Nitroxide mediated radical polymerization NMRP

Nitroxide mediated radical polymerization (NMRP) was pioneered by Riz-zardo and Solomon in the mid-1980s [1]. Their work went unnoticed for almost a decade until Georges et al. reported the preparation of narrow polydispersity (PD) (<1.2) polystyrene using NMRP [2]. This report initiated an explosion of research aimed at both understanding the mechanism of NMRP and also utilizing it to prepare block copolymers. This chapter describes the application and limitations of NMRP for making styrene-containing block copolymers. 

The first step for the core-first stars is the synthesis of multifunctional initiators. Since it is difficult to prepare initiators that tolerate the conditions of ionic polymerization, mostly the initiators are designed for controlled radical polymerization. Calixarenes [39, 58-61], sugars (glucose, saccharose, or cyclodextrins) [62-68], and silsesquioxane NPs [28, 69] have been employed as cores for various star polymers. For the growth of the arms, mostly controlled radical polymerizations were used. There are only very rare cases of stars made from nitroxide-mediated radical polymerization (NMRP) [70] or reversible addition-fragmentation chain transfer (RAFT) techniques [71,72], In the RAFT technique one has to differentiate between approaches where the chain transfer agent is attached by its R- or Z-function. ATRP is the most frequently used technique to build various star polymers [27, 28],... 

There are several techniques for performing CRP, but the most popular and successful ones so far are as follows stable free radical (SFR) or nitroxide-mediated radical polymerization (NMRP) [44, 45, 49], atom transfer radical polymerization (ATRP) [50, 51], and degenerative transfer techniques, including particularly reversible addition-fragmentation transfer (RAFT) polymerization [3]. These are examined in some detail in the following sections. 

Stable Free Radical Polymerization (SFRP) or Nitroxide-Mediated Radical Polymerization (NMRP)... 

In the field of living radical polymerization, MALDI-TOF has been highly useful for characterization of polymers prepared by nitroxide-mediated radical polymerization (NMRP) [35, 36], atom transfer radical polymerization (ATRP) [37], and reversible addition-fragmentation chain transfer polymerization (RAFT) [38, 39]. The modern MALDI-TOF-MS permits fast and accurate determination of a variety of polymer characteristics [40]. 

Nitroxide-mediated Radical Polymerization (NMRP) Approach to Polymer-grafted CNTs... 

Stable free radical polymerization (SFRP) or nitroxide-mediated radical polymerization (NMRP or NMP)... 

The DA chck reaction, a [4 + 2] cycloaddition system based on the coupHng of a diene and a dienophile, represents an alternative strategy in chck chemistry [26-28]. This reaction has been applied successfully to the preparation of block copolymers with PEO and PSt or PMMA segments, prepared using AROP and ATRP or nitroxide-mediated radical polymerization (NMRP) [26-28]. Thus, PEO-b-PSt and PEO-h-PMMA block copolymers were synthesized via DA chck reaction of maleimide- and anthracene end-functionahzed polymers. The overall strategy is represented in Scheme 11.3, as the preparation of PEO-h-PSt... 

Figm 7 Synthesis of mixed brushes by sequential ATRP and nitroxide-mediated radical polymerization (NMRP) using Y-shaped initiators that have functionalities for both polymerization techniques. Reprinted with permission from Zhao, B. Zhu, L. Macromolecules 2009,42.9369. Copyright 2009 American Chemistry Society. 

Currently, most of the LFRP research is focused on the use of nitroxides as the stable freeradical. The main problems associated with nitroxide mediated radical polymerizations (NMRP) are slow polymerization rate and the inabihty to make high molecular weight narrow polydispersity PS. This inabihty is hkely due to side reactions of the living end leading to termination rather than propagation (243). The polymerization rate can be accelerated by the addition of acids or anhydrides to the process (244). The mechanism of the accelerative effect of the acid is not certain. Another problem with nitroxides is that they work well for vinylaromatic monomers, but not for acrylate and diene monomers. This has... 

Fig. 46. In situ synthesis of alkoxyamine initiator during nitroxide mediated radical polymerization (NMRP) of styrene.

Braslau and coworkers [83] synthesized cyclic PSTY through the combination of nitroxide-mediated radical polymerization (NMRP) and CuAAC click reaction. The synthesis procedure was relatively complex compared with other strategies. 1 - [4-(Chloromethyl)phenyl] ethyl alkoxyamine was used to mediate the styrene polymerization, followed by successive azidation and oxidative cleavage with ammonium cerium(IY) nitrite in the presence of propargyl alcohol. The azide and alkyne groups were then introduced to each end of the polymer. Finally, the cyclization reaction was carried out in toluene with CuBr and PMDETA as catalyst at 100°C (Scheme 35). The cyclization results showed about 64% click product, as derived from Gaussian curve fitting. 

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