Penultimate Effects and Charge-Transfer Complexes

Complex formation is particularly helpful in explaining free-radical copolymerizations in systems such as styrene-maleic anhydride. This system forms a 1 1 copolymer over most of the range of monomer composition, and the addition of maleic anhydride greatly enhances the rate of polymerization over that of pure styrene, despite the fact that maleic anhydride will not horaopolymerize at a noticeable rate. These observations are consistent with the formation of a strong, readily polymerized complex between the monomers. The general equations to describe such copolymeiizations have been presented by Seiner and Litt and applied in a number of special cases.  

Crud Chemicals wishes to make a copolymer for beverage bottles using styrene for low cost and processability and acrylonitrile (AN) for its barrier (to CO2 and O2, mainly) properties. They want a uniform composition of 75 wt % acrylonitrile (component 1) and 25% styrene (component 2). This is 86%(1), 14%(2) on a mole basis. With more AN, the copolymer becomes difficult to process (recall from Chapter III that homopolyacrylonitrile, though linear, is not thermoplastic). With less, barrier properties suffer. For this system, = 0.040, T2 = 0.40. Crud proposes to use the semibatch technique of Fig. 12.4a. 

For a product that is to contain 100 total moles monomer at complete conversion, address the following  

Most commercial copolymers of styrene and acrylonitrile, known as SAN, are about 75% styrene and 25% acrylonitrile, just the reverse of the composition in Problem 3. Why is this copolymer much easier to make at approximately uniform composition  

We wish to make an acrylonitrile (l yrene (2) copolymer (see Problem 3) by the technique shown in Fig. 12.4c. The range of instantaneous copolymer composition must be limited to 0.40 Fi 0.60. 

There are certain cases in which the equations developed in Section 11.2 do not adequately describe copolymerization. Two approaches have been taken to remedy these deficiencies invoking penultimate effects and postulating the formation of charge-transfer complexes. 

In the former, the next-to-last (or penultimate) monomer in a growing chain also exerts an influence on the addition of the next monomer molecule [7-10]. In the latter, a 1 1 complex forms reversibly between electron-donating and electron-accepting comonomers (introducing an equilibrium constant to the analysis). This complex may then polymerize (from either end—introducing four more reactivity ratios) with itself or with the uncomplexed monomers. 

The possibilities (and complexities) introduced by copolymerization are seemingly infinite. Monomers can be added at different ratios, but even so, the reactivity ratios must be considered to evaluate the arrangement of the different repeat units in the polymer. Also, the polymer chains that form early on in a reactor may have a different composition from those formed later. The kinetics also become more and more difficult to accurately describe. While copolymers are certainly interesting and valuable materials, this chapter has only broken the surface in the analysis of these reactions. Further complexities can be introduced—three or more different monomers, the addition of a crosslinking agent, and step-growth polymers all require detailed analysis that is beyond the scope of this book. 

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