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Chap 9. Chain-growth Polymerization

1. Free radical Initiation Processes. 2. Cationically Initiated Processes. 3. Anionically Initiated Processes. 4. Group Transfer Polymerization. 5. Coordination Polymerization. Chap 9. Chain-growth Polymerization. Chain-Growth Polymerization (Addition) Processes.

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Chap 9. Chain-growth Polymerization

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  1. 1. Free radical Initiation Processes • 2. Cationically Initiated Processes • 3. Anionically Initiated Processes • 4. Group Transfer Polymerization • 5. Coordination Polymerization Chap 9. Chain-growth Polymerization Chain-Growth Polymerization (Addition) Processes

  2. Chain-Growth Polymerization • Only growth reaction adds repeating units one at a time to the chain • Monomer concentration decreases steadily throughout the reaction • High Molecular weight polymer is formed at once; polymer molecular • weight changes little throughout the reaction. • Long reaction times give high yields but affect molecular weight little. • Reaction mixture contains only monomer, high polymer, and about 10-8 • part of growing chains. Step-Growth Polymerization • Molecular weight increases steadily. • High molecular weight polymers are found at the end. • Long reaction time needs to synthesize high conversion and high molecular weight.

  3. Bond Energy= 46 kcal/mole Heat (60ºC) UV kd Primary radical AIBN Chain Growth Polymerization (1) Initiation kd : Initiator decomposition rate constant : 10-4 ~ 10-6 L/mole sec Unstable radical

  4. Chain Growth Polymerization (2) Propagation (Repetition of similar reaction) kp : 102 ~ 104 L/mole sec (much faster than step-growth polymerization)

  5. Chain Growth Polymerization (3) Termination (a) Coupling or combination

  6. Chain Growth Polymerization (3) Termination (b) Disproportionation kt=ktc+ktd 106 ~ 108 L/mole sec

  7. Chain Growth Polymerization (4) Chain Transfer Physical chain length Monomer, Polymer, Solvent or Chain transfer agent Kinetic chain length

  8. Chain Growth Polymerization • Kinetic Chain Length : • kinetic chain length υ of a radical chain polymerization is defined as the average number of monomer molecules consumed (polymerized) per each radical, which initiates a polymer chain. • ex) Monomer # 4000 Disproportionation • υ =4,000/4 =1,000 • Determined by steps 1, 2, 3. (Initiation, propagation, and termination) • (No chain transfer) • Physical Chain Length : • This condition contains Step 1, 2, 3, 4. Radical 1,2,3,4

  9. Non-Polymerization Reaction • Peroxide induced Bromination of Toluene • 1) Initiation • Two types of reaction •  R-O-O-R 2RO• (1) •  R-O• + Br2 ROBr + Br• (2) •  R-O• + ФCH3 ROH + ФCH2•(3) • Two radicals and two kinetic chains are formed by decomposition of • each ROOR molecules Kinetic Chain Reaction

  10. Kinetic Chain Reaction • 2) Propagation •  Br• + ФCH3 HBr + ФCH2 (4) •  ФCH2• + Br2 ФCH2Br + Br • (5) • Two special features  The number of active species is fixed. •  Same reactions are repeated during the kinetic chain reaction.

  11. Kinetic Chain Reaction • 3) Termination •  2 Br• Br2 •  2ФCH2• ФCH2 CH2Ф •  ФCH2• + Br • ФCH2Br + Br • • Net Effect of Kinetic Chain Reaction: • One ROOR molecule can cause formation of Br2, CH2CH2, CH2Br,HBr, ‥.

