Alkoxyamine macroinitiators

Many mono- and poly(N-alkoxyamines) have been synthesized and investigated in NMP reactions (161,173-185). Halogen-terminated polymers have been converted into alkoxyamine macroinitiators (186). 

Delaittre, G., Nicolas, J., Lefay, C., Save, M., and Charleux, B. 2006. Aqueous suspension of amphiphilic diblock copolymer nanoparticles prepared in situ from a water-soluble polyfsodium acrylate) alkoxyamine macroinitiator. Soft Matter 2 223-31. 

Alkoxyamines containing a polymerizable group, (I), were prepared by Nesvadba et al. (2) and used to prepare macroinitiators. 

Another example of the macroinitiator approach to making block copolymers is shown in Scheme 8.4. Since methyl methacrylate (MMA) polymerization cannot effectively be initiated by TEMPO-based alkoxyamine initiators, a poly(methyl methacrylate) macroinitiator (XIII) was prepared using conventional free radical polymerization [16]. However, the azo initiator was functionalized with a TEMPO-based alkoxyamine. Since the main mechanism of termination during bulk MMA polymerization is by radical coupling, most of the MMA polymer chain-ends are functionalized with alkoxyamine groups. 

In another example, Yildirim et al. photochemically generated anthracene radical cations in the presence of TEMPO [29]. TEMPO immediately trapped the radical to form the TEMPO-anthracene cation, which was subsequently used to initiate cationic polymerization of cyclohexene oxide (CHOX). The resulting alkoxyamine-functional polycyclohexene oxide was used to macroinitiate styrene polymerization, resulting in the formation of S-6/-CHOX (Scheme 8.9). 

Reactions that convert the alkoxyamines to hy-droxylamines and alkenes can strongly limit the monomer conversion. These are either usual radical disproportionations between the nitroxide and the propagating radicals or concerted alkoxyamine decays. Both pathways lead to an exponential decrease of the concentration of the dormant chains with rate constant kdec = f)kd, where fn is the fraction of the side reaction concurring with radical coupling of alkoxyamine decay.57 kdec can be measured from the decay of the dormant alkoxyamine chains under nonscavenging conditions, and its relation with kd provides fn. From data of Fukuda et al. one can deduce fn = 0.4% for a TEMPO-polystyryl compound and fn = 1.1% for a di-tert-butylnitroxide-poly-terf-butylacrylate macroinitiator both at 120 °C.53,55 Similar small values of fn hold for TEMPO-cumyl (Scheme 10),22 TEMPO-1-phenylethyl,112 and a better mimetic compound for TEMPO-polystyryl.113 In these cases, fn probably represents the usual radical disproportionation. A much larger fn 25% holds for TEMPO—... 

Fig. 1 a Linear increase of the experimental molecular weight of PvDMTPA vs conversion (determined by 1H-NMR) for the NMRP of vDMTPA using a monomer-alkoxyamine-TIPNO ratio of 200 1 0.1. The extrapolated experimental molecular weight of 3.2kgmol 1 at conversion = 0% arises from calibration with polystyrene standards, b SEC curves of the elution volume of different well-defined PvDMTPA macroinitiators... 

Scheme 6 One-pot synthesis of P3HT-MI macroinitiators for NMRP using the McCullough method followed by in situ endcapping with the Grignard derivative of a common alkoxyamine initiator. Starting from these macroinitiators, the acceptor monomer PerAcr is polymerized to give fully functionalized, double-crystalline block copolymers P3HT-/ -PPerAcr...

Although block copolymers such as polystyrene-h-poly(butyl acrylate) (PS-Z>-PBA) can be prepared by NMP, the reaction is much more successful if the (PBA) alkoxyamine-terminated block is used to initiate the styrene polymerization than vice versa. However, a PS macroinitiator can be used to prepare well-defined diblocks with isoprene as the second monomer, i.e., (PS-h-PI), using NMP techniques and 2,2,5 trimethyl-3-(l-phenyl ethoxy)-4-phenyl-3-azahexane (TMPAH) as the nitrox-ide mediator. 

