monomer, albeit slowly.99 100 This side reaction provides a mechanism for mopping up the excess stable radical formed as a consequence of termination between propagating radicals and may be essential to maintaining polymerization rates. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

Ring-opening metathesis polymerization (ROMP) of 1,4-cyelooctadiene was used to prepare poly(l,4-B) terminated with halo end groups.647 This was then used as a macroinitiator of ATRP with heterogeneous Cu bpy catalysts to form PS- >/ti /r-poly(l,4-B)-WoeA -PS and PMMA-Moc.T-poly(l,4-B)-Wof A-PMMA. 

Polymers prepared with the trichloromethyl-functional initiators648 or with chloroform or carbon tetrachloride as a transfer agent649 have been used as macroinitiators for ATRP. The method has been used to make PVAc-block-PS... 

Irichloromethyl-I unctional initiators, as ATRP macroinitiators 546 trichloromethyl radicals Hammett parameters... 

Polystyrene-Woc -polysulfone-/ /oc -polystyrene and poly(butyl acrylate)-Woc -polysulfone-/ /oc -poly(butyl acrylate) triblock copolymers were prepared using a macroinitiator.214 The hydroxyl-terminated polysulfone was allowed to react with 2-bromopropionyl bromide, an atomic transfer radical polymerization (ATRP) initiator, in the presence of pyridine. The modified macroinitiator could initiate die styrene polymerization under controlled conditions. 

Using these macroinitiators PDMS-polystyrene and PDMS-poly(methyl methacrylate) multiblock copolymers were synthesized 305). Due to the backbone Structure of these macroinitiators and their thermolysis mechanisms, the copolymers obtained... 

Macroinitiators 56 Macromers see Macromonomers Macromolecular monomers 157 Macromolecules, at-co difunctional 151 Macromonomers 21-23, 54-56, 155. 157... 

Scheme 8 Preparation of PEG-polypeptide diblock copolymers by macroinitiation...

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