  12. Kinetic Chain Reaction • Comparison between Chain Polymerization & Chain Reaction Init. propagation termination Chain reaction Ri= Rt Reaction Rate Steady state Time Induction period In proportion to the O2 concentration

  13. Kinetic Chain Reaction • In the case of Chain reaction, there are induction periods, due to the existence of inhibitor. • When an active center is formed, the reaction rate would be faster and then go to steady state. • The whole reaction rate is reaching a plateau region. • After that, reaction rate decreases due to a loss of monomers or initiators. • Linear Chain-Growth: • Polymer of high DPn found easily in early reaction • Linear Step-Growth: • high extent of reaction value required to obtain high DPn

  14. Kinetic Chain Reaction • Comparison Free Radical Reaction &Ionic Reaction • - Ionic Initiation – multiple bond addition,ring opening polymerization • - Radical Initiation – Ring-opening polymerization is not initiated. For the cationic initiation, it will not be free radical. isobutylene Ex) Because of resonance stability

  15. Kinetic Chain Reaction • Comparison between Free Radical Reaction &Ionic Reaction

  16. Kinetic Chain Reaction • Comparison between Free Radical Reaction & Termination Step of Ionic Reaction • A) Free Radical Termination Two molecules involved = bimolecular reaction

  17. Kinetic Chain Reaction • B) Cationic Termination Anionic capture is similar to combination offree radical reaction. But, this reaction can’t include increasing of MW because of unimolecular reaction

  18. Kinetic Chain Reaction • The proton release is similar to disproportination offree radical.. • But, one chain joins in the reaction unimolecular reaction • C) Anionic Termination

  19. Kinetic Chain Reaction

  20. Kinetic Chain Reaction • Free Radical Initiated Polymerization of Unsaturated monomers • Kinetic Scheme • Initiation • Two step sequence-Both enter into overall rate • Initiator decomposition • I2 2I • 2. Addition of Initiator fragment to the monomer, Initiation of Chain growth. • I+M IM • The efficiency of Initiator - Determined by competition of desired reaction and side reaction kd ki Primary radical species Generally, 0.5 << f << 1

  21. Kinetic Chain Reaction • A.Cage Effect –primary recombination • Initiator fragments surrounded by restricting cage of solvent • Ex) • (acetyl peroxide)

  22. Kinetic Chain Reaction • I) Recombination possible I2 2I • II) If elimination reaction occurs while the free radical in-cage, • Formation of stable molecules due to Radical combination. • And formation of Inactive Species.

  23. Kinetic Chain Reaction B. Induced Decomposition –Secondary combination I) Through Radical attack on peroxide molecules R + R-O-O-R RH + ROOR R=O + RO Finally, R + ROOR ROR+ RO Total number of radical does not change, but among them half molecules were wasted. II) Chain Transfer to Solvent (In this case, since just one radical was obtainedhalf molecules were wasted.)

  24. . . I + M I M n . I I M n I . I + + I M n 2 Kinetic Chain Reaction • III) Reaction with Chain Radical • Since not all Molecules participate in the initiation →Efficiency factor • f: Initiator Efficiency • = mole fraction of initiator fragments that actually initiate polymer chains. • 0.5 < f < 1.0

  25. Kinetic Chain Reaction C. Reaction Rate If [M] is representative for the concentration of chain radical, That is , M = IM or = I [M] f  1 Riis unrelated with [M] f=[M] f < 1 Ri is related with [M] [M] , f [I2] , f due to induced decomposition by convention, tworadical formation.

  26. Kinetic Chain Reaction • D. Initiator • - containing compounds. Acetyl peroxide 80~100C Benzoyl peroxide 80~100C • Cumyl peroxide 120~140C

  27. Kinetic Chain Reaction • t-butyl peroxide • Hydroperoxides, cumyl or t-butyl • 80~100C 50~70C • AIBN 2,2 azobisisobutyronitrile

  28. . . M M k k tc td Kinetic Chain Reaction • Propagation • Termination By convention Since 2 radical elimination

  29. Kinetic Chain Reaction • Overall Rate of Polymerzation • Radical concentration • Difficulty of measurement, low concentration. (~10-8molar) • Thus, it is impractical using this therm. • [M]elimination is desirable. (# of propagation step >>> # of initiation step)