Initial studies utilized a poly(tert-butyl acrylate)-SGl macroinitiator for the chain extension of isoprene, but no kinetic data was reported (Matsuoka et ah, 2009). More recently, Nicholas and coworkers reported the polymerization of isoprene with a variety of SGI-based alkoxyamine initiators, and investigated the effect of temperature, alkoxyamine concentration, and structure on the polymerization kinetics (Harrisson et ah, 2011). Optimal control over the polymerization was observed at 115°C where 40% conversion was attained after 16h,butonlymodestmolecular weights were observed (M 8350).Thebest control over the polymerization was observed with SGI-based alkoxyamines which contain secondary and tertiary nonacid groups. 

For CRP based on a reversible termination process, the strategy was to synthesize water-soluble macroinitiators (with either an alkoxyamine or an alkyl halide chain-end) by direct CRP or by post-functionalization, before using them as both initiators and stabilizers in emulsion polymerization of a hydrophobic monomer. 

On the other hand, ABA triblock of PSt-b-PEO-Z7-PSt was also synthesized by a modified synthetic pathway. In this case, PEO with hydroxyl groups at both ends was functionalized with N-tert-butyl-N-( 1 -diethylphosphono-2,2-dimethylpro-pyl)nitroxide-based alkoxyamine (AMA-SGl). Functionalized PEO can behave as a difimctional macroinitiator. Indeed, upon heating in the presence of St, the macroinitiator produced the desired block copolymer (Scheme 42). 

Cheng, C., and Yang, N.-L. (2001). Advances of well-defined alkoxyamine-based poly functional macroinitiators for structural control of graft polymers. Polym. Mater. Sci. Eng., 85 512-513. 

As recently demonstrated by Grubbs et al. [30] and by Yin et al. [31], amphiphilic block copolymers could be prepared by NMP from alkoxyamine functional macroinitiators such as ... 

In the second method, the alkoxyamine-ftmctionalized backbone is prepared by a chemical modification of a preformed polymer. Abbasian and Entezami prepared alkoxyamine-functionalized poly(vinyl chloride) (PVC) in a three-step procedure. PVC was first arylated with toluene by Friedel-Crafts acylation followed by a bromination step using N-bromosuccinimide. The bromine atom was finally reacted via nucleophilic substitution by the TEMPO hydro-xylamine anion. PVC-g-PS was finally obtained after TEMPO-mediated polymerization of styrene. A TEMPO-functionalized isotactic poly(l-butene) macroinitiator was synthesized by Jo et al. who used a rhodium-catalyzed activation of the alkane C-H bonds and subsequent transformations of the boronate ester group into an hydroxyl pendant group. This reactive moiety was then used to attach a TEMPO-based alkoxyamine bearing another hydroxy function by an ether linkage. A method to prepare PE-g-PS from a poly(ethylene-co-m,p--methylstyrene) obtained by metallocene-catalyzed polymerization was also reported. The macroalkoxya-mine was synthesized after bromination with N-bromosuccinimide followed by a nucleophilic reaction with the TEMPO hydroxylamine anion. 

Eventually, alkyl radicals can be randomly produced along the polymer chain after its exposure to irradiations (electron beam or °Co y-ray irradiation). This was demonstrated with a N-phthaloylchitosan (the soluble intermediate for the modification of chitosan) followed by the grafting of either S or Ss s 9.59o stmcture and/or the location of the aminoxyl function was, however, unclear. Holmberg et irradiated poly(vinylidene fluoride) (PVDF) in the presence of TEMPO. The macroinitiator was then used to graft styrene or a styrene/ N-phenylmaleimide mixture onto the PVDF membranes. Such a result was surprising since the alkoxyamine formed should be very stable and not prone to decomposition. 

For nitroxide-mediated radical polymerizations and in the RAFT process, the same synthetic strategy as for ATRP can be used in the synthesis of AB and ABA block copolymers. The first step is coupling a functionalized alkoxyamine with a telechelic or monofunctional nonvinylic polymer to give a macroinitiator. This macroinitiator can be used in standard controlled free-radical polymerization procedures. This approach is best illustrated by the preparation of PEO-based block copolymers [81-84]. One example is the preparation of macroinitiator LMI-7 by the reaction of a monohydroxy-terminated PEO with sodium hydride followed by reaction with the chloromethyl-substituted alkoxy amine as shown in Scheme 3.16. 

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