Stability of radicals 2 resonance

1
Stability of radicals 2-resonance
Conjugation to double and triple bonds stabilizes
radicals
2
Hyperconjugation

• Important for reactive species like radicals
• Like p-conjugation through double/triple bonds
  but through s-bonds

More resonance forms as number of b Hs increases
3
Sterics

• Not all stabilities/reactivities reconcilled by
  electron distribution models
• Sterics very important
• hindered radicals are stabilzed

NO conjugation between rings possible Puckering
takes up space
4
Reactions of radicals
Combination R. R.
Abstraction R.
RX RX
R. Addition R.
Addition-fragmentation
R-R
5
Examples
a) termination of
styrene

polymerization

b) halogenation
6
Examples

• Transfer to thiol in
• polymerization of methyl methacrylate

reinitiates
b) Formation of short branches in Polyethylene
synthesis
7
Examples

• Reinitiation of MMA following abstraction

b) Propagation in radical polym.
8
c) cascade cyclizations
9
Examples
New radical-kinetic chain continues
10
The RAFT process Radical Addition Fragmentation
Termination Rizzardo, Moad, Thang et al
11
In Polymerization
Initiation Addition
Propagation Addition
Termination Copmbination
12
Transfer