Polymerization stable free radical

As living polymerization implies that during the process, side reactions such as irreversible termination and transfer reactions are virtually absent, in free radical polymerization it can be achieved by a reversible termination by reaction between the active center and another radical, using photoactivation (Otsu and Kuriyama, 1984) or thermal activation (Bledzki et al., 1983 Crivello et al., 1986 Otsu et al., 1987). An example of the latter is provided by stable nitroxyl radicals like TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) (I). 

in stable free radical polymerization (SFRP), also called nitroxide-mediated polymerization or NMP (which was discovered while using TEMPO as a radical scavenger in investigating the rate of initiation during free radical polymerization), it is believed that reversible combination of a polymer radical, P, with a stable niUoxyl radical, N, takes place forming an adduct, P-N, that exists as a dormant species  

The equilibrinm between dormant chains (P-N) and active chains (P ) is designed and the temperature is adjusted so as to heavily favor the dormant state, which effectively reduces the radical (P ) concentration to suf ciently low levels that allow controlled polymerization. For example, the equilibrium constant K in Eq. (11.11) for the polystyrene (PSt)/TEMPO reversible reaction in the bulk polymerization of styrene at 125°C in the presence of a PS-TEMPO adduct 

The key to success in synthesizing polymers with narrow polydispersity and well-de ned chain end structure by carrying out free-radical polymerization in the presence of nitroxide SFRs such as TEMPO, is the essentially simultaneous initiation and reversible termination of the polymer radical with the SFR (Georges et al., 1994). However, the dissociation such as depicted in Fig. 11.4 for the polystyrene (PSt)-TEMPO adduct is known to occur in a limited number of systems (at high temperatures). A versatile use of the simple TEMPO-based SFRP is therefore not possible. For example, attempts to perform SFRP of monomers such as acrylonitrile (AN), methyl and ethyl acrylates (MA and EA), and 9-vinylcarbazole (VCz) with benzoyl peroxide (BPO) and TEMPO have not been successful. Interestingly, however, styrene has been successfully copolymerized (see Section 11.2.4) with these monomers using BPO initiator and TEiMPO under a living fashion (Fukuda etal., 1996). 

The features of each are outlined briefly below interested readers can find extensive discussions of the features and kinetics of each system in reference 2. 

Solomon et al. [75] first used nitroxyl radicals and alkoxyamines in a radical polymerization, but their work was limited to production of low-molecular weight polymers. In 1993, Georges et al. [76] used a mixture of benzoyl peroxide (BPO) initiator and 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) to produce low-polydispersity and high molecular weight polystyrene. Since then many papers about SFRP (also known as nitroxide mediated polymerization or NMP), mainly focused on styrene polymerization in the presence of TEMPO, have been published. Other nitroxide mediators are being developed that are better suited to polymerization of more polar monomers such as meth(acrylates) [77]. 

The basic mechanism of SFRP is the alternating activation-deactivation process between large amounts of dormant species and small amounts of propagating radicals  

Dormant species Pn—X form propagating radicals P through the carbon-oxygen bond cleavage typical values for activation rate coefficient are 10 -10 s . In the active 

The equilibrium of Equation 3.75 dictates that the concentration of active free radicals in the system, [Ptot] remains low. For the system to remain living, the reversible deactivation reaction with nitroxide must be dominant compared to irreversible radical-radical termination (Equation 3.5). However, as it is impossible to totally eliminate the loss of radicals through termination, an imbalance between [X] and [Ptot] arises  

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Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

Stable free radical polymerization (SFRP), 20 442, 443 Stable node(s)... 

The first workable capping agents for controlled radical polymerization were discovered by Rizzardo et al. [77, 78] who used nitroxides. The nitroxide reacts reversibly with radical chain ends but itself does not initiate the monomer. They called their new system Stable Free Radical Polymerization (SFRP). Scheme 32a depicts an example of SFRP using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy). SFRP was developed independently by Georges at Xerox for the synthesis of styrene block polymer as dispersing agents [79]. 

These methods are based on the idea of establishing equilibrium between the active and dormant species in solution phase. In particular, the methods include three major techniques called stable free-radical polymerization (SFRP), atom transfer radical polymerization (ATRP), and the degenerative chain transfer technique (DCTT) [17]. Although such syntheses pose significant technical problems, these difficulties have all been successively overcome in the last few years. Nevertheless, the procedure of preparation of the resulting copolymers remains somewhat complicated. 

In a stable free-radical polymerization (SFRP), the initiated polymer chains are reversibly capped by a stable radical, for example, the 2,2,6,6-tetra-methylpyridin-l-oxyl radical (TEMPO). Stable PS dispersions via miniemulsion polymerization were prepared by MacLeod et al. with an optimized ratio... 

Title Copolymers of Maleic Anhydride by Stable Free Radical Polymerization... 

A stable free radical polymerization using 2,2,6,6-tetramethyl-l-piperidinyloxy with maleic anhydride and styrene was used to prepared moderate molecular weight copolymers with polydispersities less than 1.5. Thermal re-activation of these copolymers in the presence of other monomers produced block polymers. 

Bifunctional stable free radical polymerization reactions were prepared by Georges [4] using divinylbenzene. 

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. 

Controlled Polymerization of Styrene Using 1,1-Diphenylethylene as Controlling Agent [Stable Free Radical Polymerization SFR]... 

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). ... 

Fig. 2. The mechanisms of (1) stable free radical polymerizations, (2) reversible redox polymerizations (i.e., ATRP), and (3) degenerative chain transfer...

Stable Free Radical Polymerization and Nitroxide Mediated Polymerization (SFRPandNMP)... 

Other teams worked on the functionalization of the aminoxyl group situated at the co position. For instance, the method of Ding et al. [342] is original for the synthesis of a novel series of poly(sodium styrenesulfonate) (PSSNa) macromonomers (compound 3 in Scheme 74) based on stable free radical polymerization in the presence of TEMPO. 

Matyjaszewski et al. systematically investigated the effect of electron donors (ED), such as pyridine and triethylamine, on the CRP of VAc with Co(acac)2. They proposed that the polymerization mechanism of VAc with Co(acac)2 in the absence of electron donor was a degenerative transfer process as shown in scheme 3(a). The polymerization in the presence of electron donor was a stable free radical polymerization controlled by the reversible homolytic cleavage of cobalt(III) dormant species as shown in scheme 3 (b). ... 

Random copolymers of styrene/isoprene and styrene/acrylonitrile were prepared by the stable free radical polymerization process. The molecular weight of the polymers increased as a function of conversion, as expected for a living radical polymerization. The microstructure of the copolymers and reactivity ratios of the monomers were found to be very similar to what would be obtained for a conventional free radical polymerization. The propagating living radical chain reacts similarly to a conventionally propagating chain. 

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