Chloroprene, oxidation

Bailey also found that more hydroperoxides were produced from the octene than form polychloroprene and suggested that chloroprene oxidation proceeded via peroxide formation involving carbon atoms located on non-adjacent double bonds... 

Uninhibited chloroprene suitable for polymerisation must be stored at low temperature (<10° C) under nitrogen if quaUty is to be maintained. Otherwise, dimers or oxidation products are formed and polymerisation activity is unpredictable. Insoluble, autocatalytic "popcorn" polymer can also be formed at ambient or higher temperature without adequate inhibition. For longer term storage, inhibition is required. Phenothiasine [92-84-2] / fZ-butylcatechol [2743-78-17, picric acid [88-89-17, and the ammonium salt of /V-nitroso-/V-pheny1hydroxy1 amine [135-20-6] have been recommended. 

During World War II, polychloroprene was chosen as a replacement for natural rubber because of its availability. Two copolymers of chloroprene and sulphur which contain thiuram disulphide were available (Neoprene GN and CG). One of the first successful applications of these polychloroprene adhesives was for temporary and permanent sole attachment in the shoe industry. However, these polychloroprene cements show a decrease in viscosity on ageing and a black discolouration appears during storage in steel drums. Discolouration was produced by trace amounts of hydrochloric acid produced by oxidation of polychloroprene... 

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

FIGURE 4.8 Comparative tensile stress-strain plot of polychloroprene-ordinary zinc oxide (ZnO) and poly-chloroprene-nano-ZnO system. (From Sahoo, S., Kar, S., Ganguly, A., Maiti, M., and Bhowmick, A.K., Polym. Polym. Compos., 2007 (in press). Courtesy of Smithers Rapra Technology Ltd.)... 

Chlorobutadiene or chloroprene rubbers (CRs), also called neoprene rubbers, are usually vulcanized by the action of metal oxides. The cross-linking agent is usually zinc oxide in combination with magnesium oxide [27]. CR can be vulcanized in the presence of zinc oxide alone, but magnesium... 

II. B polyethylene glycol, ethylene oxide, polystyrene, diisocyanates (urethanes), polyvinylchloride, chloroprene, THF, diglycolide, dilac-tide, <5-valerolactone, substituted e-caprolactones, 4-vinyl anisole, styrene, methyl methacrylate, and vinyl acetate. In addition to these species, many copolymers have been prepared from oligomers of PCL. In particular, a variety of polyester-urethanes have been synthesized from hydroxy-terminated PCL, some of which have achieved commercial status (9). Graft copolymers with acrylic acid, acrylonitrile, and styrene have been prepared using PCL as the backbone polymer (60). 

Natural rubber is resistant to dilute mineral acids, alkahes, and salts, but oxidizing media, oils, and most organic solvents will attack it. Hard rubber is made by adding 25 percent or more of sulfur to natural or synthetic rubber and, as such, is both hard and strong. Chloroprene or neoprene rubber is resistant to attack by ozone, sunlight, oils, gasoline, and aromatic or halogenated solvents but is... 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

Chloroprene (2-chloro-l,3-butadiene 105), which is a mass-produced, inexpensive industrial material, is an excellent precursor to a variety of terminal allenes 107 [97]. The palladium-catalyzed reaction of 105 with pronucleophiles 106 in the presence of an appropriate base gave the terminal allenes 107 in good yields (Scheme 3.53). The palladium species generated from Pd2(dba)3-CHC13 and DPEphos was a good catalyst for these reactions of chloroprene. In contrast, (Z)-l-Phenyl-2-chloro-l,3-buta-diene, which is isostructural with the bromo-substrate 101, was nearly inert under these conditions. There is no substituent at the vicinal ris-position to the chloride in 105, which allows oxidative addition of the C-Cl bond in 105 to the Pd(0) species. 

Chlorine (from the Greek chloros for yellow-green ) is the most abundant halogen (0.19 w% of the earth s crust) and plays a key role in chemical processes. The chlor-alkali industry has been in operation since the 1890s and improvements in the technology are still important and noticeable, for example, the transition from the mercury-based technology to membrane cells [60]. Most chlorine produced today is used for the manufacture of polyvinyl chloride, chloroprene, chlorinated hydrocarbons, propylene oxide, in the pulp and paper industry, in water treatment, and in disinfection processes [61]. A summary of typical redox states of chlorine, standard potentials for acidic aqueous media, and applications is given in Scheme 2. 

Chloroprene was fractionally distilled under a reduced pressure of nitrogen. It was stored at — 80°C. in vacuo, and when required small amounts were distilled in vacuo into a subsidiary reservoir and from thence directly into the oxidation reactor. In this way chloroprene could be obtained completely free of peroxide, dimers, and higher polymers. 

Oxidations were carried out using about 2 ml. of chloroprene in an apparatus similar to that described by Bolland (5), modified for automatic recording. In some instances a gas-circulating system was used, in which oxygen from the reaction vessel passed through water in a conductivity cell, which was used to record the formation of hydrogen chloride (2). 

Solutions of peroxide were prepared by oxidizing to the required extent, quenching the oxidation by cooling, and adding an excess of an inert diluent such as toluene. More than half the toluene was then pumped off while the oxidate was kept at — 20°C. After this procedure had been repeated twice, solutions of peroxide in toluene could be prepared in which the residual chloroprene concentration was about 0.5% (w./w.) of the peroxide. Complete removal of solvent gave faintly yellow viscous peroxidic material which was mildly explosive at room temperature. 

