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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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         K10478.indb   196                                                                    9/14/2010   3:39:33 PM
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