Nitroxide-mediated polymerization nitroxides used

They can be synthesized either by block copolymerization of monomers having acceptor pendant units or by synthesis of BCP precursors at first stage, followed by chemical grafting at later stage to incorporate the pendant acceptor units. For example, poly(bisphenyl-4-vinylphenylamine)-fo/ocfc-poly(perylene diimide acrylate) (PvTPA-h-PPerAcr, PI), poly(bis(4-methoxyphenyl)-4 -vinylphenylamine)-b/ocA -poly(perylene diimide acrylate) (PvDMTPA-it-PPerAcr, P2), and poly(N,N-bis(4-methoxyphenyl)-N-phenyl-N -4-vinylphenyl-(l,l -biphenyl)-4,4 -diamine)-b/ock-poly(perylene diimide acrylate) (PvDMTPD-h-PPerAcr, P3) were prepared by nitroxide-mediated polymerization (NMP) using monomer having acceptor pendant rmits [229,282,302]. 

The tendency of nitrones to react with radicals has been widely used in new synthetic routes to well-defined polymers with low polydispersity. The recent progress in controlled radical polymerization (CRP), mainly nitroxide-mediated polymerization (NMP) (695), is based on the direct transformation of nitrones to nitroxides and alkoxyamines in the polymerization medium (696, 697). In polymer chemistry, NMP has become popular as a method for preparing living polymers (698) under mild, chemoselective conditions with good control over both, the polydispersity and molecular weight. 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Various stable radicals such as nitroxide, triazolinyl, trityl, and dithiocarbamate have been used as the mediating or persistent radical (deactivator) for SFRP. Nitroxides are generally more efficient than the others. Cyclic nitroxide radicals such as 2,2,6,6-tetramethyl-l-piper-idinoxyl (TEMPO) have been extensively studied. SFRP with nitroxides is called nitroxide-mediated polymerization (NMP). Polymerization is carried out by two methods that parallel those used in ATRP [Bertin et al., 1998 Georges, 1993 Flawker, 1997 Flawker et al., 2001], One method involves the thermal decomposition of an alkoxyamine such as... 

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. 

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

Di- and triblock polymers have been prepared by nitric oxide mediated polymerization using t-butyl l-diethylphosphono-2,2-dimethylpropyl nitroxide. Materials prepared from this process are useful as paint thickeners and viscosity index improvers in paint. 

Nitroxide mediated polymerization using 1- and 2-nitroso-naphthol were used by Ma [4] to regulate the free radical polymerization of styrene. 

The controlled emulsion polymerization of styrene using nitroxide-mediated polymerization (NMP), reversible addition-fragmentation transfer polymerization (RAFT), stable free radical polymerization (SFR), and atom transfer radical polymerization (ATRP) methods is described. The chain transfer agent associated with each process was phenyl-t-butylnitrone, nitric oxide, dibenzyl trithiocarbonate, 1,1-diphenylethylene, and ethyl 2-bromo-isobutyrate, respectively. Polydispersities between 1.17 and 1.80 were observed. 

Preparation of Styrene Block Copolymers Using Nitroxide Mediated Polymerization... 

Since polystyrene is one of the oldest commercial polymers with over 9 million tonnes/yr of sales, there have been thousands of patents issued covering all aspects of its manufacture and property enhancement. The styrene monomer readily polymerizes to polystyrene either thermally or with free-radical initiators (see Chapter 6 on free-radical polymerization and Chapter 8 on nitroxide-mediated polymerization). Commercial processes for the manufacture of polystyrene are described in Chapter 3 while process modelling and optimization of styrene polymerization is examined in Chapter 5. Styrene also can be polymerized via anionic and Ziegler-Natta chemistries using organometallic initiators. Using free radical and anionic polymerization chemistries, the... 

Controlled free-radical polymerization (CFRP) has been used successfully to produce block, graft, and other controlled architecture copolymers within the last decade for a variety of free radically polymerizable monomers. The main techniques include reversible addition fragmentation and transfer (RAFT) polymerization, stable free-radical polymerization (SFRP) mediated by nitroxide/alkoxyamine based radicals, atom transfer radical polymerization (ATRP), diphenyl ethylene (DPE) mediated polymerization, and novel precipitation/emulsion polymerization based methods like free-radical retrograde precipitation polymerization (FRRPP). ... 

