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Lecture 5 Polymerization Reactions

Polymer Science and Engineering I. Lecture 5 Polymerization Reactions. Condensation Polymerization. Condensation Polymerization Characteristics. Gives off a small molecule (often H 2 O ) as a byproduct Stepwise polymerization long reaction times

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Lecture 5 Polymerization Reactions

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  1. Polymer Science and Engineering I Lecture 5 Polymerization Reactions

  2. Condensation Polymerization

  3. Condensation Polymerization Characteristics • Gives off a small molecule (often H2O) as a byproduct • Stepwise polymerization • long reaction times • Bifunctional monomers linear polymers • Trifunctional monomers cross-linked (network) polymers

  4. Polymerization mechanisms - Step-growth polymerization

  5. Condensation Polymerization • Examples: • Polyesters • Nylons • Polycarbonates

  6. Polyesters from ahydroxy-acid • Acid and base functionality on one monomer: e.g., n .HO-CH2CH2CH2CH2COOH -(-CH2CH2CH2CH2COO-)n- + nH2O • General reaction: n . HO-R-COOH  -(-R-COO-)n- + nH2O

  7. Condensation Polymerization — Esterification • Polyester from a di-alcohol and a di-acid • Example (Callister eq. 16.8):

  8. Another polyester: PETE • terephthalic acid+ ethylene glycol n.HOOC-C6H4-COOH + n.HO-CH2CH2-OH  -(-OOC-C6H4-COO-CH2CH2-)n + 2nH2O

  9. Nylon 6 frompolymerization of an amino acid • acid and base groups on one monomer H2N-(CH2)5-COOH n H2N-(CH2)5-COOH  (-(CH2)5-CONH)n + nH2O • 6-carbon monomer  6-carbon mer

  10. Nylon 6,6— 2 monomers: 6-carbon diamine + 6-carbon diacid hexamethylene diamine + adipic acid

  11. Problem — Nylon 6,6 Calculate the masses of hexamethylene diamine and adipic acid needed to produce 1000 kg of nylon 6,6. • Solution: • The chemical reaction is: nH2N-(CH2)6-NH2 + nHOOC-(CH2)4-COOH -(-HNOC-(CH2)4-CONH-(CH2)6-)n+ 2nH2O • The masses are in proportion to molecular weights. Per merof nylon 6,6: C6H16N2+ C6H10O4C12H22O2N2 + 2H2O 72+16+28 + 72+10+64 144+22+32+28 + 2x18 116 + 146 226 + 36 116x103kg146x103kg103 kg 226 226 513 kg HMDA + 646 kg adipic acid Note: 159 kg of H2O byproduct

  12. Free Radical Polymerization

  13. Polymerization mechanisms - Chain-growth polymerization

  14. Characteristics of Chain Reaction • each polymer chain grows fast. Once growth stops a chain is no longer reactive. • growth of a polymer chain is caused by a kinetic chain of reactions. • chain reactions always comprise the addition of monomer to an active center (radical, ion, polymer-catalyst bond). • the chain reaction is initiated by an external source (thermal energy, reactive compound, or catalyst).

  15. Stages of Free Radical Polymerization • initiation (start) • propagation (growth) • chain transfer (stop/start) • termination (stop) • During initiation active centers are being formed. • During termination active centers disappear. • Concentration of active centers is very low (10-9 - 10-7 mol/L). • Growth rate of a chain is very high (103 - 104 units/s). • Chains with a degree of polymerization of 103 to 104 are being formed in 0.1 to 10 s.

  16. Example Initiators Benzoyl Peroxide Azobisisobutyronitrile (AIBN)

  17. Initiation Kinetics The rate of radical production is then given by:

  18. The actual rate of initiation Ri is expressed in terms of the rate of radical production that leads to actual polymer chains growing!: where f is the efficiency factor: the fraction of radicals that really leads to initiation. The rate constant ki is NOT used in the mathematical description of the polymerization.

  19. Propagation This reaction is responsible for the growth of the polymer chain. It is the reaction in which monomer is added at the active center: The rate of this reaction Rp can be expressed as:

  20. Termination • Chain growth stops by bimolecular reaction of two growing chain radicals: • termination by combination (ktc) • termination by disproportionation (ktd) The general kinetic equation reads:

  21. Termination Every reaction consists of two steps: 1) approach of both reactants 2) chemical reaction The second step in the termination reaction is very fast. This means that the rate of approach (significantly) determines the overall termination rate.

