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Free Radical Polymerization Introduction to Macromolecular Chemistry aka polymer chemistry Mondays, 8.15-9.45 am, NC 02/99 Dr. Christian Merten, Ruhr-Uni Bochum, 2019 www.ruhr-uni-bochum.de/chirality Polymerization reactions: Summary STEP GROWTH REACTION CHAIN GROWTH REACTION . Monomer with 2 . Monomer has one polymerizable groups polymerizable group . Multiple reactive chain . Active chain end, i.e. ends growth in one direction . Molecular weight increases slowly . High molecular weight Different mechanisms: only at high conversion . Radical polymerization . Cationic polymerization . Anionic polymerization . Coordinative polym. Macromolecular Chemistry | Dr. C. Merten, 2019 72 Free radical polymerization Typical examples monomer polymer Ethylene Vinyl chlorid Cl Acrylo nitrile C N Three step reaction: . Initiation Vinyl acetate . Chain growth . Termination Methyl acrylic acid Styrene Macromolecular Chemistry | Dr. C. Merten, 2019 73 Free radical polymerization: Initiation Photochemical activation of initiator Thermal activation of initiator Thermal initiation of polymerization Macromolecular Chemistry | Dr. C. Merten, 2019 74 Free radical polymerization: Initiation Important: Initiator radical lifetime at polymerization temperature Isopropyl percarbonate Azobisisobutyro nitrile Dibenzoyl peroxide Di-tert-butyl peroxide Cumolhydro peroxide Macromolecular Chemistry | Dr. C. Merten, 2019 75 Free radical polymerization: Growths and termination Chain growths Termination mechanisms a) Combination b) Disproportionation Macromolecular Chemistry | Dr. C. Merten, 2019 76 Free radical polymerization: Kinetics Initiation: In-In 2 In Chain start: In + M In-M Initiation is much slower than chain start: ≫ InIn Reaction kinetics of 1st order in concentration of initiator: InIn In 2 InIn Growth: Mi + M R-Mi+1 M For polymerization rate: ⋅ M ⋅∑M i Macromolecular Chemistry | Dr. C. Merten, 2019 77 Free radical polymerization: Kinetics Termination: Mi + Mj M(i+j) for recombination Mi + Mj for disproportioning M The rates of determination thus are: i 2 ∑M M i j M i 2 ∑M M i j with and assuming that Mi and Mj M are indistinguishable it follows that i 2 ∑M i Simplification: steady state in which number of radicals formed and consumed are equal d In d Mi InIn ⇒ 2 InIn 2 ∑Mi ⇒ ∑ Mi d d Macromolecular Chemistry | Dr. C. Merten, 2019 78 Free radical polymerization: Kinetics Simplification: steady state in which number of radicals formed and consumed are equal d In d Mi InIn ⇒ 2 In 2 ∑Mi ⇒ ∑ Mi d d We can thus write for the rate of polymerization: d M / ⋅ M ⋅∑Mi ⋅ M ⋅ InIn d but, as is small, [InIn] remains approximately constant during the reactions, so that the rate of polymerization becomes 1st order in [M]! Macromolecular Chemistry | Dr. C. Merten, 2019 79 Free radical polymerization: Degree of polymerization Kinetic chain length: How many monomers are added to each started radical chain before termination?: d M Rate of monomer addition to chain growth rate ̅ ̅ d Rate of new chain formation initiator rate d In d Substituting the equations from above gives in case of steady state: M⋅∑ M M ̅ i 2 InIn 2 InIn The degree of polymerization is linked with the kinetic chain length by ⋅ where is a factor which depends on the mechanisms of termination (1for disproportionation, 2for recombination) Macromolecular Chemistry | Dr. C. Merten, 2019 80 Free radical polymerization: Degree of polymerization The degree of polymerization is linked with the kinetic chain length by ⋅ where is a factor which depends on the mechanisms of termination (1for disproportionation, 2for recombination) M So we end up with ⋅ ⋅ InIn For the number average molecular mass, ⋅, and by simplifying b , we get M InIn Macromolecular Chemistry | Dr. C. Merten, 2019 81 Free radical polymerization: Mass distribution Mass distribution of radical polymerizations depend on kinetic chain length: Rate of monomer addition to chain growth rate ̅ ̅ Rate of new chain formation initiator rate and on the mechanism of termination – disproportion or recombination: For disproportion: ln1 ⋅⋅ 1 For recombination: ln1 ⋅ ⋅ 1 mP represents mass fraction of polymers with certain degree of polymerization. Macromolecular Chemistry | Dr. C. Merten, 2019 82 Free radical polymerization: Temperature effects Temperature dependence can be described with Arrhenius equations: ⋅exp ⋅exp ⋅exp For the T-dependence of the rate of polymerization we get: d M d M ⋅ M ⋅ InIn ⇒ ⋅exp 2 2 ⋅ M ⋅ InIn d d Generally, the exponential term is negative reaction rate goes up, if T is raised. But: M 2 2 M ⋅ ⇒ ⋅exp ⋅ 2 InIn 2 InIn Here the exponential term is often positive average molecule weight decreases! Macromolecular