Page 61 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GQT/LBX P2: GQT/MBQ QC: FYD Final Pages
Encyclopedia of Physical Science and Technology EN008C-602 July 25, 2001 20:31

876 Macromolecules, Structure

by means of a resonant radio frequency field having a fre-
quency ν 0 . In modern instruments, this resonant field is
supplied as a pulse of radiofrequency energy and the res-
onance signal appears as an oscillating current in the time
domain (“free induction decay” or fid), which must be
Fourier transformed by computer to the normal spectrum
in the frequency domain.
The value of NMR to the chemist lies in the fact that at
any particular value of ν 0 all nuclei of a given species—
say, all protons—do not resonate at exactly the same value
of B 0 (or vice versa). Resonance actually occurs at slightly
different values of B 0 for each type of proton, depending
upon its chemical binding and position in the molecule.
The cause of this variation is the cloud of electrons about
each nucleus. Protons attached to or near electronegative
groups such as OR, OH, OCOR, CO 2 R, or halogens expe-
rience a lower electron density and appear at the left in the
NMR spectrum, while those removed from such groups, as
in hydrocarbon chains, appear at the right. Similar struc-
13
tural relationships are observed for C nuclei, which also
show a marked dependence on conformation. These vari- FIGURE 19 500-MHz proton NMR spectra of predominantly (a)
syndiotactic and (b) isotactic samples of poly(methyl methacry-
ations are termed chemical shifts and are commonly ex-
late), observed in 10% chlorobenzene-d 5 solution at 100 C. The
◦
pressed in relation to tetramethylsilane (TMS) as the zero
methoxyl resonance is not shown, but appears as 3.4 ppm.
of reference. For protons the total range of chemical shifts
13
in organic compounds is ∼10 ppm. For C it is much
13
greater—over 200 ppm. The C nucleus constitutes only
12
1.1% of naturally occurring carbon ( C has no spin and is (b)withfluorenyllithiumintoluene,ananionicinitiatorare
shown. The profound effect of the nature of the initiator
not observable by NMR) and consequently requires sub-
is evident in the marked differences between these spec-
stantial spectrum accumulation.
tra. To interpret these spectra, let us consider the chain
For any nucleus, the separation of chemical shifts, ex-
in terms of sequences of two monomer units or dyads.
pressed in hertz (Hz), is proportional to B 0 . An important
There are two possible types of dyads and they have dif-
advantage of high-field magnets is the improved resolu-
ferent symmetry properties. The syndiotactic or racemic
tion of resonances and finer discrimination of structural
(r) dyad has a twofold axis of symmetry and consequently
features. Spectrometers employing superconducting mag-
the two methylene protons are in equivalent environments.
nets, operating at 200 to 500 MHz for protons, are best
They therefore have the same chemical shift and appear
suited for studies of polymers in solution.
as a singlet resonance despite strong two-bond or gem-
Another important parameter in NMR spectra is nu-
inal coupling between them. The isotactic or meso (m)
clear coupling. If a nucleus has n sufficiently close equiv-
dyad has no symmetry axis, and so the two protons are
alently coupled neighbors, its resonance will be split into
nonequivalent and should in general give different chem-
n + 1 peaks, with intensities given by the coefficients
ical shifts.
of the binomial expansion. Thus, one neighboring spin
splits the observed resonance to a doublet, two produce H A R R H A R
a 1:2:1 triplet, three a 1:3:3:1 quartet, and so on. The
strength of the coupling is denoted by J and expressed R H A H B
in hertz. Proton–proton couplings through three interven- r m
(PLANAR ZIGZAG CONFORMATION)
ing bonds—termed “vicinal”—are strongly dependent on
(a) (b)
the dihedral angle between the protons. This is dealt with
in Section IV. When there is no vicinal coupling to neighboring protons,
as is the case in poly(methyl methacrylate), the syndio-
tactic sequences should exhibit a methylene singlet while
C. Stereochemical Configuration
the isotactic form should give two doublets, each with a
In Fig. 19 the 500-MHz proton spectra of poly(methyl spacing equal to the geminal coupling, ∼15 Hz. We see
methacrylate) prepared (a) with a free radical initiator and in Fig. 19 that the methylene spectrum of the anionically