Redox Initiation

A CASE OF POSSIBLE TERMINATION BY PRIMARY RADICALS IN HOMOGENEOUS POLYMERIZATION Earlier (1) we identified the radical. +NH s-c 'NH generated from the reaction between thiourea (TU) and a suitable oxidant by trapping them a s polymer endgroups. Kinetic studies of the Fe(ClO,),-TU initiated polymerization of methyl methacrylate in tbutyl alcohol (TBA) at 4OoC. have revealed valuable information about the reactivity of the radical toward addition to the 77 bond of the monomer and its role in the polymerization reaction aside from initiation. The following results are noteworthy. (1) The polymerization is inhibited by the stable free radical 2,2diphenyl-1-picryl-hydrazyl showing the free-radical nature of the reaction. component concentrations (range lO-'M to 10-3M) while the reaction Fe(II1)-TU is kinetically of second order.

Protection of acetylenic monomers is a common practice to avoid parasitic side reactions during polymerization. Herein, we report that redox‐initiated RAFT polymerization allows the direct, room temperature synthesis of a variety of single‐chain nanoparticle precursors (displaying narrow molecular weight dispersity, $\overline{M}_{\rm w}$/$\overline{M}_{\rm n}$ = 1.12 –1.37 up to $\overline{M}_{\rm w}$ = 100 kDa) containing well‐defined amounts of naked, unprotected acetylenic functional groups available for rapid and quantitative intrachain cross‐linking via metal‐catalyzed carbon–carbon coupling (i.e., C–C “click” chemistry). To illustrate the useful “self‐clickable” character of the new unprotected acetylenic precursors, single‐chain nanoparticles have been prepared for the first time in a facile and highly efficient manner by copper‐catalyzed alkyne homocoupling (i.e., Glaser–Hay coupling) at room temperature under normal air atmosphere.

Low molar mass poly(acrylic acid) (PAA) is generally obtained by free radical polymerization of acrylic acid (AA) in aqueous solution, using thermal initiators and some chain transfer agent. However, under such conditions it is rather difficult to efficiently produce molar masses as low as those required for obtaining an effective dispersant. In this work, the semibatch polymerization of AA at 45 °C is considered, using potassium persulfate (KPS) and sodium metabisulfite (KPS/NaMBS), or alternatively KPS and sodium hypophosphite (KPS/NaHP) as redox initiators to produce PAA of controlled low molar masses. These initiation systems allow the production of PAA with M n as low as 2.0 kDa, relatively narrow molar mass distribution (1.5 < M w /M n < 3.0), and low branching degree. Most of the investigated polymerizations reach almost complete conversions (>95%); and it is verified that both reductants, NaMBS and NaHP, also behave as chain transfer agents. Finally, the investigated process with redox couples allowed the production of PAA with acceptable dispersant and antiscaling properties.

Protection of acetylenic monomers is a common practice to avoid parasitic side reactions during polymerization. Herein we report that redox initiated RAFT polymerization allows the direct, room temperature synthesis of a variety of single-chain nanoparticle precursors (displaying narrow molecular weight dispersity, M w / M n = 1.12-1.37 up to M w = 100 kDa) containing well-defined amounts of naked, unprotected acetylenic functional groups available for rapid and quantitative intrachain crosslinking via metal-catalyzed carbon-carbon coupling (i.e. CC "click" chemistry). To illustrate the useful "self-clickable" character of the new unprotected acetylenic precursors, single-chain nanoparticles have been prepared for the first time in a facile and highly-efficient manner by copper-catalyzed alkyne homocoupling (i.e. Glaser-Hay coupling) at room temperature under normal air atmosphere.

Protection of acetylenic monomers is a common practice to avoid parasitic side reactions during polymerization. Herein we report that redox initiated RAFT polymerization allows the direct, room temperature synthesis of a variety of single-chain nanoparticle precursors (displaying narrow molecular weight dispersity, M w / M n = 1.12-1.37 up to M w = 100 kDa) containing well-defined amounts of naked, unprotected acetylenic functional groups available for rapid and quantitative intrachain crosslinking via metal-catalyzed carbon-carbon coupling (i.e. CC "click" chemistry). To illustrate the useful "self-clickable" character of the new unprotected acetylenic precursors, single-chain nanoparticles have been prepared for the first time in a facile and highly-efficient manner by copper-catalyzed alkyne homocoupling (i.e. Glaser-Hay coupling) at room temperature under normal air atmosphere.

