Functional initiators nitroxides

Functional homopolymers can be synthesized by essentially two different methods. The first and more preferred way is to use a functional initiator which will ensure a high rate of chain end functionality. For instance, the polymerization of St initiated by a unimolecular terpyridine-functionalized nitroxide initiator yields well-defined PS homopolymers. The second technique is based on post-polymerization modifications. In this case, the reaction between mPEG and chloroterpyridine yields terpyridine-functionalized PEG building blocks, as illustrated in Scheme 13. 

Schubert and co-workers have reported the preparation of terpyridine (terpy) end-group functionalized polystyrene (PS) and polyethylene oxide (PEO) from a nitroxide-mediated terpy functionalized initiator and OH-terminated PEO transformation, respectively (Scheme 3.1,... 

One of the limitations of anionic polymerization with respect to preparation of block copolymers is the rather limited range of monomers that can be polymerized anionically to form polymers with well-defined stmctures. One solution to this problem is to utilize anionic polymerization to form a well-defined polymer that is functionalized with an end group that can be used to initiate polymerization via another polymerization method, for example, controlled free-radical polymerization. One such functional group is the aminoxy group which can be used to initiate nitroxide-mediated radical polymerization (NMP). °° PSLi has been reacted with 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (MTEMPO), a stable nitroxide free radical, in THF at -78 °C as shown in eqn [30]. The mechanism of this functionalization was presumed to occur... 

NMP is based on the concept of a dynamic equilibration between dormant alkoxyamines and propagating radicals as shown in eqn [55].The choice of the persistent radical is cmcial for controlled polymerization. While styrene can be easily moderated by 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO), other monomers required the development of nitroxides that contain hydrogen atoms at the a-C. There are two different initiation methods for NMP. Conventional radical initiators (i.e., AIBN, BPO) in conjunction with a persistent radical were initially used to prepare polymers by NMP, but these systems were limited in the choice of monomer. Functionality could be incorporated via a functionalized initiator or a functionalized persistent radical. For example, Baumert and Mulhaupt prepared carboxylic acid-terminated polystyrene, poly(styrene-co-acrylonitrile), and polystyrene-b-poly (styrene-co-acrylonitrile) by the use of the functionalized initiator 4,4 -azobis(4-cyanopentanecarboxylic acid). The polymerization was controlled by the addition of 2,2,6,6-tetramethyl-l-piperidyloxyl radical, and polymers with... 

SI-NMP can also be carried out using an initiator/nitroxide bimolecular system [22]. In this strategy, an azo-based initiator functionalized with chlorosilyl groups was first attached to the sihcon wafer after immersion of the substrates in a toluene solution containing the azo derivatives. The formation of PBA brushes with thickness of 4—14 nm using two different azo initiators (AMCl and ATCl) was performed... 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

In chloroprene containing 0.05M azobisisobutyronitrile and 0.02M 2,2,6,6-tetramethyl-4-piperidone-l-oxyl an induction period of 22 minutes was observed, followed by retarded oxidation. In the absence of the initiator 110 p.p.m. of N,N-dimethyl-4-nitrosoaniline inhibited oxidation for 1 hour. Nitroxide radicals and their nitroso precursors (17) do not function as peroxy radical traps since they cause no inhibition and little retardation of the initiated oxidation of cumene at 60°C. 

Fig. 10.6 Free nitroxide concentration as a function of time during P—T initiated bulk polymerisation of...

For styrene-based random copolymers, functional groups can be introduced into the polymer chains via copolymerization with functional styrene derivatives, because the electronic effects of the substituents are small in the metal-catalyzed polymerizations in comparison to the ionic counterparts. Random copolymer R-6 is of this category, synthesized from styrene and />acetoxystyrene.372 It can be transformed into styrene// -vinylphenol copolymers by hydrolysis.380 The benzyl acetate and the benzyl ether groups randomly distributed in R-7 and R-8 were transformed into benzyl bromide, which can initiate the controlled radical polymerizations of styrene in the presence of copper catalysts to give graft copolymers.209 Epoxy groups can be introduced, as in R-9, by the copper-catalyzed copolymerizations without loss of epoxy functions, while the nitroxide-mediated systems suffer from side reactions due to the high-temperature reaction.317... 

Becker et al. [64] functionalized a peptide, based on the protein transduction domain of the HIV protein TAT-1, with an NMP initiator while on the resin. They then used this to polymerize f-butyl acrylate, followed by methyl acrylate, to create a peptide-functionahzed block copolymer. Traditional characterization of this triblock copolymer by gel permeation chromatography and MALDI-TOF mass spectroscopy was, however, comphcated partly due to solubility problems. Therefore, characterization of this block copolymer was mainly hmited to ll and F NMR and no conclusive evidence on molecular weight distribution and homopolymer contaminants was obtained. Difficulties in control over polymer properties are to be expected, since polymerization off a microgel particle leads to a high concentration of reactive chains and a diffusion-limited access of the deactivator species. The traditional level of control of nitroxide-mediated radical polymerization, or any other type of controlled radical polymerization, will therefore not be straightforward to achieve. 

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