Chemistry | Dr. C. Merten, 2019 83 Free radical polymerization: Thermodynamic aspects A reaction takes place, if . This is the case, if Δ 0 and Δ 0 Δ 0 and Δ Δ Δ ≪ 0 even with Δ 0 typical for polymerizations! Ceiling-temperature Tc: Δ Δ At Tc, polymerization and depolymerization are equally likely: examples: MMA 493 K If Tc is reached, the reaction stops ( ). styrene 583 K (Diagram redrawn based on Tieke, „Markomolekulare Chemie“) Macromolecular Chemistry | Dr. C. Merten, 2019 84 Free radical polymerization: Autoacceleration For bulk polymerization or in concentrated solutions, strong deviations from steady state kinetics are possible: increased viscosity prevents termination mechanisms. termination rate decreases significantly decrease in reaction rate once glass temperature is reached gel effect (Trommsdorf Norrish effect) d M ⋅ M ⋅ InIn ⇒ ln ⋅ d Schematic change in log with time at different concentrations of MMA in benzene: A linear correlation is only observed for low concentrations. (Diagram redrawn based on Tieke, „Markomolekulare Chemie“) Macromolecular Chemistry | Dr. C. Merten, 2019 85 Free radical polymerization: Chain transfer Chain transfer: Transfer of radical from growing chain to other species, from which the growth can continue kind of termination reaction (a) Chain transfer to monomer (≠ regular polymerization step) Competition with polymerization: P + M P + M transfer constant / M + n M P Macromolecular Chemistry | Dr. C. Merten, 2019 86 Free radical polymerization: Chain transfer Consequence of chain transfer: Autoinhibition 4 Monomer CM / 10 Acrylamide 0.5 Allyl compounds , propene, isobutylene, vinyl ether cannot Acrylonitrile 0.26 be polymerized via radical polymerization. Ethene 0.4 - 4.2 Styrene 0.3 - 0.6 Formation of resonance stablized monomer radical is Vinyl acetate 1.75 - 2.8 Vinyl chloride 10.8 – 16 thermodynamically favored over polymer radical! Macromolecular Chemistry | Dr. C. Merten, 2019 87 Free radical polymerization: Chain transfer (b) Chain transfer to polymer Example: polyethylene: Example: Poly vinylacetate Crosslinking of polymer chains Macromolecular Chemistry | Dr. C. Merten, 2019 88 Free radical polymerization: Chain transfer (c) Chain transfer to solvent . Can be utilized for end group functionalization: Cl Cl NH HOOC HOOC 2 Cl3C n hydrolysis n NH3 n Macromolecular Chemistry | Dr. C. Merten, 2019 89 Free radical polymerization: Chain transfer (d) Chain transfer to regulators e.g. to mercaptanes … allows control of molecular mass (e) Chain transfer to retarders and inhibitors example: chinone as inhibitor Macromolecular Chemistry | Dr. C. Merten, 2019 90 Free radical polymerization: Industrially relevant polymers and processes Emulsion polymerization An important process for polyacrylates, poly(vinvyl chloride), poly(vinyl acetate) Advantages over homogeneous (bulk) polymerization: . No heat formation (respectively good discipation) . No increase in viscosity . No organic solvents Reaction mixture: Monomers, emulsifier, water, water-soluble initiator Polymerization almost exclusively in the micelles, new monomer molecules are provided from droplets Technically relevant for instance for wall paint Macromolecular Chemistry | Dr. C. Merten, 2019 91 Free radical polymerization: Industrially relevant polymers Polystyrene (PS) . Commerciallized by BASF in 1930s . Polymerization either in • highly concentrated solution (5-25% ethylbenzene) with thermal initiation • Suspension with peroxid initiation . In both cases, preparation of granules/pellets, which can be molten for further processing Polyethylene (PE) . High pressure PE = LDPE: commerciallized by ICI in 1939 . Radical polymerization in bulk at 1400-3500 bar, 130-330 °C . Initiation: oxygen, organic peroxides . Characteristic: short crosslinks (butyl/ethyl) through chain transfer Macromolecular Chemistry | Dr. C. Merten, 2019 92 Free radical polymerization: Industrially relevant polymers Polyvinylchloride (PVC) . Commerciallized by IG Farben in 1931 . Polymerization either in suspension (75%), emulsion (15%), or bulk (10%) . In each case, atactic polymers with short syndiotactic sequences; crystallinity of 3-10% . Monomer with tendency for chain transfer, therefore polymerization at low temperatures . Suspension polymerization: 10-200 µm particles, between 30-130 kg/mol Commerically relevant: . Rigid PVC („Hart-PVC“): Tm ~ 150-200°C, processed > 220°C . Flexible PVC („Weich-PVC“): up to 50% softener added (e.g. diisopropyl phthalate) Both used for foils, flooring, cable insulation, tubing … Macromolecular Chemistry | Dr. C. Merten, 2019 93 Free radical polymerization: Industrially relevant polymers Polymethylmethacrylate
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