larger range of diffusion coefficients (1 x 10 -11 – 1 x 10 -8 ) when the microreactor radius is lower than ~0.25 mm.
Iwasaki & Yoshida (2005) reported the solution free-radicalpolymerization of acrylates in a tube reactor of 500 µm of internal diameter using AIBN (azobisisobutyronitrile) as the chemical initiator. The monomers butyl acrylate (BA), methyl methacrylate (MMA), styrene (St), benzyl methacrylate (BMA) and vinyl benzoate (VBz) were polymerized at two different temperature conditions of 80 and 100 degrees C. It was concluded that the use of small reactor dimensions did not lead to significantly different results when comparing the polydispersity index obtained in standard laboratory-scale reactors for low exothermic monomers like St or VBz. However, for highly exothermic monomers like BA, the molecular weight distribution was found to be considerably narrower (PDI ~3.14) than for the batch system (PDI ~10.3). They claimed that this effect is caused directly from the high surface-to-volume ratio of the tube reactor which allows a better removal of the heat released by the polymerization. Even though the PDI of polystyrene was not significantly improved (i.e. ~1.74 for all mean residence times), the conversion achieved in the microscale reactor at 100 degrees C was considerably higher (34.3% max.) for all the flow conditions investigated. The increase in conversion was more pronounced for mean residence times lower than 10 minutes reaching 31.1% of conversion at that point compared to 18% achieved in a batch reactor. No fouling was assumed after 360 minutes of continuous operation during PS polymerization since no significant increase of pressure drop was detected. Nonetheless, this was a four stages process in which a union-tee (i.d. 800 µm) was used to bring the reactants into contact and three coiled tube sections were used: complete mixing of reactants (i.d. 250 µm x 2 m at 25C), reaction (i.d. 500 µm x 9 m at 100C) and termination (i.d. 500 µm x 1 m at 0C).
1) Chemical compounds:
4-diphenylphosphinobenzoic acid (4-DPPBA), ethyl 4-(dimethylamino)benzoate (EDB), phenyl-N-tert-butylnitrone (PBN) and N-phenylglycine (NPG) were obtained from Sigma Aldrich (Scheme 2). Bis-(4-tert-butylphenyl)iodonium hexafluorophosphate (Iod or Speedcure 938) was obtained from Lambson Ltd (UK). Trimethylolpropane triacrylate (TMPTA) was obtained from Allnex and used as benchmark acrylate monomer for freeradicalpolymerization. Bisphenol A-glycidyl methacrylate (BisGMA) and triethyleneglycol dimethacrylate (TEGDMA) were obtained from Sigma Aldrich and used with the highest purity available . For the family of flavones, all these compounds are purchased from TCI.
Moreover, this photolysis characterization would be useful to explain the distinct photoinitiating abilities of the ketone derivatives containing the same central cyclohexanone core but different peripheral substituting groups. For example, Ami-2 and Ami-6 shared the same cyclohexanone cores with Anth-2 and Anth-6, respectively, but different peripheral substituting groups: tertiary amine or anthracene. However, the photoinitiating abilities of Ami-2 and Ami-6 based three-component systems for the freeradicalpolymerization of acrylates are much better than those of Anth-2 and Anth- 6 based systems, especially in the thick films. Further photolysis characterization revealed that upon the same LED irradiation, only a very tiny percent amount of Anth- 2 and Anth-6 were consumed under the same condition with EDB or iodonium salt, as illustrated in Fig. S8.
Herein, five ketone derivatives substituted by the same peripheral 1,3-
bis(allyloxy)benzene group are elaborately designed and synthesized for the first time.
In combination with an amine and an iodonium salt, these ketones can form three- component photoinitiating systems for the freeradicalpolymerization of acrylates. The different central cyclohexanone structures would lead to distinct photoinitiation abilities, which are studied by real-time Fourier transform infrared spectroscopy. Besides, the ketone/Iod two-component system was also investigated for the cationic polymerization of epoxides. Photoreactivity of representative ketones in the presence of an amine and an iodonium salt, respectively, was systematically investigated with steady state photolysis and fluorescence characterizations. In addition, the migration of ketones from the synthesized photopolymer was investigated. Finally, the application of these ketones-based three-component photoinitiating systems for the LED projector photocuring 3D printing on acrylates is also studied.
