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B. Configuration in Chain-Growth Polymerization

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. 2) Radical Stability Considerations.

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B. Configuration in Chain-Growth Polymerization

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  1. 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

  2. 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

  3. H No resonance stabilization P CH C X 2 ______________________________________________ CH 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

  4. (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

  5. 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

  6. 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

  7. 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.

  8. 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

  9. 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)

  10. 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)

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

  12. 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!

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