Essentials of Computational Chemistry

6.5 CASE STUDY: POLYMERIZATION OF 4-SUBSTITUTED AROMATIC ENYNES 199
is the economical MIDI! basis set for which, as noted above, heteroatom d exponents
were specifically optimized so that high-quality geometries and charge distributions (instead
of minimal energies) are obtained from the HF wave function. Electrostatic potentials
computed with MIDI! also give good agreement with correlated levels of electronic structure
theory.
A more complete discussion of charge distributions is deferred until Chapter 9. The performance
of HF theory for other molecular properties is also presented in more detail there.
6.5 Case Study: Polymerization of 4-Substituted
Aromatic Enynes
Synopsis of Ochiai, Tomita, and Endo (2001) ‘Investigation on Radical Polymerization Behavior
of 4-Substituted Aromatic Enynes. Experimental, ESR, and Computational Studies’.
One strategy for making highly functionalized polymers is first to carry out polymerization
of a system bearing functionalizable appendages, and then after polymerization
to react those appendages to introduce new functionality into the polymer. Such an
approach can be advantageous in instances where the monomer that would in principle
lead directly to the functionalized polymer fails itself to be useful as a polymerization
substrate.
Ochiai and co-workers developed an experimental protocol for the radical polymerization
of one such reactive monomer, 4-phenylbut-1-en-3-yne. As illustrated in Figure 6.13, this
polymerization creates a polyethylene chain functionalized with phenylethynyl substituents.
A factor that affects the kinetics of the polymerization, and, more critically, the utility
of the monomer in copolymerizations with other monomers, e.g., methyl methacrylate,
is the stability of the radical formed from addition of the growing polymer chain to
the vinyl terminus. In order to gauge the stabilizing effect of the phenylethynyl group,
and the sensitivity of the stabilization to substitution at the para position of the aromatic
ring, Ochiai and co-workers carried out calculations at the UHF/3-21G level to evaluate
(i) the spin density in the 1-phenylprop-1-yn-3-yl radical and (ii) the reaction energy for the
process
ž ž
RCH3 + CH3 → RCH2 + CH4
(6.14)
where R was varied over a number of different functional groups. This so-called isodesmic
reaction (see Section 10.4.3) essentially computes the C–H bond energy for the substituted
system relative to the C–H bond energy in methane, thereby reducing absolute errors that
would be associated with a small HF calculation for an absolute bond energy.
The spin density calculation, which analyzes the difference between each atom’s Mulliken
population (see Section 9.1.3.2) of α and β electrons, indicated the unpaired electron to
be highly delocalized, with populations on the ortho and para carbons of the phenyl
ring nearly equal to that found on the formal radical position (these large positive spin
densities were balanced by large negative spin densities on the intervening carbon atoms,
which is a typically observed situation). HF theory tends to overpolarize spin, so the
magnitude of the spin polarization is probably not trustworthy, but the large degree of
delocalization is probably qualitatively reasonable. The prediction that the ring para position
carries substantial spin was found to be consistent with copolymerization reactivity