Percarbonates, initiators

Chloroformates are versatile, synthetic intermediates, based on the affinity of the chlorine atoms for active hydrogen atoms. Chloroformates should be considered as intermediates for syntheses of pesticides, perfumes, dmgs, polymers, dyes, and other chemicals. Some of these products, eg, carbonates, are used as solvents, plastici2ers, or as intermediates for further synthesis. A significant use of chloroformates is for conversion to peroxydicarbonates, which serve as free-radical initiators for the polymeri2ation of vinyl chloride, ethylene, and other unsaturated monomers. The most widely used percarbonate initiators are diisopropyl peroxydicarbonate (IPP), di-2-ethyIhexylperoxydicarbonate (2-EHP), and di-j -butylperoxydicarbonate (SBP). The following Hst includes most of the commercially used percarbonates. 

Fig. 1. Bulk polymerization of diethylene glycol bis(aHylcarbonate) at 45°C with initial addition of 3.0% diisopropyl percarbonate. Rates of polymerization as measured by density and catalyst consumption decrease with time at a given temperature (14).

Scratch resistance of polymer from DADC is improved by novel mixtures of peroxide initiators such as 5% isopropyl percarbonate with 3.5% benzoyl peroxide (16). In order to force completion of polymerization and attain the best scratch resistance in lenses, uv radiation is appHed (17). Eyeglass lenses can be made by prepolymerization in molds followed by removal for final thermal cross-linking (18). 

The DADC monomer has been copolymerized with small amounts of polyfunctional methacryflc or acryflc monomers. For example, 3% triethylene glycol dimethacrylate was used as a flexibiflzing, cross-linking agent with a percarbonate as initiator (26). CR-39 and diethylene glycol diacrylate containing isopropyl percarbonate were irradiated with a mercury lamp to a 92% conversion and then cured at 150°C (27). By a similar two-step process DADC was copolymerized with methyl methacrylate and tetraethylene glycol dimethacrylate (28). 

The influence of substituents on the rates of degradation of arylazo reactive dyes based on H acid, caused by the action of hydrogen peroxide in aqueous solution and on cellulose, has been investigated [43]. The results suggested that the oxidative mechanism involves attack of the dissociated form of the o-hydroxyazo grouping by the perhydroxyl radical ion [ OOH]. The mechanism of oxidation of sulphonated amino- and hydroxyarylazo dyes in sodium percarbonate solution at pH 10.6 and various temperatures has also been examined. The initial rate and apparent activation energy of these reactions were determined. The ketohydrazone form of such dyes is more susceptible to attack than the hydroxyazo tautomer [44]. 

Recently, similar telomerisation was extensively investigated in our laboratory [135]. Whatever the nature of the initiator (azo, peroxides, peresters, percarbonates) or even the way of initiation (photochemical, thermal or in the presence of redox catalysts) rather high molecular weight-telomers were obtained in medium yields. 

Their polymerizafon and copolymerization with vinyl acetate or vinyl methyl carbonate was also studied. Whatever the radical initiator used (AIBN, benzoyl peroxide, dicyclohexyl percarbonate) the monomers are too hindered to self-polymerize. 

Figure Ic. GPC of dicyclohexyl percarbonate prepared with large amounts of an initiator. Abscissa shows chart distance in centimeters, ordinate shows differential...

The efficiency is >0.9 using azo initiators and percarbonates if the monomer concentration is high. 

The behavior of ethylene is discussed below with the initiation by percarbonates as an example. 

In the initiation reaction a carbonate group is fixed at the polyethylene chain-end, and in the termination reaction a saturated end-group is formed. Although these experiments failed to give telechelics they showed that primary radicals from percarbonates will give carbonate end-groups, and telechelics should be obtainable if this side reaction can somehow be avoided. 

Percarbonates are another class of initiators capable of introducing two functional end-groups into butadiene... 

The most important radical initiators used for polymer polyol synthesis are azoderivatives, such as azoisobutyrodinitrile (AIBN) [1, 3,13, 22]. Other initiators used successfully are peroxides (tert-amyl peroxides are very efficient), hydroperoxides and percarbonates, but the half life has to be lower than two minutes, at the polymerisation temperature (115-125 °C) [23-28]. 

CR-39 monomer Is generally thermally polymerized with dllso-propyl percarbonate as Initiator Into crossllnked polymer samples with allyl side chains. The presence of allyl side chains can result In several undesirable properties In terms of the use of CR-39 polymer In dosimetry. Allyl groups are sensitive to oxidation by air. Surface oxidation of polymer films can be expected and Indeed has been observed by Tarle. This will shorten the... 

FIGURE 3.1 Half-lives r 2 of selected peroxide initiators. Curve 1, isopropyl percarbonate curve 2, 2,2 -azo-bis-isobutyronitrile curve 3, benzoyl peroxide curve 4, di-tertiary butyl peroxide curve 5, cumene hydroperoxide. 

The butyl groups have been spectroscopically identified, but proof of ethyl side groups is disputed. According to these measurements, about 8-40 branch points per 1000 main-chain atoms are formed. The many branch points reduce crystallizability, and, consequently, the density, also. Thus, LDPE [low-density poly(ethylenes)] have X-ray crystallinities of only about 60% and densities of about 0.92 g/cm Higher densities are obtained if percarbonates are used instead of oxygen as initiator, since the free radicals from the former tend to produce fewer transfer reactions. 

There are various additives in PVC plastics, such as antioxidants, light stabilizers, initiators, plasticizers, flame retardants, pigments and others. As initiators, potassium persulfate, benzoyl peroxide, lauryl peroxide, percarbonate and some azo-compounds can be used. The presence of chlorine in the hydrocarbon backbone gives rigidity and toughness to the polymer, but PVC liberates hydrogen chloride when exposed to high temperatures. To prevent this, stabilizers are added to the polymer. There are several kinds of stabilizers on the market. The most important contain lead, tin, calcium and zinc and... 

Macromolecular substances with labile groups in pendant, in-chain or terminal positions have been used to initiate block and graft copolymerizations. They include polycarbonates with pendant azo groups, macromolecular percarbonates, polyethylene oxide with perester end-groups, and nylon-6 with in-chain azo groups. ... 

The monomer is dispersed in demineralised water by the combination of mechanical stirring and surfactants. Partially hydrolysed polyvinyl acetates are the most widely used type of suspension agent. The polymerisation takes place inside the VCM droplets under the influence of VCM soluble initiators such as peresters, percarbonates or peroxides. A phase of solid PVC primary particles builds up. The PVC particles present at the end of the polymerisation process, result from the complex aggregation of such primary particles, giving S-PVC its characteristic cauliflower aspect under an optical microscope. 

The initiators used in aqueous solution polymerizations of the acrylic acids usually are the water-soluble initiators commonly used in emulsion polymerization, such as persulfate, percarbonate, and perphosphate salts. Monomer-soluble initiators have also been used, usually at sufficiently low concentrations or in the presence of water-soluble solvents to form a homogeneous system. 

---