MOLECULAR WEIGHT
       "Drop the idea of large molecules.
       Organic molecules with a molecular 
       weight higher than 5000 do not exist."
       —Advice given to Hermann Staudinger* 
            MOLECULAR WEIGHT
     WHY IS IT IMPORTANT ?
   TENSILE             MELT
   STRENGTH            VISCOSITY
                  MOL.WT.           MOL.WT.
             Molecular Weight - Some Initial Observations  
                                                How does the molecular weight of a high polymer differ
                                                 from that of a low-molecular-weight substance ?
                                                                                                            But for most polymers there is a
                                                                                                               Distribution of Chain Lengths
                  CH                                                     16
                       4
                  CH CH                                                  30
                      3        3                         Gases
                  CH CH CH                                               44
                      3        2        3
                  CH CH CH CH                                            58
                      3        2        2        3
                  CH (CH) CH                            Liquid           114
                      3          2 6        3
                  CH (CH) CH "Semi-solid" 450
                       3         2 30        3                                                                 We must therefore define an
                                                         Solid                                       Average Degree of Polymerization ( DP )
                  CH (CH)                   CH                            420030                 The average number of structural units in the polymer chain
                       3         2 30000        3
                                                                Increasing                                                          And an
                                                          Molecular Weight                                    Average Molecular Weight (M )
                             Molecular weight M simply:                                           The average degree of polymerization times the molecular
                                                                                                                     weight of the structural unit, M
                                                                                                                                                                 0
                   (# C atoms x 12) + (#  H atoms x 1)
                                     MOLECULAR WEIGHT
                                            12                    M ≅ 108,500
                                                                    n
                                                                    M  ≅ 118,200
                                            10                        w
                                             8
                                             6
                                        Moles
                                             4
                                             2
                                             00            100000          200000         300000
                                                                Molecular Weight
                                             Number Average                 Weight Average
                                                          N  M                                2
                                                       ∑                                N  M
                                              M  =           x   x        M   =  ∑ x          x
                                                 n       ∑ N                 w         N  M
                                                               x                    ∑ x x