Predicting and Measuring the Sequence Distribution of Condensation Polymers - Mathematical Approaches to Polymer Sequence Analysis and Related Problems - page 239

11.12 Nuclear Magnetic Resonance

NMR can be applied to the analysis of condensation polymers. For instance, a
copolymer with units of etherketone and ethersulfone [ 48 ] was sequenced by NMR.
The sample was treated as a mixture of chains and, more specifically, a binary mix-
ture characterized by a mole fraction

.1/ of exactly alternating chains and a mole

fraction of bernoullian chains. Condensation terpolymers can be sequenced by

NMR too. Su and Shih analysed by NMR a ternary mixture made of a poly(ethylene
naphthalate)/poly(trimethylene terephthalate) copolymer and a poly(ether imide)
[ 49 ]. Kricheldorf and Hull sequenced by NMR a terpolyamide [ 50 ]. However, NMR
turns out to be less powerful for condensation polymers than for addition poly-
mers. This is due to the fact that condensation polymers possess bulky repeat units.
Unfortunately, NMR can hardly discern beyond the triad level. When this statement
is translated in the “repeat unit” language, it becomes: NMR can hardly discern be-
yond 1.5 repeat units. This implies that the penultimate model, the pen-penultimate
model, and other models with a high number of degree of freedom cannot be used.

./

11.13 Mass Spectrometry

In the previous sections, we briefly introduced MS and the theoretical mass spectra
for a copolymer, a terpolymer, and a tetrapolymer were shown. We also consid-
ered condensation copolymerization with cyclic dimers from a theoretical point of
view. MS can be applied to the analysis of the above copolymers. In particular
co-polydepsipeptides are currently under consideration as biodegradable materi-
als. Ring-opening copolymerization of lactide and 6-methyl-2,5-morpholinedione
(the cyclic glycine-lactic acid dimer) affords a series of co-polydepsipeptides corre-
sponding to Fig. 11.4 . It is apparent that the synthetic route yields a quasi-random
copolymer with a peculiar sequence distribution, since it deviates from the standard
form in which the two repeat units (glycine and lactic acid residues) are found at
random along the copolymer chain. The deviation is due to chains having sequences
containing two consecutive glycine units, GG, which are not produced. This devia-
tion is small and is therefore difficult to detect. Figure 11.7 reports the MALDI-TOF
spectrum of a copolymer containing units of glycine (G) and lactic acid (L) [ 51 ].
The peaks in the mass spectrum were assigned to oligomers bearing the same end
groups, namely:

H . O CH . CH 3 / CO / m . NH CH 2 CO / n OH (11.19)

The peak at m= z D 401 is due to the lactic pentamer ( L 5 ). The lactic hexamer

falls at , and it displays a more than double intensity. An intense peak

in the mass spectrum is at

m= z D 473

and it corresponds to an oligomer with five
lactic acid residues and two glycine residues (

m= z D 515

L 5 G 2 L). The peak at m= z D 530

is slightly less intense and it is due to L 6 G 1 . Pentamers, heptamers, and other