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Radical Chain Polymerization: “Molecule ‘Empire Building’ by ‘Radical’ Groups”

Radical Chain Polymerization: “Molecule ‘Empire Building’ by ‘Radical’ Groups”. Chain-Growth Polymerization (Addition) Processes. 1. Free radical Initiation Processes. 2. Cationically Initiated Processes. 3. Anionically Initiated Processes. 4. Group Transfer Polymerization.

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Radical Chain Polymerization: “Molecule ‘Empire Building’ by ‘Radical’ Groups”

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  1. Radical Chain Polymerization: “Molecule ‘Empire Building’ by ‘Radical’ Groups” Chain-Growth Polymerization (Addition) Processes • 1. Free radical Initiation Processes • 2. Cationically Initiated Processes • 3. Anionically Initiated Processes • 4. Group Transfer Polymerization • 5. Coordination Polymerization

  2. Characteristics of Chain-Growth Polymerization 1. Only growth reaction adds repeating units one at a time to the chain 2. Monomer concentration decreases steadily throughout the reaction 3. High Molecular weight polymer is formed at once; polymer molecular weight changes little throughout the reaction. 4. Long reaction times give high yields but affect molecular weight little. 5. Reaction mixture contains only monomer, high polymer, and about 10-8 part of growing chains.

  3. The Chemistry of Free Radical Polymerization Radical Generation R R 2 R - Initiator Radicals Initiation R C C R C C + Monomers Propagation R C C C C R C C C + Termination C C C R R C C + R C C C C C R Polymer

  4. Free Radical Polymerization Mechanisms 1. Overview –Free radical polymerization processes involve at least three mechanistic steps. • A. Initiation • 1. Radical Formation (Generation) • D In In In In + h v , etc. 2. Initiation In M In M +

  5. B. Propagation In-M1. + M2 In-M1M2. In-M1M2. + M3 In-M1M2M3. In-M1M2M3…MX. + MY In-M1M2M3…MXMY.

  6. C. Termination 1) Radical Coupling (Combination) In-MX. + .MY-In In-MX-MY-In In In In In + 2) Disproportionation (-hydrogen transfer) H H H H In M In M C C C C + y x H H H H CH In M M In H C CH CH + x y 3 2 2

  7. D. Chain Transfer (sometimes)– An atom is transferred to the growing chain, terminating the chain growth and starting a new chain. Chain Transfer to Chain Transfer Agent: R P H R P + x + x Chain Transfer to Monomer: Px. + H2C=CH-(C=O)OR Chain Transfer to Polymer: Causes Branching H P P P P + x + y x y

  8. E. Inhibition and Retardation– a retarder is a substance that can react with a radical to form products incapable of reacting with monomer. An inhibitor is a retarder which completely stops or “inhibits” polymerization. 2. Monomers that are susceptible to free radical addition A. Vinyl Monomers H C CHX H C CH Cl 2 2 Vinyl chloride H F H X H F H Y Vinylidene fluoride

  9. B. Allyl Monomers Cl X Allyl Chloride C. Ester Monomers 1) Acrylates OR OH O O Acrylic Acid Acrylate Esters

  10. 2) Methacrylates O O OH OR Methacrylic Acid Methacrylate Esters 3) Vinyl Esters O Vinyl Acetate O D. Amide Monomers O O NH NH 2 2 Acrylamide Methacrylamide

  11. 3. Monomers that are not susceptible to Free Radical Addition A. 1,2-a-olefins (Polymerize to oils only) x B. Vinyl ethers R O O methyl vinyl ether C. 1,2-disubstituted Ethylenes Cl Cl 1,2-dichloroethylene H H

  12. 4. Initiation – “Getting the thing started!” • A. Radical Generators (Initiators) 1. Benzoyl Peroxide O O 0 80-90 C C O O C O 2 CO C O + 2 2 (continued)

  13. + New Active Site Initiator End-Group Ph Ph 2) t-Butyl Peroxide CH CH CH 3 3 3 0 0 120 -140 C H C C H C C O O C CH 2 3 3 3 CH CH CH 3 3 3 (continued)

  14. CH 3 O H C C + 3 O CH 3 O O 3) Azobisisobutyronitrile (AIBN) CH3 CH3 H3C – C – N=N – C – CH3 CN CN ~60oC or hn (continued)

  15. CH CH 3 3 Ph H C C H C C C CH N + 2 3 3 H 2 CN CN 4) Cumyl Hydroperoxide CH 3 C O OH OH + Ph O CH 3 (continued)

  16. (continued) O + Ph O O O Ph O O

  17. Hydroperoxides can generate radicals by “induced decomposition” from growing polymer chains: + P H O O R PH + O O R R OO 2 2 RO O + R-OO-OO-R 2 What effect does this have on the polymerization process? Acting as a chain-transfer agent, it reduces the degree of polymerization and molecular mass.

