Group contribution techniques using

An area that has used chemical stmctures for predictive purposes quite successfully is the estimation of thermophysical properties of compounds. There has been an extensive compilation of estimation methods (81), and prediction of physical properties has been automated using these techniques (82). More recendy, the use of group contribution techniques to design new molecules that have specified properties has been described (83). This approach to compound design is being used to develop replacement materials for chloroduorocarbons. 

No matter how broad the scope of the experimental data is, there will always be a need for data that have not yet been measured or that are too expensive to measure. Another objective of this Handbook is to provide accurate, predictive techniques. Predictive techniques not only furnish a source of missing experimental data, they also aid in the understanding of the physical nature of the systems of interest. The most useful predictive methods require as input data only the structure of the molecules or other data that are easily calculated or have been measured. Many of the methods present in this Handbook are based on the concept of group contributions which use as input only the structure of the molecules in terms of their functional groups or which use group contributions and readily... 

Starting with a compound s molecular structure, the T, P, and are estimated first, using the three group contribution techniques indicated above. These values are then used in an equation-oriented technique to yield the final estimate for P. ... 

We need to employ an estimation procedure for and the associated independent physical properties, such as those described in Section I.B [see Eqs. (2) and (3a-c)], in order to determine each candidate molecule s vapor pressure at 273 K. Starting with a compound s molecular structure, and P are first estimated using the three group-contribution techniques [Eqs. (3a), (3b), and (3c), respectively]. These values are then used in an equation-oriented technique to yield the final estimate for P [Eq. (2)1. 

The framework of interactive design is entirely built on the premise of additivity of group contributions for the estimation of physical properties. Let us look at some typical illustrations. Table XV shows the estimation of normal boiling points T, and normal melting points T , using Joback s group contribution techniques (Joback and Reid, 1987), along with the... 

Additive or group contribution techniques are commonly used to predict the properties of polymers from their molecular structures. These techniques provide many extremely useful simple correlations to predict the properties. Group contribution techniques will be reviewed briefly in Section l.B. The main approach used in the scheme of correlations developed in this book is based on topological techniques. The philosophy of this approach, and the scope of the work presented in this book, will be discussed in Section l.C. The detailed technical implementation of the scheme of correlations will be postponed to later chapters. Equations for converting between mole, weight and volume fractions will be listed in Section l.D. The subjects which will be covered in the later chapters will be summarized in Section l.E. 

Van Krevelen [3] published the classic textbook on group contribution techniques in the QSPR of polymers. This book contains a compendium of useful QSPR relationships for polymers, as well as tables of large amounts of experimental data. The information contained in this book has been extremely valuable in the development of many of our correlations. We will, consequently, refer very frequently to this book as well as to a later review article by van Krevelen [4] containing a significant amount of additional or revised information. While we will refer to the third edition of van Krevelen s book (published in 1990), the reader should be aware that some of our correlations developed with his hook as a key resource used its second edition (published in 1976). In general, we did not revise correlations that we developed by using the information in the second edition as a resource, unless the third edition contained information suggesting qualitatively different conclusions. 

The values of the properties which will be fitted by using equations 2.9 and 2.10 will be selected from available and apparently reliable experimental data whenever there are sufficient amounts of such data. Some important properties of polymers, such as the van der Waals volume (Chapter 3) and the cohesive energy (Chapter 5), are not directly observable. They are inferred indirectly, and often with poor accuracy, from directly observable properties such as molar volume (or equivalently density) and solubility behavior. When experimental data are unavailable or unreliable, the values of the properties to be fitted will be estimated by using group contributions. The predictive power of such correlations developed as direct extensions and generalizations of group contribution techniques will then be demonstrated by using them... 

As with most of the new correlations developed in this book, the objective of developing Equation 7.17 was to transcend the intrinsic limitations of the predictive capabilities of group contribution techniques, which were discussed in Chapter 1. Examples of the predictive use of Equation 7.17 are given in Table 7.5, where this equation is combined with equations 3.10 and 3.11 for the van der Waals volume, and with equations 3.13 and 3.14 for the molar volume at room temperature, to provide a mutually consistent set of predictions of the y of ten polymers at room temperature. No values were available for the contributions of silicon atoms and sulfone groups to P in the group contribution tables [3,4] which provide the best values of P for polymers. The ten P values calculated by using Equation 7.17 and listed in Table 7.5 could therefore not have been estimated from group contribution tables for the P of polymers. 

For the material and energy balances, pure-component heat capacity and density data are needed. These are among the most widely measured data and are available on process simulators for more than a thousand substances. (See Chapter 13 for details of process simulators.) There are also reasonably accurate group-contribution techniques for use when no data are available [8]. The enthalpies of mixtures require an accurate equation of state for gases and nonionic liquids. The equations of state available on process simulators are accurate enough for these systems. However, additional heat of solution data are needed for electrolyte solutions, and these data may not be as readily available. For these systems, care should be taken to use accurate experimental data, because estimation techniques are not as well defined. 

Group contribution methods These methods have been used to estimate the solubility parameter (17,20,28,33,35, 58,60,61,96,112,121,122). van Krevelen (123), Fedors (35), and Barton (12) have reviewed these techniques and given tables of group values. The molar volume of solvents and polymers can also be estimated by group contribution techniques (1 OS). 

Bicerano (164) and Porter (165) have developed new group contribution techniques for a wide variety of polymer properties. These approaches consider how the different functional groups are connected in the molecule or in the polymeric repeating unit. Bicerano s method uses Fedor s (35) and van Krevelen s (123,163) group contribution values. Both references provide solubility parameter predictions of a number of polymers. 

Values of the free energy of formation as a linear function of temperature can be estimated by group contribution techniques (Reid et al). The uncertainties, however, in the obtained values - combined with the sensitivity of the equilibrium conversion to the value of the standard free energy of the reaction (see Problem 15.28) - make such estimates useful for examining the feasibility of a given reaction only. 

Numerous other methods have been used to predict properties of gases and Hquids. These include group contribution, reference substance, approaches, and many others. However, corresponding states theory has been one of the most thoroughly investigated methods and has become an important basis for the development of correlation and property estimation techniques. The methods derived from the corresponding states theory for Hquid and gas property estimation have proved invaluable for work such as process and equipment design. 

The enthalpy of formation can be determined theoretically and experimentally. The theoretical methods can be defined as those which use bond contributions and the ones which use group contributions. The bond contribution techniques can be characterized as zero, first, second, or higher order methods, where zero is elemental composition only, first adds the type of bonding, second adds the next bonded element, and higher adds the next type of bond. A survey of typical theoretical methods is shown in Table 2.6. 

The chapter is divided into the following sections. First, a brief introduction to group contribution methods is given with a major emphasis on the concept and limitations of this technique. An introduction to the use of chemical graph theory and how it applies to polymers and in particular to the dielectric constant is given next. Application of the method to a number of polyimides is then demonstrated and predictions are compared to experimental results. 

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