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Free radical polymerization entropy

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. [Pg.33]

Table III - Experimental activation entropy AS and calculated entropy change ASp for the chain propagation in the free radical polymerization of some monomers. Table III - Experimental activation entropy AS and calculated entropy change ASp for the chain propagation in the free radical polymerization of some monomers.
Turning to the comparison between the rate constants for the chain propagation in the free radical polymerization of methyl and butyl acrylayes, it can be observed that both these reactions should occur with the same entropy decrease, because identical double bonds are involved. From the experimental data by Melville and Bickel (1 3) and by Bengough and Melville (14) relative to butyl acrylate, 4 pairs of activation energy and entropy can be calculated, which are collected in Table IV. It is evident that the experimental activation entropy which is closest to the calculated ASp for alkyl acrylates (i.e. the ASp value reported for methyl acrylate in Table III) is -12.+. f j/mol K, whereas all the other activation entropies seem to be too high. The rate constant calculated at JO°C from... [Pg.22]

S(conj) is the decrement of the molar entropy of the monomer due to conjugation effects which are lost in the reaction a is the symmetry number of the monomer molecule and v is the number of optical isomers which can be formed in the reaction. For the intermolecular chain propagation reaction occurring in the free radical polymerization of DPHD the following values must be used ... [Pg.203]

The entropy change accompanying the ring closure in the free radical polymerization of DPHD is considered (8,1) as similar to the entropy change accompanying the following reaction carried out in the ideal gas phase ... [Pg.204]

The stereocontrol of the free radical polymerization of a monomer at a given temperature is still weakly dependent on the solvent. In this case, however, there is a linear relationship between activation enthalpy differences and the corresponding activation entropy differences for each of the possible six differences of the total four possible elementary steps of a first-order Markov statistics (Figure 20-8). These relationships are each independent of the solvent used, and, so, also, of conversion. The straight lines are parallel to each other, that is, the compensation temperature is independent of the kind of diad formation occurring. The stereocontrol for methyl methacrylate at this temperature of about 60° C is therefore independent of the solvent used. [Pg.232]

Free radical polymerization is generally exothermic because it involves conversion of n bonds to ct bonds. Thus, the change in enthalpy AH is negative. Also, because there is a decrease in randomness in conversion of monomers to polymer, the change in entropy AS is also negative. The overall change in free energy of the free radical polymerization process is,... [Pg.131]

The stereocontrol of most free radical polymerizations appears to be governed by an end-controlled mechanism. It generally follows first-order Markov statistics with respect to diads (see also Section 16.5.2.3). The tactic-ity of the formed polymer is also influenced by the solvent used. The cause of this solvent control effect is unclear, and possibly is due at least partly to different degrees of solvation. A compensation effect (see Section 16.5.4.) exists in the relationship between the activation entropies and enthalpies for diad formation in various solvents. The compensation temperature TJj varies with monomer constitution (Table 20-11). The compensation enthalpies AAHI vary strongly according to both monomer and placement type. [Pg.729]

Table 20-12. Proportion of 1,2 and 3,4 Links Together with the Activation Entropies and Enthalpies in the Free Radical Polymerization of 1,3-Dienes... Table 20-12. Proportion of 1,2 and 3,4 Links Together with the Activation Entropies and Enthalpies in the Free Radical Polymerization of 1,3-Dienes...
Gore-Tex nonabsorbable monofilament suture is made from a highly crystalhne hnear PIPE. This fully fluorinated thermoplastic is an addition polymer and is formed by the free radical polymerization route in aqueous dispersion under pressure with persulfates and hydrogen peroxide as initiators. The monomer (tetrafluoroethylene) is made from a two-step process the fluorination of chloroform by HF to produce CHClFj which is subsequently dimerized by pyrolysis to form tetrafluoroethylene. FI FE has the highest enthalpy and entropy of polymerization (-156 kJ/mol and -112 J/ mol-deg, respectively) among the vinyls (Joshi and Zwolinski, 1967). Its molecular weight can be as high as 5 x l(p. [Pg.300]

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. [Pg.6966]

Activation Enthalpies and Entropies of Stereocontrol in Free Radical Polymerizations... [Pg.479]

U/448 ACTIVATION ENTHALPIES AND ENTROPIES OF STEREOCONTROL IN FREE RADICAL POLYMERIZATIONS... [Pg.482]

Activation enthalpies and entropies of stereo-control in free radical polymerization See corresponding chapter of this Handbook ... [Pg.766]

Now, according to the transition-state theory of chemical reaction rates, the pre-exponential factors are related to the entropy of activation, A5 , of the particular reaction [A = kT ere k and h are the Boltzmann and Planck constants, respectively, and An is the change in the number of molecules when the transition state complex is formed.] Entropies of polymerization are usually negative, since there is a net decrease in disorder when the discrete radical and monomer combine. The range of values for vinyl monomers of major interest in connection with free radical copolymerization is not large (about —100 to —150 JK mol ) and it is not unreasonable to suppose, therefore, that the A values in Eq. (7-73) will be approximately equal. It follows then that... [Pg.268]

To be polymerized, vinyl monomers use the property that with the addition of each monomer, the resulting free radical maintains the same structure as that of the attacking radical and sustains the ability to add new molecules. In the formation of monomeric unit chains, the variation of entropy is negative, that is, the monomer-to-polymer conversion entails a reduction of disorder, while a compensation of the enthalpy term is observed. The alteration results in a negative variation of enthalpy therefore, the reaction is exothermic. [Pg.65]

Polymerizations are generally exothermal reactions with specific energies up to 3600 kJ kg , corresponding to an adiabatic temperature rise of up to 1800 K. Some typical reaction enthalpies are presented in Table 11.2, together with the specific heat of reaction and adiabatic temperature rise obtained for mass polymerization. Most free-radical and ionic polymerizations have negative standard enthalpies and standard entropies thus at higher temperatures these reactions must be considered reversible [Eq. (15)]. [Pg.565]

As for step-growth polymerization, the presentation of the kinetics of radical polymerization must be followed by a description of its equilibrium. The Gibbs free energy, G, for any system at temperature T is defined as H — TS, where H and S are the enthalpy and entropy of the system, respectively. The change in Gibbs free energy, AGp, for the formation of a polymer... [Pg.210]


See other pages where Free radical polymerization entropy is mentioned: [Pg.380]    [Pg.3]    [Pg.57]    [Pg.74]    [Pg.19]    [Pg.197]    [Pg.205]    [Pg.166]    [Pg.199]    [Pg.6878]    [Pg.6910]    [Pg.313]    [Pg.2527]    [Pg.480]    [Pg.484]    [Pg.13]    [Pg.708]    [Pg.101]    [Pg.536]    [Pg.197]    [Pg.295]    [Pg.612]    [Pg.286]    [Pg.176]    [Pg.1204]    [Pg.62]    [Pg.139]    [Pg.1343]    [Pg.140]   
See also in sourсe #XX -- [ Pg.385 ]




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