Autoxidation of Chloroprene. The oxidation was autocatalytic and up to about 5 mole % oxidation—i.e., 5 moles of oxygen absorbed per 100 moles of chloroprene initially present—the quantity (mole % oxidation)172 was a linear function of time, as observed by Kern (10). Beyond this extent the oxidation continued at a rather greater rate than given by this relation and was still accelerating at 25 mole % oxidation. Values of K in the expression... 

Initiated Oxidation. The initial rates of oxidation of chloroprene, initiated with 2,2 -azobisisobutyronitrile, were measured in the range 20° to 40°C. at a total pressure of 700 mm. of Hg. The difficulty with these measurements was that chloroprene autoxidizes so readily that even when the initiator is used at the fairly massive concentration of 0.462M, the rate of oxidation is constant for only a few minutes before acceleration, resulting from a contribution to initiation from chloroprene peroxide. 

Table II. Initial Rates of Oxidation of Chloroprene Initiated with 0.462M Azobisisobutyronitrile...

Inhibition of Oxidation. Several antioxidants were tested in chloroprene at 45°C. Those which can be classified as mainly suppressors of initiation (I), because of their ability to destroy hydroperoxides—namely, zinc dialkyldithiophosphates, zince dialkyldithiocarbamates, triphenyl-phosphine, and the like—had no inhibiting effect at the 100-p.p.m. level. 

Various aromatic secondary amines, substituted phenols, and pyrazoli-dones (3) that function as traps for the propagating peroxy radicals gave dead-stop induction periods when used at a concentration of 50 p.p.m. An indication of the ease of oxidation of chloroprene is that 50 p.p.m. of 2,6-di-ferf-butyl-4-methylphenol gave an induction period of only 15 minutes, while the same concentration of antioxidant prevented n-hexadecane from oxidizing for 2 hours at 160°C. 

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. 

During the induction periods caused by adding antioxidants, a small contraction in volume occurred because of the formation of dimers of chloroprene (14). This reaction occurs during the oxidation but was most easily studied by dilatometry in the absence of oxygen. A few values of the initial rate of dimerization of chloroprene, inhibited against polymerization with 2,2,6,6-tetramethylpiperidine-l-oxyl, are given in Table III. Their dependence on temperature is given by... 

Chloroprene Peroxide. The efficiency of conversion of oxygen to total peroxides and hydroperoxide at various extents of oxidation was determined by iodometric methods. At up to 12% oxidation the proportion of hydroperoxide was constant at 20% of the whole. Ferrous thiocyanate likewise estimated a constant proportion (40%) of the total peroxide. Direct analysis of oxidates was somewhat difficult since the chloroprene tended to continue oxidizing during manipulation. Total peroxide estimates on chloroprene-free solutions of peroxide in toluene showed that at 20% oxidation 84% of the oxygen absorbed was present as peroxide groups. This is a minimum value since a small amount of the peroxide may have decomposed while chloroprene was being removed at —20°C. 

It was confirmed that no volatile peroxides were formed (10). Chloroprene was oxidized to 10% at 45°C. and then flash-distilled in... 

When the NMR spectrum of a 30% (w./v.) solution of peroxide in toluene was recorded at 34°C., absorption was observed between 8 2.74 and 5.46. There were seven main resonances, all multiplets, which were interpreted in terms of aliphatic hydrogen shifted by oxygen. Resonance from ethylenic hydrogen amounted to only a fraction of a proton. However, the sample darkened while in the instrument and probably decomposed extensively. When the spectrum of a solution of peroxide prepared by oxidation to 10.4 mole % was recorded using a cold probe at —35°C. a different picture was obtained. There was complex absorption from both ethylenic and saturated hydrogen which was interpreted as arising from a mixture of 1,2 and 1,4 oxygen copolymers in an approximate jatio of 1 to 2. In this sample the residual chloroprene amounted to 0.15% of the monomer units in the peroxide and dimers of chloroprene to 0.6% of the peroxide. 

In the early stages of the autoxidation of chloroprene the amount of oxygen absorbed increased as the square of the time. This dependence on time is frequently observed in autoxidations and is an approximation to that expected for an oxidation of long chain length, initiated by the first-order decomposition of the peroxidic product and terminated by a bimolecular reaction of the propagating peroxy radicals. 

This decomposition usually shows little dependence on solvent, so if Ed for decomposition in chloroprene is likewise 30.9 kcal. per mole, then since Eox = 25.1 kcal. per mole Ep = 9.6 kcal. per mole, assuming termination to require no energy of activation. This is 1.2 kcal. per mole larger than kp for styrene oxidation (8). Values of e for azobisisobutyronitrile in oxidation systems usually lie in the range 0.6 to 0.8 if e = 0.7, the above equation for the decomposition of the azonitrile and that given earlier for the initiated oxidation of chloroprene permit calculation of kp/kt1/2 for chloroprene and also the kinetic chain lengths of the oxidations (Table IV). 

Table V. Calculated Values for for Decomposition of Chloroprene Peroxide and Kinetic Chain Lengths at 5 Mole % Oxidation...

Chloroprene is a monomer used almost exclusively for the production of polychloroprene elastomers and latexes. It readily forms dimers and oxidizes at room temperature. Occupational exposures occur in the polymerization of chloroprene and possibly in the manufacture of products from polychloroprene latexes. 

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