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.All vacuum-line work should be performed while standing behind a protective Perspex screen.Never use flat-bottomed flasks with rotary evaporators.AIBNScheme 5 Polymerization of styrene initiated by AIBN.Equipment● Dual manifold (nitrogen/vacuum)● Lab-jack● Vacuum source● Polymerization tube● Source of dry nitrogen● Dewar containing liquid nitrogen● Thermostatted water-bath● Beaker (500 mL)● Separating funnel● Vacuum oven● Erlenmeyer flasks (250 mL)● Apparatus for filtration under reduced pressure:● FunnelBuchner funnel, flask, and water aspiratorMaterials● Sodium hydroxide solution (1 M)causes severe burns● Magnesium sulfatedo not breath dust, avoid contact with skin and eyes● Styrene-d7, 2 g, 19 mmolflammable, irritating to eyes and respiratory system● AIBN, 0.032 g, 0.2 mmoltoxic, harmful, explosive, highly flammable● Methanol, ca.300 mLhighly flammable, toxic by inhalation and if swallowed● Dichloromethane, 45 mLharmful by inhalation, potential carcinogen● Liquid nitrogen for cooling.extremely cold liquid, vapour can cause rapid suffocationMethod1.The styrene is freed of inhibitors by shaking with sodium hydroxide in a separating funnel.It is then dried with magnesium sulfate a.A protective netting sleeve is then placed over the tube to minimize danger from glass should the tube shatter.Attach the tube to the manifold and close the Young’s tap at the top of the tube.2.Styrene-d7 (2 g, 19 mmol) is added to a polymerization tube (Figure 2.2) and AIBN (0.032 g, 0.2 mmol) is then added.562: General procedures in chain-growth polymerization3.The monomer and initiator are then degassed using the freeze–pump–thaw technique described in Protocol 1 (steps 6–11).The tube containing thedegassed monomer is then placed in a thermostatted water-bath at 55⬚C forabout 15 h.After this time the styrene has been converted into a glassy solid lump of polystyrene at the bottom of the tube.4.The glassy polymer is dissolved by adding dichloromethane (15 mL) and allowing to stand for 2 h.The highly viscous solution is then allowed to drop slowly into a flask containing cold methanol.The solid polymer is collected, dried, and then further purified by twice dissolving in dichloromethane (15 mL) and re-precipitating into methanol.Finally, the polymer is dried in a vacuum oven at 40⬚C.a Styrene can be purified by distillation at reduced pressure, but it does exhibit a marked tendency to bump and with a small sample such as used here losses may be rather significant.2.2 CopolymerizationThe introduction of a second monomer into a free-radical polymerization is a useful tool to modify the properties of the resultant polymer.Such an approach may offer advantages over the blending of the two polymers since the latterprocedure does not guarantee a miscible material due to the poor entropy ofmixing of large molecules.One simple application of copolymerization mightbe the introduction of chemical reactive units to allow the incorporation of other units following polymerization.Two examples might be the incorporation of chromophores containing nitro compounds to generate a liquid crys-talline compound with specific optical properties or the introduction ofhydroxyl or other units to provide site for subsequent cross-linking.We shall see in Chapter 9 how this approach can be useful in providing materials that display a permanent memory of their orientation at the time of cross-linking.For free-radical copolymers the incorporation of a second monomer is notstraightforward.The composition of the final copolymer is determined by the kinetics in a way first described by Dorstal19 but later elaborated by Alfrey,20Mayo and Walling21, and others.The kinetic model assumes that the kineticsdepends on the end group of the radical chain and the new monomer in a waycommonly described for monomers M1 and M2 as shown in Scheme 6.MM1•+ M11•k 11MM1•+ M22•k 12MM2•+ M11•k 21MM2•+ M22•k 22Scheme 6 Reactions occurring in a copolymerization; in the simplest model, the reactivity of the propagating chain is considered to be dependent only on the terminal monomer unit.57N.Aragrag