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.42 2General procedures inchain-growth polymerizationNAJIB ARAGRAG, DARIO C.CASTIGLIONE, PAUL R.DAVIES,FRED J.DAVIS, and SANGDIL I.PATEL1.IntroductionThis chapter is intended to provide a general introduction to the laboratorytechniques used in polymer synthesis, by focusing on some relatively well-known polymerizations that occur by chain-growth processes.In this waysome of the more commonly used procedures in polymer chemistry aredescribed.Due to the nature of the intermediates produced,1 such as free rad-icals, carbanions, carbocations, together with a range of organometallicspecies, the techniques often involve handling compounds in the completeabsence of oxygen and moisture.Because of this the best results may requirequite sophisticated equipment and glassware;2 however, it is our intention toshow that the general procedures are accessible to any reasonably equippedlaboratory, and indeed some of the techniques are suitable for use in anundergraduate teaching laboratory.3 5Chain-growth polymerization involves the sequential step-wise addition ofmonomer to a growing chain.Usually, the monomer is unsaturated, almostalways a derivative of ethene, and most commonly vinylic, that is, a mono-substituted ethane, 1 particularly where the growing chain is a free radical.For such monomers, the polymerization process is classified by the way inwhich polymerization is initiated and thus the nature of the propagatingchain, namely anionic, cationic, or free radical; polymerization by coordina-tion catalyst is generally considered separately as the nature of the growingchain-end may be less clear and coordination may bring about a substantiallevel of control not possible with other methods.6H XH H1Structure 1 N.Aragrag et al.Ring-opening polymerizations exhibit many of the features of chain-growthpolymerization, but may also show some of the features expected from step-growth polymerizations.7 However, it is probably fair to say that from a prac-tical point of view the techniques involved are rather similar or the same asthose used in chain-growth processes and consequently some examples ofring-opening processes are provided here.It is particularly instructive to consider the requirements of chain-growthcompared to step-growth processes in terms of the demands for reagentpurity and reaction conditions.In both cases monomer purity is of the highestimportance, but it should be remembered that for a chain-growth processonly a tiny fraction (depending, of course, on the molecular weight) ofmonomer molecules react with initiator and consequently there is the poten-tial for massive inhibition of the reaction by traces of impurities, the classicexample being the inhibition of free-radical chain-growth polymerization bysmall quantities of phenolic compounds.Indeed, use is made of this propertyin the stabilization of monomers and compounds such as 2 are added to com-mercial monomer samples to prevent polymerization prior to use.These needto be removed prior to use; vide infra.Water and oxygen may also need to beremoved and often this necessitates the use of vacuum lines and other equip-ment used typically for air- and moisture-sensitive materials.In the case ofstep-growth polymerization, the crucial need is to avoid the presence ofmonofunctional units that can cap the polymerization and of course anyunknown material that might inadvertently effect the stoichiometry herehigh molecular weights are only possible at high conversions as dictated byCarother s equation (see Chapter 4).OHCH32Structure 22.Free-radical chain polymerizationThis is perhaps the most well-known method of polymerization, and as the nameimplies, involves the continuous addition of monomer units to a growing free-radical chain.8 The general mechanism of this process in relation to the polymer-ization of a vinyl monomer is shown in Scheme 1.As Scheme 1 shows, initiationis a two-stage process in which, first a free radical is formed, and second thisradical adds on to a monomer unit.The second stage is essentially the same for44 2: General procedures in chain-growth polymerizationall the related processes; however, the first step can be achieved in a variety ofways; and the type of initiator depends on the nature of the polymerizationexperiment.In a laboratory, 2,2 -azo-bisisobutyronitrile (AIBN), in a sealedtube is usually the initiator of choice for this and other free-radical processes pre-sumably because of the convenient timescale of its decomposition (Scheme 2) ofabout 18 h at 60 C [ Pobierz całość w formacie PDF ]