  30. Kinetic Chain Reaction [M]elimination methods Steady-State Assumption Radical concentration increases at the start, comes to steady state simultaneously and thenreaction rate change becomes 0. (active centers created and destroyed at the same time) Ri = Rt

  31. MMA using BPO Rp Vinyl Acetate using AIBN [I2]1/2 H . Kinetic Chain Reaction Mostly in case of f<1system → [I2]1/2 (Square Root Dependence of [I2]) ※ Odian Fig. 3-4 -CO2 2 300C BPO + N2 Azobisisobutyronitrile

  32. + BPO + C H 3 Kinetic Chain Reaction In case f < 1, but SRD is not applicable, Because f is‘dependent’ on [M] Why? Due to induced decomposition of toluene + [I2]

  33. Kinetic Chain Length (KCL) At S-S assumption (1) Disproportionation Knowing that (2) Coupling or combination

  34. Kinetic Chain Length (KCL) (3) Both (1)+(2)

  35. Kinetic Chain Length (KCL) Degree of Polymerization • The more concentration of monomer, • The less concentration of initiator, Monomer consumption rate polymer formation rate (1) Dispropotionation (2) Coupling

  36. Kinetic Chain Length (KCL) (3) Dispropotionation& Coupling Polymer formation rate Monomer consumption rate

  37. Kinetic Chain Length (KCL) • From (1),(2),(3) • In case of noChain transfer, and valid S-S assumption

  38. Chain Transfer • M + XY MX + Y • Chain transfer agent • If Chain transfer occurs Rpisunchangable buthas an effect on DPn • (∵ Since [Y] instead of Rp=kp[M][M] ) ex) • (1) Chain transfer occurs by solvents or additives • In this case,Highchain transfer coefficient. • (2) Transfer occurs by monomer or polymer

  39. Chain Transfer Inhibitor and Retarder • Inhibitor • When Y take part in chain opening reaction, polymer moves from one site to another. • In this case, hydroquinone etc. are used as inhibitor. • Retarder • When the reactivity of Y is low, controlling the MW of the monomer including • these two materials, Mercaptan etc. are used as Retarder. • Like this, when chain transfer condition arises

  40. Chain Transfer 1 See Odian P.235 DPn From the slope of a graph Chain transfer coefficient ‘Cs’ [5]/[M]

  41. Temperature Dependence of Rp and DPn • Assume : no chain transfer

  42. Temperature Dependence of Rp and DPn • slope of lnRp/T is ( + ) • as T  lnRp • but Rate of Increase  as d lnRp/dT 

  43. Temperature Dependence of Rp and DPn

  44. Ceiling Temperature Polymer-Depolymerization Equilibria ㆍCeiling Temperature Polymerization and Depolymerization are in equilibrium ΔGp = ΔHp – TΔSp ΔHp : Heatof polymerization ΔSp : Molecular arrangement changes between monomer andpolymer At eq. State ΔGp=0 Monomers can no longer be persuaded to form polymers by chain polymerizationabove a certain temperature.ceiling Temperature(Tc)

  45. Rate Eq. of Polymerization Reactions at Depolymerization prominent Temperature If, M Tc

  46. Ceiling Temperature Polymer-Depolymerization Equilibria k sec-1 kdp Tc : No reaction above Tc Stable blow Tc kp[M] kp[M]- kdp 300 400 500 Tc

  47. Ceiling Temperature Polymer-Depolymerization Equilibria • ※Odian Fig 3-18 • Entropy changes for all polymers are not so different. • Sp= Sp- Sm (–) value of Sp is higher • Hp= Hp- Hm if (–) , exothermic.

  48. Trommsdorff Effect or Gel Effect The increasing viscosity limits the rate of termination because of diffusional limitations restricted mobility of polymer radical kp( relative to [M] ) ( ∵ kp const. in reaction progress, ktdrop off in reaction progress) → Autoaccerelation effect

  49. Trommsdorff Effect or Gel Effect one would expect ξ  as t Butξ  as [[M0]  80%  60% • autoacceleratioan 40% as [M0] drastic in . 10% t

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