A CASE OF POSSIBLE TERMINATION BY PRIMARY RADICALS IN HOMOGENEOUS POLYMERIZATION Earlier (1) we identified the radical. +NH s-c 'NH generated from the reaction between thiourea (TU) and a suitable oxidant by trapping them a s polymer endgroups. Kinetic studies of the Fe(ClO,),-TU initiated polymerization of methyl methacrylate in tbutyl alcohol (TBA) at 4OoC. have revealed valuable information about the reactivity of the radical toward addition to the 77 bond of the monomer and its role in the polymerization reaction aside from initiation. The following results are noteworthy. (1) The polymerization is inhibited by the stable free radical 2,2diphenyl-1-picryl-hydrazyl showing the free-radical nature of the reaction. component concentrations (range lO-'M to 10-3M) while the reaction Fe(II1)-TU is kinetically of second order.

Cannabis indica fibre has been graft copolymerized with methyl acrylate using ceric ammonium nitrate as initiator. The effect of monomer, initiator and HNO) concentrations, amount of water, reaction temperature and reaction time on per cent grafting has been studied. The swelling behaviour in DMF, n-butanol and water follows the pattern DMF > H 2 0> nbutanol for the grafted fibres and H 2 0> DMF > n-butanol for the ungrafted fibre. Both the ungrafted and grafted fibres degrade thermally in multiple and single stage respectively. Grafting has not been found to improve the thermal stability of the fibre. A plausible explanatio n to account for the trends in results has been provided.

Low temperature grafting of methyl methacrylate (MMA) on to coir fibre was carried out in aqueous medium using Potassium per sulphate (PPS) as an initiator under the catalytic influence of Ferrous ammonium sulphate (FAS). Optimization of various parameters of grafting viz. monomer, initiator and catalyst concentration, time and temperature was carried out to obtain the maximum tensile properties. Evidence of grafting was characterized from Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Thermal analysis (TGA). The maximum breaking stress (BS) of control and grafted coir fibre were 213.08 and 365.00 N/mm2 respectively. Hence the percentage of improvement of grafted coir fibre was found to be 71.30%. Increase in tensile properties with maximum BS observed under monomer (25%), initiator (0.75%) and catalyst (0.75%) concentration, time (150min) and temperature (500C) respectively.  The t-test and Analysis of variance (ANOVA) were studied for statis...

A study has been made of the grafting of acrylonitrile onto a bleached kraft pulp by application of the xanthate method developed by Faessinger and Conte. Acrylonitrile was grafted to the cellulose in aqueous suspension at 25W, with cellulose xanthates of gamma numbers from 3 to 13. The reaction system contained 1-2 percent xanthogenated pulp, 0.24.9 mole/l. hydrogen peroxide, 0.45-1 .O mole/l. acrylonitrile, and the reaction time was 45 min. In some cases 0.660 mmole/l. sulfuric acid was added to the reaction system. The grafting yield was calculated from the nitrogen content of the grafts. The degree of polymerization (DP) of the grafted polyacrylonitrile, after hydrolytic removal of the cellulose, was calculated from the limiting viscosity number of dimethylformamide solutions of the polymer at 25"C, and use of the formula [q] = 2.43 x (M)0.75. The grafts were found to contain 10-35 percent polymer of DP = 350-1000, and an average number of 1-4 grafted polymer chains per cellulose chain, i.e., less than 5 percent of the number of xanthate groups initially present in the cellulose xanthate. The number of grafted polymer chains was dependent upon the pH, and the DP of the grafted polymer was dependent upon the monomer and xanthate concentrations; the higher the monomer concentration was, and the lower the xanthate concentration was, the higher it became. Possible mechanisms for grafting are discussed.

The grafting of methacrylic acid (MAA) and other vinyl monomers onto cotton cellulose in fabric form was investigated in an aqueous medium with a potassium peroxydiphosphate–metal ion– cellulose thiocarbonate redox initiation system. The graft copolymerization reaction was influenced by peroxydiphosphate (PP) concentration, the pH of the reaction medium, monomer concentration, the duration and temperature of polymerization, the nature of vinyl monomers, and the nature and concentration of metallic ions (activators). On the basis of a detailed investigation of these factors, the optimal conditions for the grafting of MAA onto cotton fabric with the said redox system were as follows: [Fe2+]= 0.1 mmol/L, [PP] = 2 mmol/L, [MAA] = 4%, pH=2, grafting time = 2 h, grafting temperature= 70°C, and material/liquor ratio = 1 : 50. Under these optimal conditions, the graft yields of different monomers were in the following sequence: MAA > acrylonitrile > acrylic acid > methyl acrylate >methyl methacrylate. The unmodified cellulosic fabric (the control) had no ability to be grafted with MAA with the PP–Fe2+ redox system. The percentage of grafting onto the thiocarbonated cellulosic fabric was more greatly enhanced in the presence of iron salts than in their absence. This held true when the lowest concentrations of these salts were used separately. A suitable mechanism for the grafting processes is suggested, in accordance with the experimental results.

