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.With the flask connected to the inert gas supply by means of a syringeneedle inserted through the septum, dry toluene (5 mL) is added by means ofa clean dry syringe.2.A dry two-necked round-bottomed flask (100 mL) is equipped with a Teflon®-coated magnetic follower, a condenser, and a septum cap.The flask is922: General procedures in chain-growth polymerizationconnected to the dual manifold by means of a gas-inlet tube, and is evacu-ated and then maintained under an atmosphere of argon.3.MAO (12.5 mL, 21 mmol with respect to aluminium) is added to the reaction flask using a gas-tight syringe followed by the metallocene solution (0.2 mL0.004 mmol).The mixture is placed in a dry ice/acetone bath and allowed tostand at ⫺78⬚C for 1 h.4.The monomer is prepared for addition by cooling to ⫺78⬚C in the presence of a portion of MAO.A two-necked round-bottomed flask (100 mL) fitted with arubber septum is attached to the double manifold by means of a gas-inlettube and flushed with argon.1-Hexene (2 mL, 16 mmol) and MAO (0.7 mL,12 mmol with respect to aluminium) are added using gas-tight syringes ofappropriate sizes.The flask is then place in a dry ice/acetone bath andallowed to stand at ⫺78⬚C for 30 min.5.The monomer solution prepared in step 4 above is then added to the metallocene/MAO solution using a syringe as before.The solution is then stirredfor 5 days at ⫺78⬚C.6.After 5 days, the polymerization is terminated by addition of ice-water (50 mL) and the toluene layer removed.The aqueous layer is placed in aseparating funnel and extracted with three portions of CH2Cl2 (ca.3 ⫻ 25 mL).The combined organic extracts are then dried using anhydrous magnesiumsulfate filtered and the solvent removed on the rotary evaporator.7.The crude polymer is dissolved in a small amount of dichloromethane and precipitated by dropping this solution into a cold stirred solution ofmethanol.This procedure is repeated and the polymer collected by filtration at the pump and dried in the vacuum oven at 40⬚C for 24 h.A different type of organometallic process that has also seen many import-ant developments in recent years is the ring-opening polymerization of cyclic alkenes induced by organometallic catalysts, also known as ring-openingmetathesis polymerization (ROMP).For example, cyclopentene is polymer-ized by a WCl6/AliBu3 catalyst to produce an acyclic polymer as shown inScheme 20.64 It is impossible here to do justice to the outstanding work that has been done in this area,65,66 but a simple example is given below; this utilizes a catalyst invented by Professor R.Grubbs67 to polymerize norbornene(Scheme 21).68 This catalyst is particularly easy to use; and such polymerizations may, for example, be performed in an undergraduate laboratory.3WCl6iAl Bu3Scheme 20 Ring-opening polymerization of cyclopentene.93N.Aragrag et al.Protocol 18.Ring-opening polymerization of norbornene (Scheme 21)Caution! Carry out all procedures in a well-ventilated fume-hood (or where appropriate in a dry-box) wear appropriate disposable gloves, a lab-coat, and safety glasses.All vacuum-line work should be performed while standingbehind a protective Perspex screen.P(C H)6 11 3ClPhClP(C H)6 11 3Scheme 21 Ring-opening polymerization of norbornene.Equipment● Dual manifold (argon/vacuum)● Source of dry argon● Single-necked round-bottomed flask (25 mL)● Dry-box● Gas-tight syringes and narrow bore needles● Apparatus for filtration, Buchner funnel, flask,(1, 5, and 50 mL)and water pump● Two-necked round-bottomed flask (100 mL)● Teflon®-coated magnetic stirrer bar● Magnetic stirrer● Rubber septa● Condenser● Gas-inlet adapter● Beakers and Erlenmeyer flasks (various)● Vacuum ovenMaterials● Norbornene, 1 g, 11 mmolhighly flammable● Benzylidene-bis(tricyclohexylphosphine)irritating to eyes, skin, and respiratory systemruthenium dichloride, 0.1 g, 0.1 mmol a● Dichloromethane, ca.45 mLharmful by inhalation, potential carcinogen● Ethyl vinyl ether, 0.05 mL, 0.5 mmolhighly flammable, repeated exposure may cause skindryness or cracking, vapours may cause drowsiness or dizziness● Methanol, ca.200 mLhighly flammable, toxic by inhalation and if swallowedMethodPreparation: All glassware is dried in an electric oven at 125 ⬚C for 24 h prior to use; apparatus is constructed whilst still hot, and allowed to cool under an atmosphere of inert gas.Dichloromethane is pre-dried with CaCl2 followed by distillation from CaH2.1.A catalyst solution is prepared by weighing out benzylidene-bis(tricyclohexylphosphine)ruthenium dichloride (0.1 g, 0.1 mmol) in a dry-box under an942: General procedures in chain-growth polymerizationargon atmosphere and placing in a dry single-necked round-bottomed flask(25 mL).The flask is then fitted with an appropriate septum cap and removed from the dry-box.With the flask connected to the inert gas supply by meansof a syringe needle inserted through the septum, dry dichloromethane(3 mL), is added by means of a clean dry syringe.b2.A dry two-necked round-bottomed flask (100 mL) is equipped with a Teflon®-coated magnetic follower, a condenser, a gas-inlet adapter, and a septumcap.The flask is then charged with norbornene (1 g, 11 mmol).The flask isconnected to the dual manifold by means of the gas-inlet adapter, and isevacuated c and then maintained under an atmosphere of argon.3.By means of a gas-tight syringe, dichloromethane (40 mL) is added the reaction flask, and the mixture stirred until the monomer has dissolved.The polymerization is then initiated by the addition of 0.6 mL of the catalyst solution via a gas-tight syringe, and the solution stirred for 24 h.4.The polymerization is then halted by the addition of ethyl vinyl ether (0.05mL, 0.5 mmol).d The polymer is then precipitated into methanol and purified by dissolving in chloroform and re-precipitation.The polymer is then dried in the vacuum oven at 40⬚C.a Research-scale quantities of this catalyst can be obtained from the Sigma-Aldrich Corporation.b This procedure can be performed using a glove bag, but in such circumstances it is recommended that the exact quantities required for the polymerization (step 2) should be measured, to avoid any subsequent decomposition of the catalyst solution.c The flask must be cooled at this stage to prevent the monomer subliming.d This stage is necessary to cleave the polymer from the ruthenium.6
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