  18. 5) Redox Initiator Systems 2+ 3+ Fe HO + OH + Fe H O O H + OR 2- - O S O O SO SO SO + + 3 3 3 4 2- - SO + S-SO 4 3

  19. 6) Photoinitiators(Photocleavage – Norrish I) O O h v C HO + OH benzoin C H H C + Ph OH OH Ph Ph H

  20. (continued) OR O O h v C C O benzil 2 C

  21. 7) Photoinitiators (Photo-Abstraction) O Photosensitizer * O h v Ph Ph benzophenone excited state R R R R H C N OH + C N R R Coinitiator Ph Ph R R

  22. 5. Propagation- “Keeping the thing going!” A. The addition of monomer to an active center (free radical) to generate a new active center. H X R C C CH C R C CH 2 H H H 2 2 2 X X X H X X R C C CH C H H etc. etc. 2 2 n X X (continued)

  23. Examples: Polystyrene H Ph R C CH R C C CH C 2 H H H 2 2 2 n Ph Ph Ph O CH R C C CH 3 H H 2 2 O C O Polymethyl Acrylate O CH 3 H R C C C CH C H H H 2 2 2 C O C O O O CH CH 3 3

  24. B. Configuration in Chain-Growth Polymerization 1) Configuration Possibilities favored CH H C P C C -attack -attack H 2 H X 2 X P . H HC CH X P CH C 2 2 X X sterically and electronically unfavored

  25. 2) Radical Stability Considerations Which possible new active center will have the greatest stability? . P C CH P C CH 2 H H 2 2 P C CH -attack produces resonance stabilized free radical H 2

  26. H No resonance stabilization P CH C X 2 ______________________________________________ CH2 CH O 2  P CH HC C O CH X 3 O C O CH 3 P  P C CH H C C C O CH 2 3 H H H 2 C O O P C CH H H Secondary radical is resonance stabilized O CH 2 3 C O O CH 3

  27. (more examples) Cl Cl H  X P C CH 2 Cl H Cl P Cl H Cl  P C C H 2 Cl H Cl Cl Cl P C C P C C H H 2 2 Cl Cl Tertiary radical is resonance stabilized

  28. 3) Steric Hinderance Considerations HC CH X 2 X P • For large X, -substitution • is sterically favored CH H C 2 X 4) Radical Stability 3o > 2o > 1o

  29. 5 ) “Bottom Line” • Resonance and steric hinderance considerations lead to the • conclusion that -substitution(head-to-tail) is strongly • preferred in chain-growth polymerization. H H H H C C C C C C C C H H H H 2 2 2 2 X X X X Alternating configuration

  30. 6. Termination -“Stopping the thing!” A. Coupling (most common) H H P C P C C C + y x H H 2 2 X X H H P C P C C C y x H H 2 2 X X • - occurs head-to-head • produces two initiator fragments (end-groups) • per chain.

  31. B. Disproportionation H H H H In M In M C C C C + y x H H H H CH In M M In H C CH CH + x y 3 2 2 - Production of saturated chain and 1 unsaturated chain per termination - Produce one initiator fragment (end-group) per chain

  32. C. Factors affecting the type of termination that will take place. 1) Steric factors -large, bulky groups attached directly to the active center will hinder coupling 2) Availability of labile -hydrogens 3) Examples – PS and PMMA H H P C C P C C + x y H H 2 2 Combination (coupling) Polystyrene (continued)

  33. H H P P C C C C y x H H 2 2 Ph Ph Ph = CH3 H3C ~~~PX – CH2-C. + . C-CH2- PY~~~ C=O O=C O O CH3 CH3 PMMA • Sterically • hindered • 5 b-Hydrogens • Disproportion- • ation dominates (continued)

  34. CH3 H3C ~~~PX – CH2=C + HC-CH2- PY~~~ C=O O=C O O CH3 CH3 • Electrostatic Repulsion Between Polar Groups – • Esters, Amides, etc.

  35. Polyacrylonitrile (PAN) ~~~PX – CH2-CH. + . HC-CH2- PY~~~ d+ CN d- d- NC d+ One might assume electrostatic repulsion in this case. BUT, how about electrostatic attraction from the nitrogen to the carbon? Also, steric hindrance is limited. At 60oC, this terminates almost exclusively by coupling!

