Biomedical Engineering Reference
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Fig. 11.7
MALDI-TOF spectrum of a copolymer containing units of glycine (G) and lactic-acid
(L). Reprinted from [
51
] with permission from the copyright holder Wiley 1999
odd-membered oligomers are present in the sample. The polymerization reaction
(Fig.
11.4
) should produce exclusively even-membered oligomers. Possible expla-
nations for the presence of odd oligomers may be the hydrolysis of the polymer
chain or an ester-ester exchange reaction occurring during polymerization. This
phenomenon has already been observed in poly(lactic) produced by ring opening
of the dilactide. The fact that the intensity of MS peak due to oligomers which
possess many lactic residues implies that the average molar fraction of L units,
c
L
,
is larger than 0.50. MS was used to measure copolymer's sequence. More specifi-
cally, sequence information was extracted from mass spectral peak intensities using
a computer program called mass Analysis of COpolymers (MACO). The output
was
c
L
D 0:77
, which compares well with the composition determined by NMR,
namely
. Even more important, the MACO method was also able to detect
subtle deviations within a sequence in which lactic and glycine residues alternate at
random along the chain. In the previous sections, we pointed out that bacteria can
produce polymers. A series of bacterial copolyester, in which
c
L
D 0:76
ˇ
-hydroxybutyrate
(HB) and
-hydroxyapatite (HV) units are found along the chain, were analysed.
In principle, using (
11.11
), MS is able to discriminate a genuine HB-HV copoly-
mer from a mixture of two copolymers. Some experiments were conducted to see
whether this is true. Three HB-HV copolyester samples, denoted as S6, S7, S3 were
considered. Sample S6 is an equimolar mixture of homopolymer HB plus HB-HV
copolymer (composition
ˇ
c
HB
,
is 0.86. Sample S7 is an equimolar mixture of HB-HV copolymer (composition
72=28
). Thus, the overall molar fraction of HB units,
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