Keywords: free-radicalpolymerization; photoinitiators; light-emitting diodes (LEDs); visible light; naphthalimide derivatives
For polymer research, the development of more eco-friendly initiating systems has received enormous attention in recent times [ 1 , 2 ]. Polymers are unavoidable materials. Synthetic or natural, their role is essential in everyday life. Because of their numerous properties, such as flexibility and chemical resistance, polymers constitute the building blocks of numerous materials. However, polymers, and in particular polymers formed by a thermal process, show a strong negative impact on Earth related to their high energy consumption for production, emission of volatile organic compounds (VOCs), and problem of after-used wastes [ 3 ]. In this context, the photopolymerization process can represent an important progress for polymer synthesis in an eco-friendlier way. Usually, formation of the polymer is induced by a thermal process, but the use of light to initiate polymerization is becoming less and less marginal. Indeed, it offers significant benefits compared to other initiation pathways such as low energy consumption, no heating needed, no or almost no VOC emission, high spatial resolution, and temporal control of the reaction [ 4 ]. Up to now, most photopolymerization processes were performed under UV (ultraviolet) light exposure. However, these types of irradiation sources lead to major concerns, notably the dangerousness of the irradiation wavelength (risk of skin cancer, skin ageing, and eye damage when wavelengths are shorter than that of visible light) and ozone release [ 5 ]. Therefore, a huge challenge today is to perform photopolymerization under milder
Fig. 8. ESR spectrum , in toluene at 52°C, for poly(tBMA) formed in water by the NaNO 2 /ascorbic acid/K 2 S 2 O 8 system. (5 mg poly(tBMA) in 0.5 ml toluene)
When a solution of the oligomers in toluene is heated at 80°C for 15 min, the hyper- fine structure is no longer observed, and the intensity of the signal is slightly de- creased (Fig. 9A). This decrease is however more important when the solution is heated for a longer period of time (Fig. 9B). Thus, at the polymerization temperature of 80°C, the amount of nitroxyl radicals decreases with time, which is in line with the loss of control observed at increasing monomer conversion. Some side reactions, such as thermal degradation of the nitroxyl radicals or β-H elimination  may explain that nitroxyl radicals disappear.
Herein, we propose new photoinitiating system (PIS) for photoinitiation under NIR light. Recently, a three-component photoinitiating system using a dye, a phosphine and an iodonium salt has been proposed for methacrylate polymerization under low light intensity at 785 nm. Through this photochemical approach, radicals able to initiate the FRP are generated by an electron transfer reaction between the excited state of the NIR dye and the iodonium salt. In parallel, a NIR absorbing dye is also used to perform the photopolymerization of a methacrylate resin, this time in combination with a thermal initiator through a photothermal mechanism. The NIR absorbing dye used in this second pathway has the property of light-to-heat conversion, particularly observed with cyanine dyes. Heat generated is sufficient to initiate a thermal polymerization. Thus, it is probably possible to merge thermal curing and photocuring and in particularly, to combine the advantages of both methods. As the light can be switched on/off on demand, the polymerization can be started and stopped whenever the user wants. On the contrary, with the conventional thermal process, preheating and cooling down are necessary and the temporal control is not possible. Moreover, the thermal polymerization is already widely used with a long history, particularly in the industrial field, therefore, the thermal initiators are easily accessible and cheap.
PVC samples synthesized by freeradicalpolymerization at
40°C in bulk (V70 as initiator) and at 80°C in suspension (confidential initiator) were considered as references. We only investigated the -CH=CH-CH2Cl structural defect, which is the most representative one for transfer to monomer and is easily detected. Comparison of the signal intensities of the polymer
3.4. Access to composites
The current composite was synthesized by impregnation of one or two layers of glass fibers with the organic resin (50% glass fibers/50% organic resin) and then cured with the UV lamp (Hg-Fe doped lamp, I = 850 mW/cm 2 ) using the UV bench conveyor. When using one layer of glass fibers, a very fast curing polymerization was observed. The sample surface and bottom are both tack-free after only one pass. As evidenced in Fig. 6, different content of TMPTA can be used to produce composites. The polymerization ability of theses hybrid systems can be confidently assigned to an improved reactivity and efficiency of the G1 based PIS, resulting from the presence of TMPTA; the optimum content of TMPTA is 10% wt. A lower performance is observed for the pure cationic polymerization of Model resin in the same experimental conditions (8 passes are required to reach a tack-free bottom of the sample). These results highlight that hybrid system, incorporating both multifunctional acrylate and multifunctional epoxide should display marked acceleration effects for the cationic polymerization. Fundamentally, this suggests that the freeradicalpolymerization of TMPTA monomer takes place rapidly and exothermically providing a heat source accelerating the frontal polymerization of the epoxide monomer.
