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196 Carraher’s Polymer Chemistry
TABLE 6.2
Energies of Activation for Propagation (E ) and
p
Termination (E ) in Free Radical Chain Polymerization
t
Monomer E p (kJ/mol) E t (kJ/mol)
Methyl acrylate 30 22
Acrylonitrile 17 23
Butadiene 39 –
Ethylene 34 –
Methyl methacrylate 26 12
Styrene 33 12
Vinyl acetate 31 22
Vinyl chloride 15 18
TABLE 6.3
Typical Free Radical Kinetic Values
Specifi c Rate Activation Energies
Constant (kJ/mol)
−3
10 s −1 80–160
k d E d
10 L/mol-s 20–30
3
k i E i
3
10 L/mol-s 20–40
k p E p
10 L/mol-s 0–20
7
k t E t
Using only the specific rate constants involved with propagation from Equation 6.28 we have
2
kk
p d
R α
p
k t (6.33)
so that the overall activation energy using average values is a positive value (Equation 6.34) so the
overall rate of polymerization increases as temperature increases.
E p(overall) α 2E + E – E = 2 × 30 + 120 – 10 = 170 (6.34)
d
p
t
For chain length, from Equation 6.30 we have
k p
DP α
2kk
d t (6.35)
so that the overall activation energy using average values is
DP (overall) α E – E – E = 30 – 120 – 10 = –100 (6.36)
p
t
d
so that DP decreases at temperature increases as pictured in Figure 4.4.
The Gibbs free-energy relationship for a reversible process at constant temperature for polymer-
ization is described by
ΔG = ΔH – TΔS p (6.37)
p
p
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