The cellulose thiocarbonate, in the fabric form, was treated first with a freshly prepared ferrous ammonium sulphate (FAS) solution. The so-treated fabric formed, with N-bromosuccinimide (NBS), an effective redox system capable of initiating grafting of methyl methacrylate (MMA) and other vinyl monomers onto the cotton fabric. The effect of the polymerization conditions on the polymer criteria, namely, graft yield, homopolymer, total conversion, and grafting efficiency, was studied. These polymer criteria were found to depend extensively upon concentrations of the Fe2+ ion (activator), NBS (initiator), and MMA; pH of the polymerization medium, and duration and temperature of polymerization. Based on detailed investigation of these factors, the optimal conditions for grafting were as follows: Fe2+, 1x 10-3 mol/L; NBS, 1 x 10-2  mol/L; MMA, 4%; pH, 2; polymerization time, 150 min; polymerization temperature, 60oC; material/liquor ratio, 1 : 100. Under these optimal conditions, the rates of grafting of different vinyl monomers were in the following sequence: methyl methacrylate % methyl acrylate > acrylonitrile. Other vinyl monomers, namely, acrylic acid, and methacrylic acid have no ability to be grafted to the cellulosic fabric using the said redox system. A tentative mechanism for the polymerization reaction is suggested.

The cellulose thiocarbonate-ferric nitrate redox system was investigated as the initiator for the graft copolymerization of methyl methacrylate (MMA) and other vinyl monomers on to cotton fabric. Other monomer were methacrylic acid (MAA), acrylonitrile (AN), methyl acrylate (MA), acrylamide (Aam), ethyl acrylate (EA), and allyl acrylate (llyl-A). A number of variables in the grafting reaction were investigated, including ferric nitrate and monomer concentrations, pH of the polymerization medium, duration and temperature of polymerization, and composition of the water/solvent polymerization medium, solvents used being methanol, ethanol, isopropanol, tertiary butanol, and acetone. The grafting reaction was studied with respect to various polymer criteria, namely, graft yield, homopolymer formation, total conversion, and grafting efficiency. The optimal conditions for low homopolymer formation and high grafting efficiency were [Fe(NO3)3.9H2O], 6M mol/litre; MA, 2%; Ph, 2; polymerization time, 2 h; polymerization temperature, 60oC ; material/liquor ratio 1:50. The rates of grafting under these optimal conditions were in the order: methyl methacrylate>methyl acrylate> ethyl acrylate> ally acrylate> acrylamide> acrylonitrile> methacrylic acid. A tentative mechanism for the grafting reaction is proposed.

Polymerization of acrylic acid (AA) with native and hydrolyzed maize starches was carried out via a potassium bromate-thiourea dioxide redox initiation system. The factors affecting the efficiency of the redox system and, in turn, the polymerization process were studied. These factors included the concentrations of AA, KBrO3, thiourea dioxide (TUD) (and equimolar ratio of the latter two), polymerization temperature and the extent of starch hydrolysis. The behaviour of the apparent viscosity of the cooked poly(AA)-starch composite paste, obtained under different polymerization conditions, was also studied. The polymerization reaction was monitored via the determination of the total percentage conversion (% TC) of AA. The poly(AA)-starch composite was evaluated by calculating the polymer yield, namely the graft yield (% GY), the grafting efficiency (% GE), the percentage homopolymer (% HP) and the total conversion. Results obtained indicated that the optimum conditions, expressed as percent of total conversion are: (a) with native starch (NS): (KBrO3), 6 mmol/l00 g NS; (TUD), 7.4 mmol/lOO g NS; (AA), 30% ofweight of starch; polymerizationtemperature, 50°C; and polymerization time 30 min. (b) With hydrolyzed starch (HS): (KBrO3), 4 mmol/l00 g HS; (TUD), 4 mmol/l00 g HS; (AA), 30%; polymerization temperature, 40°C; and polymerization time, 30 min. The results also indicated that, for a given rate of shear, while the apparent paste viscosity of the poly(AA)-NS decreases with increasing concentration of KBrO3, TUD, and AA (within the studied range), it increases with increasing polymerization temperature from 30 to 50°C. Regardless of the polymerization conditions used, the apparent viscosity of NS is higher than that of the poly(AA)-NS composite. On the other hand, the apparent viscosity of the poly(AA)-HS (hydrolyzed starch) composite dependsupon the extent of hydrolysis of the starch used as well as the polymerization conditions.