Thermodynamics
In chain growth polymerization, the position of the equilibrium between polymer and monomers can be determined by the thermodynamics of the polymerization. The Gibbs free energy (ΔGp) of the polymerization is commonly used to quantify the tendency of a polymeric reaction. The polymerization will be favored if ΔGp < 0; if ΔGp > 0, the polymer will undergo depolymerization. According to the thermodynamic equation ΔG = ΔH - TΔS, a negative enthalpy and an increasing entropy will shift the equilibrium towards polymerization.
In general, the polymerization is an exothermic process, i.e. negative enthalpy change, since addition of a monomer to the growing polymer chain involves the conversion of π bonds into σ bonds or an ring opening reaction that releases the ring tension in a cyclic monomer. Meanwhile, during polymerization, a large amount of small molecules are associated, losing rotation and translational degrees of freedom. As a result, the entropy decreases in the system, ΔSp < 0 for nearly all polymerization processes. Since depolymerization is almost always entropically favored, the ΔHp must then be sufficiently negative to composite for the unfavorable entropic term. Only then will polymerization be thermodynamically favored by the resulting negative ΔGp.
In practice, polymerization is favored at low temperatures: TΔSp is small. Depolymerization is favored at high temperatures: TΔSp is large. As the temperature increases, ΔGp become less negative. At certain temperature, the polymerization reaches equilibrium (rate of polymerization = rate of depolymerization). This temperature is called the ceiling temperature (Tc). ΔGp = 0
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