  36. D. Primary Radical Termination ~~~PX – CH2-CH.+ . In X ~~~PX – CH2-CH-In X More Likely at High [In.] So molecular mass can be controlled using chain-transfer agents, hydroperoxide initiators, OR higher levels of initiator!

  37. 7. Chain-Transfer -“Rerouting the thing!” • 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. ~~~PX – CH2-CH. + X-R  ~~~PX – CH2-CHX + R. Y Y B. Chain-transfer to solvent: ~~~PX – CH2-CH. + CCl4 ~~~PX – CH2-CHCl + Cl3C. Y Y

  38. C. Chain-transfer to monomer: ~~~PX – CH2-CH. + H2C =CH ~~~PX – CH2-CH2 + H2C =C. OR

  39. H H ~~~PX – CH - C. + H2C =CH ~~~PX – CH2=CH. + H3C - C.

  40. Propylene – Why won’t it polymerize with Free Radicals? ~~~PX – CH2-CH. + HCH=CH CH3 CH3 ~~~PX – CH2-CH2-CH3 + CH2=CH-CH2. H2C-CH-CH2  Chain-transfer occurs so readily that propylene won’t polymerize with free radicals.

  41. D. Chain-transfer to polymer: ~~~PX – CH2-CH2-CH2. + ~~~CH2-CH2-CH2~~~ ~~~PX – CH2-CH2-CH3+ ~~~CH2-CH-CH2~~~  Increases branching and broadens MWD! E. Chain-transfer to Initiator (Primary Radical Termination): ~~~PX – CH2.+ R-O-O-R  ~~~PX – CH2-OR + . OR

  42. F. Chain-transfer to Chain-transfer Agent: 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. Examples: R-OH; R-SH; R-Cl; R-Br ~~~PX – CH2-CH2.+ HS-(CH2)7CH3 ~~~PX – CH2-CH3+ . S-(CH2)7CH3 . CXH-CH2-S-(CH2)7CH3 etc., etc., etc.

  43. Inhibition and Retardation -“Preventing the thing • or slowing it down!” Definition – Compounds that slow down or stop poly- merization by forming radicals that are either too stable or too sterically hindered to initiate poly- merization OR they prefer coupling (termination) reactions to initiation reactions. ~~~PX – CH2-CH. + O= =O para-Benzoquinone Will Not Propagate ~~~PX – CH2-CH2-O- -O. ~~~PX – CH2-CH-O-O . ~~~PX – CH2-CH. + O=O

  44. Kinetics of Free Radical Polymerization 1. Initiation kd (RDS) I 2 R.Radical Generation ki R. + M M1.Initiation Assuming that ki >>kd and accounting for the fact that two Radicals are formed during every initiator decomposition, The rate of initiation, Ri, is given by: Ri = d[Mi] = 2fkd[I] dt f = efficiency of the initiator and is usually 0.3< f >0.8

  45. 2. Propagation kp M1. + M M2. kp We assume that the reactivity of the growing chain is independent of the length of the chain. M2. + M M3. kp M3. + M M4. . . . kp Mx. + M Mx+1. Rp = - d[M] = kp[M .][M] dt

  46. 3. Termination ktc Mx. + . My Mx-My (Combination) ktd Mx. + . My Mx + My (Disproportionation) Since two radicals are consumed in every termination, then: Rt = 2kt [M .]2 4. Steady State Assumption Very early in the polymerization, the concentration of radicals becomes constant because Ri = Rt  2fkd[I] = 2kt [M .]2

  47. 2fkd [I] = 2kt [M .]2 Solve this equation for [M.]: [M.] = (fkd [I]/kt)1/2 Substituting this into the propagation expression: Rp = kp[M.][M] = kp [M](fkd [I]/kt)1/2 Since the rate of propagation, Rp, is essentially the rate of polymerization, the rate of polymerization is proportional to [I]1/2 and [M].

  48. 5. Kinetic Chain Length, n Definition – The average number of monomer units polymerized per chain initiated. This is equal to the Rate of polymerization per rate of initiation: n = Rp/Ri = Rp/Rtunder steady state conditions. • = kp[M][M.] = kp[M] 2kt[M.]2 2kt[M.] n will decrease with increases in initiator concentration or efficiency. n = __kp[M]___ 2(f ktkd[I])1/2 DP = n if termination is exclusively by disproportionation. DP = 2n if termination is exclusively by coupling.

  49. 6. When Chain-transfer is Involved When chain-transfer in involved, the kinetic chain length must be redefined. Bottom Line: 1/ntr = 1/n + Cm[M] + Cs[S] + CI[I] [M] Where Cx = ktr, x /kp

  50. 7. Qualitative Effects – a Summary 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

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