Heat of reaction and entropy changes during polymerization

2 HEAT OF REACTION AND ENTROPY CHANGES DURING POLYMERIZATION 

For a hydrocarbon monomer whose double bond is not affected by substituents (ethylene), the heat of polymerization should be about —84+8 kJ mol-1, corresponding to the difference between the mean energy of a C=C bond (610 kJ mol-1) and that of two C—C bonds in the polymer (2 x 347 = 694 kJ mol-1). 

Several procedures exists for calculating the heats of polymerization for various monomers but they are not very important. It is still more useful to consider the experimentally measured values as the best basis both for theoretical studies and for practical calculations. For ethylene derivatives, the value of — AHlc (from liquid monomer to amorphous solid polymer) ranges from 33.5 kJ mol-1 for a-methylstyrene, through ca. 96 kJ mol-1 for vinyl chloride, to 174 kJ mol-1 for tetrafluoroethylene. 

All structural effects decreasing the heat of polymerization are cumulated in a-methylstyrene resonance of the double bond with the aromatic ring, —CH3 hyperconjugation stabilizing the monomer, and polymer destabilizing 1,1-disubstitution. The same is true of methacrylic acid and of all its derivatives (AHlc between 54 and 59kJmol-1) and partly even of vinylidene chloride (AHlc = 75.4 kJ mol-1). 

Saturated cyclic monomers which are opened during polymerization exhibit heats of polymerization very close to ring strain values [7]. 

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