Free radical polymerization enthalpy

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. 

The conversion of n bonds into a bonds is exothermic and leads to enthalpies of polymerization in the approximate range -60 to — lOOkJ moI for the monomers most often used in free-radical polymerizations. Clearly the heat evolved during polymerization must be removed and, although this is not a significant problem on the laboratory scale, heat transfer requirements cannot be ignored when considering large-scale production of latexes. 

Figure 15-4. Compensation effect for various placement possibilities for first-order Markov statistics in the free radical polymerization of methyl methacrylate. For better clarity, some lines have been vertically displaced by -10.5 (for AHj/, - AH /,), 4.2 (for AH, - AH /,), 6.3 (for A H /, - A H /,), and 10.5 (for A H /, - A H /,) kJ/mol. The compensation temperature, but not the compensation enthalpy, is independent of the kind of placement occurring. (From data by H.-G. Elias and P. Goeldi.)...

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. 

Figure 20-9. Influence of the relative permittivity of the solvent on the cross-over step activation enthalpy differences for the free radical polymerization of vinyl trifluoroacetate. A, Alkanes E, esters K, ketones , bulk polymerization. (From data of Matsuzawa et al.)...

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,... 

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. 

In the free radical polymerization of 1,3-dienes (Figure 20-8, Table 20-12) the difference in activation enthalpies for 1,4-trans and 1,4-cis tactic links is approximately of equal magnitude for butadiene, chloroprene, and... 

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. 

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. 

Quantum chemistry provides a powerful tool for studying kinetic and mechanistic problems in free-radical polymerization. Provided a high level of theory is used, ah initio calculations can provide direct access to accurate values of the barriers, enthalpies, and rates of the individual reactions in the process, and also provide useful related information (such as transition structures and radical stabilization energies) to help in understanding the reaction mechanism. In the following, some of the applications of quantum chemistry are outlined. This is not intended to be a review of the main contributions to this field, nor is it intended to provide a theoretical account of reactivity in free-radical polymerization (108). Instead, some of the types of problems that quantum chemistry can tackle are described, with a view to highlighting the potential of quantum-chemical calculations as a tool for studying free-radical polymerization (see Radical Polymerization). 

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. 

The reaction rate at the initial temperature must be vanishingly small but rapid at the front temperature. The front temperature is determined by the enthalpy of the reaction, heat capacity of the product and the amount of heat loss. Free-radical polymerization is ideal because for most peroxide and nitrile initiators the rate of polymerization at ambient temperature is low but high at elevated temperatures. Amine-cured epoxies suffer from the problem of short pot life but cationic cured systems are very similar to free-radical systems (10). 

Activation Enthalpies and Entropies of Stereocontrol in Free Radical Polymerizations... 

U/448 ACTIVATION ENTHALPIES AND ENTROPIES OF STEREOCONTROL IN FREE RADICAL POLYMERIZATIONS... 

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

Nxylylene system, substituents affect it only to a minor extent. AH parylenes are expected to have a similar molar enthalpy of polymerization. An experimental value for the heat of polymerization of Parylene C has appeared. Using the gas evolution from the Hquid nitrogen cold trap to measure thermal input from the polymer, and taking advantage of a peculiarity of Parylene C at — 196°C to polymerize abmptiy, perhaps owing to the arrival of a free radical, a = —152 8 kJ/mol (—36.4 2.0 kcal/mol) at — 196°C was reported (25). The correction from — 196°C to room temperature is... 

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. 

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