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.2.1 General procedures in free-radical polymerizationAlthough the presence of water is generally not an issue in free-radicalchain polymerization (indeed water may be a suitable medium for polymer-ization as in Protocols 5–7) unlike, for example, chain-growth polymeriza-tion initiated by anionic species, it is always advisable to use solvents of the highest purity and this will generally include some element of pre-drying.In general, solvents should be distilled, particularly as a number of suitable solvents for polymerization reactions contain stabilizers which usually serve to mop up free radicals and therefore inhibit the polymerization47N.Aragrag et al.process.It is usually advisable to pre-dry solvents often with calciumhydride (see below) and then distil them from another drying agent.LiAlH4is not nowadays a favoured reagent in this latter capacity.Ethereal solvents are not usually suitable for free-radical processes (unless a restricted molecular weight is required*).However, where they are used, they are gener-ally dried on a mixture of sodium and benzophenone under a nitrogenatmosphere; the deep blue colour of the sodium benzophenone ketyl indicat-ing that the solvent is dry.This ketyl also acts to remove oxygen.Thedemand for dry THF in many laboratories is such that a semi-permanentsolvent-still is required.It is important that the users remember the possibility and danger of peroxide build-up in such systems and regularly test forthese.This is done by adding 1 mL of the solvent to 1 mL of a 10% (w/v)solution of sodium iodide in acetic acid; a yellow colour indicates peroxides present, but in low concentrations; a brown colour high concentrations.†In any case, a still should be dismantled regularly and cleaned.Solvent-stills are a clear fire risk and should not be used without appropriate precautions and should be carefully monitored during use.Some informa-tion about common solvents used both for polymerization and in thepreparation and purification of common monomers is listed in Table 2.2.The list is not exhaustive and the reader is referred to the excellent bookby Perrin and Armarego,14 and some excellent textbooks on practicalchemistry.15–17After monomer and solvent purity (particularly the removal of inhibitors),perhaps the most crucial factor in determining the success of a free-radical polymerization process is the absence of oxygen.Oxygen is, of course,highly reactive towards free radicals and has a detrimental effect on polymerizations.For this reason, free-radical polymerizations need to be performed under anaerobic conditions.Solution or bulk polymerizations tend to be performed in polymerization tubes such as the one shown in Figure 2.1.Suchtubes may be sealed using a flame in which case it is usually desireable toprepare them with a constriction near the top.For less volatile monomers and solvents, particularly where only small volumes are being used, it is sufficient to seal the tube using a ‘Young’s tap’ making the whole process muchmore convenient.Even under these circumstances it is important to treatsuch tubes with considerable respect, large unexpected changes in pressurewill cause them to shatter explosively and a blast screen should be placed* The methylene unit attached to the ethereal oxygen is particularly reactive towards radical attack and thus chain-transfer processes would be expected to substantially reduce the molecular weight of the final polymer.Some reduction is noticed but both this solvent and toluene can be used for free-radical polymerization (and note the successful polymerization of benzyl acrylate in Protocol 1).†Peroxides my be destroyed by treating the solvent with a concentrated solution of ferrous sulfate.482: General procedures in chain-growth polymerizationTable 2.2Some properties and purification methods of common solventsSolventBoiling pointaPurification methodHazards18Tetrahydrofuran65Pre-dried over CaH2Highly flammable,b may(THF)then dried over Naform peroxides,and distilled from Nairritating to eyes andwith benzophenonerespiratory systemDiethyl ether35As with THF above;Extremely flammable,diethyl ethermay form peroxidessufficiently dry for thepreparation of Grignardreagents can beobtained by standingon sodium wireToluene111These solvents can beHighly flammable,dried with calciumharmful by inhalationhydride then distilled;Benzene80alternatively, they canCaution: Cancer suspectbe treated as theagent, highly flammable,etherial solvent abovetoxic by inhalationChlorobenzene131.7Relatively pureFlammable, harmful bychlorobenzene caninhalationbe prepared bywashing with H2SO4then drying withsodium bicarbonate,followed by dryingwith CaCl2 thendistillation from P2O5Hexane68.7Distillation and storingHighly flammable,over 4A molecularharmful by inhalation andsieves is usually suitablein contact with the skinfor these solventsPentane36.1Highly flammablePetroleum etherVariousExtremely flammable orhighly flammableChloroform61Chloroform can beHarmful by inhalation,simply purified bypotential carcinogenpassing through acolumn of basicalumina to removethe ethanol, whichis added as a stabilizer.Chloroform must bestored in the dark toavoid thephotochemicalgeneration ofphosgene49N.Aragrag et al.Table 2
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