growing radical chains by a transition metal species. [1,2] All along the polymerization, the dominating population of inactive chains is in equilibrium with a low amount of active ones, which strongly decreases the probability of irreversible termination reactions, abundant in a freeradicalpolymerization. Although some Chromium, Titanium and Vanadium complexes have been used as regulating agents,  the so-called Cobalt-Mediated RadicalPolymerization (CMRP) is the most efficient and versatile system, until now.  General mechanistic principles and synthetic progress of OMRP as well as remaining challenges will be presented in this communication.
Organometallic-Mediated RadicalPolymerization (OMRP).  The latter is based on the reversible deactivation of the growing radical chains by a transition metal species (See scheme below). All along the polymerization, the dominating population of inactive chains is in equilibrium with a low amount of active ones, which strongly decreases the probability of irreversible termination reactions, abundant in a freeradicalpolymerization. Some Titanium, Cobalt, Chromium and Vanadium complexes have been successfully used as regulating agents  and the so-called Cobalt-Mediated
The ultimate way to regulate the concentration of free radicals for highly reactive propagating radicals, however, is through the OMRP equilibrium. As discussed above, the OMRP activation relies only on the BDE of the metal-carbon bond in the organometallic dormant species. It is always possible in principle to design a metal complex, through an intelligent selection of ligands, metal centers, oxidation state, etc., leading to an organometallic dormant species with BDE[L n M-(m) x R] = H OMRP in a suitable range for any monomer, since the OMRP equilibrium is not contaminated by other coupled equilibria (at least not in the absence of halogen atoms in the coordination sphere). In this respect, it is pertinent to underline the recent success in the controlled polymerization of vinyl acetate by this technique using Co(acac) 2 as spin trap.  This is in contrast to other Co II systems (e.g. with porphyrines of Schiff bases as ligands), which irreversibly trap the propagating radicals of VAc (H OMRP too high), but control the polymerization of acrylates. Conversely, while the H OMRP (acac) 2 Co-(m) x R is just right for m = VAc, it is too small for m = MA, leading to an uncontrolled process. There is, therefore, in the opinion of this Author, greater promise for the development of new controlled radical processes of the “difficult” monomers in OMRP than in ATRP. The difficulty associated to the formation of metal-capped polymers may be surmounted by subsequent transformations, which may allow the recovery and recycling of the metal complex. For instance, this is the case for the Co(acac) 2 -capped PVAc, which may be converted to a R 2 NO-capped polymer and Co(acac) 2 by treatment with stable nitroxides. 
Design of Well-Defined N-Vinylamides Based Copolymers via Organometallic-Mediated RadicalPolymerization
A. Debuigne a , A. Kermagoret a , C.-A. Fustin b , R. Poli c , C. Jérôme a , C. Detrembleur a
a Center for Education and Research on Macromolecules (CERM), Chemistry
It has been found that a freeradical mechanism is involved in the cytotoxicity of most of a panel of 12 anti-colorectal cancer drugs tested against a panel of 9 colorectal cancer variants. The colorectal cancers Ht-29, LoVo and WiDr do not appear to involve freeradical oxidative stress processes, although such processes may be too small to be detected or are kinetically slower than cell membrane or other drug-target interactions.
was first reported for (porphyrin)Co II complexes, giving dormant chains containing Co III -
(polymer) bonds. 34 An interplay between ATRP and OMRP was previously observed and well
studied with Mo compounds 35-40 and also identified with Fe ATRP catalysts. 41 Therefore, the
potential interplay of these mechanisms is also addressed in this work. The mechanism in which a metal species establishes control over a radicalpolymerization has dramatic implications on the absolute amount of complex ultimately required to successfully mediate the polymerization (vide
Last, but not least, organic synthesis with the help of freeradical was significantly developed in the action. Such approaches open up new avenues for medicinal chemistry to increase the potential of three-dimensional biologically active entities. The Renaud group described a radical cascade reaction for the preparation of pyrroli- dine derivatives, using thiyl radical addition to a ter- minal alkyne function as the first step, which was followed by a 1,5-hydrogen transfer, translocating the radical center for the subsequent cyclization to afford the heterocyclic ring [ 126 ]. The Herrera group demon- strated that normally hard to functionalize methyl groups can be easily activated for cyclizations to pyrroli- dines by the iodine/iodobenzene diacetate reagent sys- tem [ 127 ]. Antiviral cyclopentane derivatives became easily accessible by new complex tandem reactions con- sisting of polar organometallic additions coupled with radical cyclizations and oxygenations [ 128 , 129 ] and a new approach toward the total synthesis of bridged diketopiperazine alkaloids using the persistent radical effect emerged [ 130 ]. Alternatively, the central diazabi- cyclo[2.2.2]octane core of several alkaloids can be approached by reaction sequences proceeding through single electron transfer (SET) oxidation steps, thus switching among multiple intermediate types of differ- ent oxidation state in one pot [ 131 ].
