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.Allow the flask to coolto room temperature.b2.Flush a syringe (50 mL) with nitrogen, by inserting the needle into a septumcap fitted to a piece of tubing connected to the inert gas supply.Withdraw theTHF (50 mL) from the receiving flask and transfer into the reaction vessel.Repeat the procedure using the styrene (10 mL, 87 mmol).Cool the reactionflask to ca.70 C using a dry ice/acetone mixture.3.A dry syringe needle is connected to the inert gas supply by means of someplastic tubing, with a gentle flow of gas passing through the needle, insertthe tip through the seal on the bottle of sec-butyllithium.Flush a syringe(1 mL) with nitrogen, by inserting this into a septum cap fitted to a piece oftubing connected to the inert gas supply.Insert the second needle throughthe seal on the butyllithium and withdraw the required amount of initiator(0.5 mL, 0.65 mmol) as shown in Figure 2.3.4.Carefully add the butyllithium to the reaction flask by inserting the syringeneedle through the septum attached to the side-arm.A dark red colourationdue to the presence of the styryl anion will be produced.Stir the polymeriza-tion mixture for 4 h.5.After 4 h the viscous solution should remain red, quench the reaction byadding propan-2-ol (2 mL) via the septum (caution!).The reaction can bethen opened to the atmosphere.6.The polystyrene is isolated by removing the solvent on the rotary evaporator,dissolving the polymer in the minimum volume of dichloromethane, andthen pouring this solution into a beaker containing aqueous methanol (1 : 4,ca.10 volume).Further purification is achieved by repeated dissolution indichloromethane, followed by re-precipitation into methanol; finally, thewhite solid is dried in a vacuum oven at 60 C.aA dry two-necked round-bottomed flask containing a magnetic stirring bar is charged with dry1-pyreneacetic acid (ca.100 mg, 0.38 mmol), and fitted with a rubber septum and a nitrogen inlet.Theflask is evacuated then flushed with nitrogen using the double manifold and dry THF (10 mL) is addedand the resulting solution stirred.A graduated syringe (1 mL) is charged with the organolithiumreagent and this is added drop-wise to the solution of pyrene acetic acid over several minutes notingthe volume added when the red colour of the dianion persists.bAlternatively, the hot system can be evacuated and then flushed with nitrogen several times to ensurecomplete replacement of the air with dry inert gas.Although the polymerization with butyllithium is an effective way of generat-ing polymer, both initiation and propagation in ionic polymerizations arerather complex processes and governed by factors such as solvation and bind-ing to counterions.For the polymerization of methacrylates, a particularproblem with butyllithium is the reactivity towards the carbonyl group, for72 2: General procedures in chain-growth polymerizationthis reason less reactive initiators such as diphenylhexyllithium (Scheme 12)have been used with some success.41 Particularly convenient are saltsbetween lithium and large aromatic molecules with acidic hydrogens, one ofthe most common examples being fluorene.42 While this compound is arather weak acid, it is of course considerably more acidic than butane andconsequently the salt can be easily prepared by reaction with butyllithium,this salt can be used to initiate the polymerization of methyl methacrylate in arelatively simple procedure.1(e),32 In the example below the formation of alargely syndiotactic polymer is described (Scheme 13), this stereochemicalcontrol is provided largely by the solvent and hence a similar procedure usingtoluene as the solvent will produce a predominantly isotactic material.BuLiTHFScheme 12 Reaction of butyllithium with 1,1-diphenylethene to form diphenylhexyllithium.Protocol 9.Preparation of predominantly syndiotactic methyl methacrylateusing 9-fluorenyllithium as the initiator (Scheme 13)Caution! Carry out all procedures in a well-ventilated fume-hood, wear appro-priate disposable gloves, a lab-coat, and safety glasses.All vacuum-line workshould be performed while standing behind a protective Perspex screen.MeO OMe MeO2C CH3 CH3 CO2MeBuLi +LiScheme 13 Polymerization of polystyrene initiated with 9-fluorenyllithium.EquipmentÏ% Ï%Dual manifold (nitrogen/vacuum) Source of dry nitrogenÏ% Ï%Two-necked round-bottomed flask (250 mL) Syringe (1 mL)Ï% Ï%Nitrogen gas inlet Syringe (10 mL)Ï% Ï%Stirrer Syringe (50 mL)Ï% Ï%Dry ice/acetone bath Vacuum oven73 N.Aragrag et al.Protocol 9
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