et al.Table 2.3Some typical data for reactivity ratiosM1M2r1r2ConditionsStyreneAcrylonitrile0.40.0460⬚CStyreneMaleic anhydride0.040.0160⬚CStyreneMethyl methacrylate0.520.4660⬚CMethyl methacrylateStyrene0.460.4260⬚CAcrylonitrileMethyl methacrylate0.151.2280⬚CThe monomer reactivity ratios r 1 (⫽ k 11/ k 12) and r 2 (⫽ k 22/ k 21) (Table 2.3) reflect the relative rate constants for a given radical adding to its precursor monomer and to the alternative.If the monomers are very similar for example two slightly different acrylates then the values of r 1 and r 2 are close to equal and unity.In such a case, the composition of the polymer is equal to the composition of the feedstock at all stages of the polymerization.If on the other hand, the values are both small as in the case of maleic anhydride and styrene then each monomer is reluctant to react with itself; the result is an alternating copolymer.Protocol 3.Preparation of a poly(styrene–acrylic acid) copolymer by free-radicalpolymerization (Scheme 7)Caution! Carry out all procedures in a well-ventilated fume-hood, wear appropriate disposable gloves, a lab-coat, and safety glasses.All vacuum-line work should be performed while standing behind a protective Perspex screen.+AIBNOOHHOOScheme 7 A copolymer formed from styrene and acrylic acid.Equipment● Dual manifold (nitrogen/vacuum)● Lab-jack● Vacuum source● Polymerization tube● Source of dry nitrogen● Dewar containing liquid nitrogen● Thermostatted water-bath● Beaker (500 mL)● Apparatus for filtration under reduced pressure:● Erlenmeyer flasks (250 mL)Buchner flask, sintered-glass● Vacuum ovenfunnel, and water aspirator582: General procedures in chain-growth polymerizationMaterials● Acrylic acid, 1.7 mL, 25 mmolflammable, causes burns● Styrene, 25.55 mL, 223 mmolflammable, irritating to eyes and respiratory system● AIBN, 0.41 g, 2.5 mmoltoxic, harmful, explosive, highly flammable● Methanol, ca.150 mLhighly flammable, toxic by inhalation and if swallowed● Acetone, ca.20 mLhighly flammable● Petroleum ether 60/80, ca [ Pobierz całość w formacie PDF ]
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.All vacuum-line work should be performed while standing behind a protective Perspex screen.Never use flat-bottomed flasks with rotary evaporators.AIBNScheme 5 Polymerization of styrene initiated by AIBN.Equipment● Dual manifold (nitrogen/vacuum)● Lab-jack● Vacuum source● Polymerization tube● Source of dry nitrogen● Dewar containing liquid nitrogen● Thermostatted water-bath● Beaker (500 mL)● Separating funnel● Vacuum oven● Erlenmeyer flasks (250 mL)● Apparatus for filtration under reduced pressure:● FunnelBuchner funnel, flask, and water aspiratorMaterials● Sodium hydroxide solution (1 M)causes severe burns● Magnesium sulfatedo not breath dust, avoid contact with skin and eyes● Styrene-d7, 2 g, 19 mmolflammable, irritating to eyes and respiratory system● AIBN, 0.032 g, 0.2 mmoltoxic, harmful, explosive, highly flammable● Methanol, ca.300 mLhighly flammable, toxic by inhalation and if swallowed● Dichloromethane, 45 mLharmful by inhalation, potential carcinogen● Liquid nitrogen for cooling.extremely cold liquid, vapour can cause rapid suffocationMethod1.The styrene is freed of inhibitors by shaking with sodium hydroxide in a separating funnel.It is then dried with magnesium sulfate a.A protective netting sleeve is then placed over the tube to minimize danger from glass should the tube shatter.Attach the tube to the manifold and close the Young’s tap at the top of the tube.2.Styrene-d7 (2 g, 19 mmol) is added to a polymerization tube (Figure 2.2) and AIBN (0.032 g, 0.2 mmol) is then added.562: General procedures in chain-growth polymerization3.The monomer and initiator are then degassed using the freeze–pump–thaw technique described in Protocol 1 (steps 6–11).The tube containing thedegassed monomer is then placed in a thermostatted water-bath at 55⬚C forabout 15 h.After this time the styrene has been converted into a glassy solid lump of polystyrene at the bottom of the tube.4.The glassy polymer is dissolved by adding dichloromethane (15 mL) and allowing to stand for 2 h.The highly viscous