Nitroxide mediated SFRP, DPE mediated polymerization, ATRP, RAFT polymerization, etc. achieve polymerization control through the use of kinetic mediators or transfer agents, which protect a propagating free radical from imdesirable transfer and termination reactions. The emulsion block copolymer method is unique in that it does not require the use of any chemical mediators to achieve this control. Polymerization control is achieved by physically trapping radicals by... 

Scheme 1.37. Formation of block copolymers using nitroxide-mediated polymerization.

In 1993, Georges et al. reported on the controlled radical polymerization of St initiated by benzoyl peroxide and mediated by 2,2,6,6-tetramethyl-l-piperidinyl-oxyl (TEMPO), a stable nitroxide radical [38]. TEMPO was able to bond reversibly to the polystyryl chain end and provide polystyrenes with predetermined molecular weights and low polydispersities. Nitroxides used earlier to control radical polymerizations were less successful [37, 57]. Scheme 3 illustrates the mechanism of the St polymerization, using a generalized structure of radical initiator I-I, and details the structure of TEMPO. Although several types of nitrox-... 

In general, TEMPO-mediated polymerizations have been successfully used to prepare copolymers of St-based monomers however, attempts to incorporate other monomers have been difficult. The major reason behind this limitation is that radicals generated by the thermal self-initiation reaction of St are required to moderate the rate of polymerization by consuming the excess nitroxide produced by termination. When the ratio of St in the monomer feed is high, copolymerization with non-St based monomers is possible however, as the level of St... 

The unsuccessful attempts to copolymerize St with EPSt using a nitroxide-mediated reaction led Jones et al. to investigate ATRP as an alternative [113]. They used the CuBr/bpy catalyst system in conjunction with methyl 4-(bro-momethyljbenzoate as the initiator. The copolymers had monomodal GPC curves and narrow molecular weight distributions (Mw/Mn<1.25) when the monomer feed contained <10% of the epoxystyrene and the polymerization temperature was kept at 100 °C. This is in contrast to the results obtained from the TEMPO-mediated polymerization where all the copolymers had bimodal GPC traces and broad molecular weight distributions [113]. 

The following sections detail the literature reports pertaining to the synthesis of block copolymers using nitroxide-mediated polymerization techniques. The sections are organized according to monomer type and generally follow the historical development of the particular subsection. Most literature on nitroxide mediated preparation of block copolymers is found for the styrene-based monomers, and is summarized first. This is followed by acrylates and dienes, as they were the next monomers to be studied. These sections are followed by more recent work with vinyl pyridine, acrylamides, and maleic anhydride. The final section deals with methacrylates. This is presented last to stress the importance of developing new nitroxides that can successfully be used for the homopolymerization of methacrylate-based monomers. 

Table 4. Summary of block copolymers prepared using nitroxide-mediated polymerizations that contain one non-styryl-based block...

Fukuda et al. used both a nitroxide-mediated polymerization as well as ATRP to polymerize a sugar containing methacrylate monomer and produce water-soluble glycopolymers however, only ATRP was used to prepare block copolymers [202,203]. The ATRP of 3-0-methacryloyl-l,2 5,6-di-0-isopropylidene-D-glucofuranose (MAIpGlc, Fig.27) was carried out to determine the characteristics of the homopolymerization. It was found that the relationship between the concentration of initiator and the rate of polymerization was not a simple first-... 

Nitroxide-mediated polymerizations have recently been used to polymerize diene-based monomers [159]. ATRP [79] has so far been less successful and therefore other routes to incorporation have been utilized. One route is to functionalize a commercially available olefin based polymer. Kops et al. reacted a hy-... 

Another series of papers has focused on combining both ATRP and nitroxide-mediated polymerizations with condensation and ring-opening polymerization reactions [360-364]. Initial reports by Hawker et al. [360,365] and Jerome et al. [361] used concepts similar to those first put forth by Puts and Sogah and prepared initiators that were dual-headed and could be used for two different polymerization techniques without a transformation step. They found that identification and use of the proper conditions could allow for the simultaneous polymerizations of two different monomers by two different routes. 

Table 17 Some unimolecular initiators used in nitroxide-mediated polymerization to lead to monofunctional oligomers...

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