and termination. 47 In these experiments, the rates of chain initiation were held constant at known and controlled values and overall rates of reduction of alkyl halides in cyclo- hexane were reproducible. 47 Small traces of oleﬁns present in some of the alkyl halides (presumably formed by HX elimination from the halide) greatly retarded the overall reac- tion rate. Even with a tertiary alkyl halide, overall reaction rates were reduced by oleﬁn addition to a reaction already underway. For example, 0.1 M cyclohexene reduced the rate of the tert-butyl bromide (2.8 M) + tributyltin hydride (0.12 M) reaction in cyclohexane at 25 1C by 97%. 47 However, cyclo- hexene did not give well-deﬁned induction periods, meaning that it functions as a retarder rather than as an inhibitor of this chain reaction. 47 (In contrast, the stable radical, galvinoxyl, is a true inhibitor because it gave well deﬁned induction periods.) 47 In this connection, it is worth noting that when the chains are terminated by the bimolecular self-reactions of the alkyl radicals (Scheme 12), as is the case for alkyl bromides and iodides, 47 oleﬁn (R H ) will be produced. The reaction rate
The thermal activation of 2 at 80 ◦ C for 24 h (entry 1 in Table 4 ) led only to a small amount of a brown-yellowish powder. However, this material does not contain any PVDF, as shown by the absence of observable resonances in the 19 F NMR spectrum. Photochemical initiation, on the other hand, led to PVDF formation, which is only in very low yields (entries 2 and 3 of Table 4 ). This is much lower than for compound 1 under the same conditions. Similar low yields were obtained by thermally initiating the polymerization with complex 3 (entries 4–6). Photoactivation experiments were not carried out with this complex. In this case, the polymers were analyzed by SEC and found to have high molar masses, which is essentially independent of the monomer/initiator ratio and of the activation temperature (90 or 100 ◦ C), which are quite similar to those of the polymers obtained in much higher yields using 1 as the initiator. However, these polymers surprisingly show low dispersities likely because of the very small quantities of the polymer that could be injected in the SEC apparatus (the signal/noise ratios were very close to unity in these cases). The polymerizations initiated by complex 5 led to slightly greater polymer yields, but still much lower than those initiated by complex 1. Thermal activation gave only a 7% yield after 24 h (entry 7, Table 4 ), while visible light (entries 8–11) and UV light (entries 12–15) irradiation gave yields that increased with irradiation time up to 23% or 21%, respectively, after 24 h. The molar masses of these polymers, like those obtained from initiation with 1 , are high and the dispersities are moderate. Once again, this suggests the absence of a controlled chain growth of the type expected from OMRP. Since we have established that the Mn-R F bond is
EPF 2011, XII GEP Congress, 26 th June - 1 st July 2011, Granada, Spain
Precision design of novel copolymers and nanohybrids by Cobalt Mediated RadicalPolymerization (CMRP) Christophe Detrembleur, Marie Hurtgen, Yasmine Piette, Jean-Michel Thomassin, Christine Jérôme, Antoine
Confined layered systems such as clay minerals have then the potential to facilitate a rich chemistry, includ- ing the transformation of CO 2 to value-added products. In addition, we showed that muon methods provide a
powerful tool for the study of confinement effects and of reactivity under confinement. Their power lies in their ability to provide efficient and relevant probes of confinement in chemistry. These probes are parameters that are proportional to the electronic structure of free radicals on different nuclei and also to reaction rates of elementary chemical reactions at a wide range of temperatures, from ultra-low temperatures where quantum effects are man- ifested, to temperatures relevant for industrial applications. Studies such as the present one play a fundamental role in the evolution of dimensionally constrained interfaces, modern catalysis, semiconductors, electron transfer processes, quantum materials chemistry and energy technologies.