solution is then allowed to drop slowly into a flask containing cold methanol.The solid polymer is collected, dried, and then further purified by twice dissolving in dichloromethane (15 mL) and re-precipitating into methanol.Finally, the polymer is dried in a vacuum oven at 40⬚C.a Styrene can be purified by distillation at reduced pressure, but it does exhibit a marked tendency to bump and with a small sample such as used here losses may be rather significant.2.2 CopolymerizationThe introduction of a second monomer into a free-radical polymerization is a useful tool to modify the properties of the resultant polymer.Such an approach may offer advantages over the blending of the two polymers since the latterprocedure does not guarantee a miscible material due to the poor entropy ofmixing of large molecules.One simple application of copolymerization mightbe the introduction of chemical reactive units to allow the incorporation of other units following polymerization.Two examples might be the incorporation of chromophores containing nitro compounds to generate a liquid crys-talline compound with specific optical properties or the introduction ofhydroxyl or other units to provide site for subsequent cross-linking.We shall see in Chapter 9 how this approach can be useful in providing materials that display a permanent memory of their orientation at the time of cross-linking.For free-radical copolymers the incorporation of a second monomer is notstraightforward.The composition of the final copolymer is determined by the kinetics in a way first described by Dorstal19 but later elaborated by Alfrey,20Mayo and Walling21, and others.The kinetic model assumes that the kineticsdepends on the end group of the radical chain and the new monomer in a waycommonly described for monomers M1 and M2 as shown in Scheme 6.MM1•+ M11•k 11MM1•+ M22•k 12MM2•+ M11•k 21MM2•+ M22•k 22Scheme 6 Reactions occurring in a copolymerization; in the simplest model, the reactivity of the propagating chain is considered to be dependent only on the terminal monomer unit.57N.Aragrag et al.Table 2.3Some typical data for reactivity ratiosM1M2r1r2ConditionsStyreneAcrylonitrile0.40.0460⬚CStyreneMaleic anhydride0.040.0160⬚CStyreneMethyl methacrylate0.520.4660⬚CMethyl methacrylateStyrene0.460.4260⬚CAcrylonitrileMethyl methacrylate0.151.2280⬚CThe monomer reactivity ratios r 1 (⫽ k 11/ k 12) and r 2 (⫽ k 22/ k 21) (Table 2.3) reflect the relative rate constants for a given radical adding to its precursor monomer and to the alternative.If the monomers are very similar for example two slightly different acrylates then the values of r 1 and r 2 are close to equal and unity.In such a case, the composition of the polymer is equal to the composition of the feedstock at all stages of the polymerization.If on the other hand, the values are both small as in the case of maleic anhydride and styrene then each monomer is reluctant to react with itself; the result is an alternating copolymer.Protocol 3.Preparation of a poly(styrene–acrylic acid) copolymer by free-radicalpolymerization (Scheme 7)Caution! Carry out all procedures in a well-ventilated fume-hood, wear appropriate disposable gloves, a lab-coat, and safety glasses.All vacuum-line work should be performed while standing behind a protective Perspex screen.+AIBNOOHHOOScheme 7 A copolymer formed from styrene and acrylic acid.Equipment● Dual manifold (nitrogen/vacuum)● Lab-jack● Vacuum source● Polymerization tube● Source of dry nitrogen● Dewar containing liquid nitrogen● Thermostatted water-bath● Beaker (500 mL)● Apparatus for filtration under reduced pressure:● Erlenmeyer flasks (250 mL)Buchner flask, sintered-glass● Vacuum ovenfunnel, and water aspirator582: General procedures in chain-growth polymerizationMaterials● Acrylic acid, 1.7 mL, 25 mmolflammable, causes burns● Styrene, 25.55 mL, 223 mmolflammable, irritating to eyes and respiratory system● AIBN, 0.41 g, 2.5 mmoltoxic, harmful, explosive, highly flammable● Methanol, ca.150 mLhighly flammable, toxic by inhalation and if swallowed● Acetone, ca.20 mLhighly flammable● Petroleum ether 60/80, ca [ Pobierz całość w formacie PDF ]