  22. Termination: which is faster? 1. + or + 2. + or + in a viscous medium in a non-viscous medium 3. + or + at 5 % conversion at 85 % conversion

  23. Polymerization Kinetics The rate of polymerization in a chain growth polymerization is defined as the rate at which monomer is consumed. Since for the production of high molar mass material Rp » Ri this equation can be re-written as: • From the beginning of the polymerization: • increasing number of radicals due to decomposition of the initiator • increasing termination due to increasing radical concentration (Rtµ [M·]2) • eventually a steady state in radical concentration:

  24. This steady state assumption leads to: From which the differential rate equation is derived: At low conversion this means: log(Rp) vs log[M] yields a slope = 1 log(Rp) vs log[I] yields a slope = 0.5

  25. The number average degree of polymerization Pn of chains formed at a certain moment is dependent on the termination mechanism: * combination: Pn = 2 * disproportionation: Pn =  chemistry:

  26. Conversion Regimes • low conversion polymer chains are in dilute solution (no contact among chains) • “intermediate” conversion • High conversion chains are getting highly entangled; kp decreases.

  27. Trommsdorff effect Somewhere in the “intermediate” conversion regime: * Polymer chains loose mobility; * Termination rate decreases; * Radical concentration increases; * Rate of polymerization increases; * Molar mass increases; This effect is called: gel effect, Trommsdorff effect, or auto-acceleration

  28. Chain Transfer • Definition – The transfer of reactivity from the growing polymer chain to another species. An atom is transferred to the growing chain, terminating the chain and starting a new one. • Chain Transfer agents are added to control molecular weight and branchning

  29. Branching: Chain Transfer to Polymer

  30. Qualitative Kinetic Effects FactorRate of RxnMW [M] Increases Increases [I] Increases Decreases kp Increases Increases kd Increases Decreases kt Decreases Decreases CT agent No Effect Decreases Inhibitor Decreases (stops!) Decreases CT to Poly No Effect Increases Temperature Increases Decreases 8/24/2014 32

  31. Thermodynamics of Free Radical Polymerization DGp = DHp - TDSp DHp is favorable for all polymerizations and DSp is not! However, at normal temperatures, DHp more than compensates for the negative DSp term. 8/24/2014 33

  32. Ceiling Temperature The Ceiling Temperature, Tc, is the temperature above which the polymer “depolymerizes”:

  33. . . . Thermodynamic rational Gp = Hp - TSp • Hp is favorable for all polymerizations and Sp is not! • At operational temperatures, Hp exceeds the negative TSp term.

  34. At and above Tc • At Tc , Gp= 0 •  Hp - Tc Sp = 0 • Hp = TcSp •  Tc = Hp/ Sp

  35. Ceiling Temperature • Depropagation has larger S • ∵ TS term increases with T ∴ T  increase; kdp increase • At T = Tc (i.e. ceiling temperature) Rp = Rdp

  36. Comparison

  37. Coplymerization

  38. Copolymers two or more monomers polymerized together random – A and B randomly positioned along chain alternating – A and B alternate in polymer chain block – large blocks of A units alternate with large blocks of B units graft – chains of B units grafted onto A backbone A – B – random alternating block graft 40

  39. Copolymers: Types Homopolymer Alternating Copolymer Random Copolymer Block Copolymer Homopolymer

  40. http://www.extremetech.com/computing/130638-mit-creates-self-assembling-3d-nanostructures-could-be-the-future-of-computer-chipshttp://www.extremetech.com/computing/130638-mit-creates-self-assembling-3d-nanostructures-could-be-the-future-of-computer-chips

  41. Example Copolymer

  42. Reactivity Ratios k11 k12 k21 k22 —M1• + M1 —M1• —M1• + M2 —M2• —M2• + M1 —M1• —M2• + M2 —M2• } }

  43. Reactivity and Type of Copolymers • Case 1: r1=0 and r2=0 • Each comonomer prefers to react with the other. • Perfectly alternating copolymer. • Case 2: r1 > 1 and r2 > 1 • Each comonomer prefers to react with others of its own kind. • Tendency to form block copolymers. • Case 3: r1 * r2=1 • There is no preference due to the chain ends. • Random incorporation of comonomers. • "Ideal" copolymerization.

  44. Typical Reactivity Ratios r1 and r2 for pairs of monomers. Note: Data are for free radical copolymerization under standard condition

  45. Monomer Reactivity and Compostion Reactivity Ration characterizes the reactivity of the 1 radical with respect to the two monomers, 1 and 2 then homopolymerization growth is preferred then only reaction with 2 will occur Composition f1, f2 : mole fractions of monomers in feed F1, F2 : mole fractions of monomers in polymer …… ③ ……④ ……⑤ From ③, ④

  46. Ideal Copolymerization Ideal Copolymerization where The two monomers have equal reactivity toward both propagating species random copolymer

  47. Ideal Copolymerization 1 F1 1 0 f1

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