Propagation polymerization thermodynamics

Polymerization thermodynamics has been reviewed by Allen and Patrick,323 lvin,JM [vin and Busfield,325 Sawada326 and Busfield/27 In most radical polymerizations, the propagation steps are facile (kp typically > 102 M 1 s l -Section 4.5.2) and highly exothermic. Heats of polymerization (A//,) for addition polymerizations may be measured by analyzing the equilibrium between monomer and polymer or from calorimetric data using standard thermochemical techniques. Data for polymerization of some common monomers are collected in Table 4.10. Entropy of polymerization ( SP) data are more scarce. The scatter in experimental numbers for AHp obtained by different methods appears quite large and direct comparisons are often complicated by effects of the physical state of the monomei-and polymers (i.e whether for solid, liquid or solution, degree of crystallinity of the polymer). 

Polymerization thermodynamics has been reviewed by Allen and Patrick, Ivin, " Ivin and Busfield, Sawada, and Busfield. In most radical polymerizations, the propagation... 

Lack of termination in a polymerization process has another important consequence. Propagation is represented by the reaction Pn+M -> Pn+1 and the principle of microscopic reversibility demands that the reverse reaction should also proceed, i.e., Pn+1 -> Pn+M. Since there is no termination, the system must eventually attain an equilibrium state in which the equilibrium concentration of the monomer is given by the equation Pn- -M Pn+1 Hence the equilibrium constant, and all other thermodynamic functions characterizing the system monomer-polymer, are determined by simple measurements of the equilibrium concentration of monomer at various temperatures. 

Projections, linearly independent, 293 Propagation, of polymerization, 158 Propane, hydrate, 10, 33, 43, 46, 47 hydrate thermodynamic data and lattice constants, 8 + iodoform system, 99 Langmuir constant, 47 water-hydrogen sulfide ternary system, 53... 

Propagation reactions in radical polymerization and copolymerization arc generally highly exothermic and can be assumed to be irreversible. Exceptions to this general rule arc those involving monomers with low ceiling temperatures (Section 4.5.1). The thermodynamics of copolymerization has been reviewed by Sawada.85... 

Thus with aMeSt, the kinetic chain is relatively short, monomer is consumed mainly by initiation and propagation, and chain transfer by the HSiCCHj CH H C Q initiator is unfavorable (see Sect. III.B.3.b.i.). In contrast, with isobutylene the kinetic chain may live longer because it is sustained by thermodynamically favorable chain transfer by the initiator. Scheme 5 illustrates the mechanism of isobutylene polymerization by the HSi(CH3)2CH2CH29>CH2Cl/Me3Al system. The kinetic chain is sustained by chain transfer loops shown on the left margin of the Scheme. 

Thus propagation must be much faster than isomerization, and the product will be determined by thermodynamics, rather than by reaction kinetics. The net results of the two processes may be quite similar, however, in that polymers of unexpected structures may be obtained, and copolymers may be prepared by polymerization of a single monomer. 

The low tendency of 1,2-disubstituted ethylenes to polymerize is due to kinetic considerations superimposed on the thermodynamic factor. The approach of the propagating radical to a monomer molecule is sterically hindered. The propagation step is extremely slow because of steric interactions between the P-substituent of the propagating species and the two substituents of the incoming monomer molecule ... 

The first basic approach to the thermodynamics of addition polymerization was presented in 1948 by Dainton and Ivin (7) and developed in their review paper (2) published ten years later. In their exposition, they stressed the significance of the propagation step in addition polymerization, emphasizing its critical role in the whole process. This is the step whereby the macromolecule is gradually formed by the sequence of reactions... 

The degenerative nature of propagation results in reformation of the same active species, but with monomer consumption and chain growth. Although the monomer s thermodynamic polymerizability is independent of the mechanism, the mechanism and structure of the active species determines the rate of monomer conversion. The structure of the active species involved in carbocationic polymerizations was discussed in Section II detailed information on the reactivities of model species was presented in Chapter 2, with the conclusion that covalent precursors do not react directly with alkenes, but must first ionize to sp2-hybridized carbenium ions. Only the resulting carbenium ions can add to double bonds. 

Acidic (toward THF) solvents form with THF loose complexes (e.g. the enthalpy of mixing, AH, of THF with CCI4 is equal to -0.7 kcal mole and with CH2CI2 as much as -1.2 kcal mole Thus, on the molecular level, the observed dependences imply that the actual momentary concentratton of THF available for polymerization (its activity in the thermodynamic sense) b lowered by the fraction of complexed monomer, provided that the complexed monomer propagates less rapidly. 

In the polymerization of dialkyldichlorosilanes with sodium in refluxing toluene, propagation and a concurrent back-biting reaction to cyclic material could give the range of products found if the products are kinetically, instead of thermodynamically, determined (12). No evidence for depolymerization has been found for the reaction in toluene solution. 

Anionic polymerization carried out under suitable conditions results in the formation of living poly-mers—i.e. species which may grow further, if a suitable monomer is present in the system. This characteristic feature of living polymers, which arises from the elimination of all the termination steps, permits the following preparation of block polymers, polymers possessing two terminal functional groups, monodispersed polymers, etc. studies of the thermodynamics of the propagation step—i.e. determination of A / < cl aS of the... 

Chain propagation of CO/ethylene copolymerization proceeds by a strictly alternating insertion of CO and olefin monomers in the growing chain. It is safe to assume that double CO insertion does not occur for thermodynamic reasons [Ic]. However, the complete absence of double ethylene insertions is remarkable because ethylene insertion in a Pd-alkyl species must be exothermic by about 20 kcal/mol (84 kJ mol). The observation of strict alternation is the more surprising since the same palladium catalysts also efficiently dimerize ethylene to butenes [25]. The perfect alternation is maintained even in the presence of very low concentrations of carbon monoxide. When starting abatch polymerization at a high ethylene/CO ratio, error-free copolymer is produced until all the CO is consumed then the system starts forming butenes (with some catalyst systems at about twice the rate of copolymerization ). 

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