Polymerization propagation reaction enthalpy

For long polymer chains the enthalpy and entropy changes in the propagation reaction are effectively those of the overall polsrmerization reaction (113,422). The polymerization enthalpies AHp of most free radical polymerizations are negative, with typical values of —30 to —100 kJ mol as can be seen in Table 8. 

In the kinetically controlled regime, the chain propagation reaction is predominant and a high molecular-weight polymer (P ) is formed (Equation 12.1). The monomer-polymer equiUbrium (Equation 12.2) is characterized by the equilibrium constant (K) which is approximately equal to the inverse equilibrium monomer concentration ([M]e) (Equation 12.3). Equation 12.4 relates the equilibrium monomer concentration and the free enthalpy of polymerization (AG°). For standard conditions ([M]o = 1 mol L ) and above a critical temperature, no polymer is obtained. The critical temperature is defined as ceihng temperature (Tc) when both AH° and AS° are negative (Equation 12.5). 

In general, it is clear that the thermodynamics controls the polymerization reactions and, in most cases, that polymerization will lead intrinsically to a decrease in the entropy of the system. To compensate for this, and in order to maintain a negative free energy, the enthalpy also should be negative this means that the monomers represent molecules that produce heat upon polymerization, and is especially evident in the case of chain-growth monomers. It follows that heat production during the propagation steps represents one of the most important features in a polymerization process. Indeed, it is the ability to control such heat production -both locally and totally - that has led to the success of the micro-reactors. 

Temperature, in terms of temperature, the first consideration in carbocationic polymerization—as in any enthalpy-driven chain reaction—is the ceiling temperature T. is the limiting temperature above which the rate of depropagation exceeds the rate of propagation at a given monomer concentration ... 

Radical Addition to C=C Bonds. Radical addition to C=C bonds are of importance for free-radical polymerization as this reaction forms the propagation step, and thus influences the reaction rate and molecular weight distribution in both conventional and controlled free-radical polymerization, and the copolymer composition and sequence distribution in free-radical copolymerization. Numerous studies have examined the applicability of high level theoretical methods for stud5dng radical addition to C=C bonds in small radical systems (32,33,37,93,94). The most recent study (37) included W1 barriers and enthalpies, and geometries and frequencies at the CCSD(T)/6-31 lG(d,p) level of theory, and is the highest level study to date. The main conclusions from this study, and (where still relevant) the previous lower level studies, are outlined below. 

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