142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

Dendrimers and Dendrimer-Polymer Hybrids 187
The nomenclature according to IUPAC rules has proven particularly unwieldy
in the case of dendrimers. Several proposals have been made [25]. The
major classes of dendrimers described here are represented in Schemes 1–4 in
short-hand form. Other particular dendritic structures will be characterized by
reaction schemes or by their branch unit in the text.
2
Analysis of Dendrimers
2.1
NMR Analysis
All standard analytical techniques of organic chemistry are applicable to dendrimers.
Of these, NMR spectroscopy is the most powerful for the analysis of low
MW dendrimers. However, as with polymers in general, the high molecular
weight of dendrimers and the similar composition of all dendritic segments often
make it difficult to detect small quantities of irregularities in the dendrimer
structure.
Small chemical shift differences between the core or focal group, the interior
spacers and the terminal groups are usually observed in the 1 H and 13 C NMR
spectra of dendrimers and these can be used for analysis of low generation dendrimers.
As the number of generations increases, signals of the core or focal
group become relatively weak and the ratio of signals from the interior spacers
and terminal groups reaches an asymptotic limit (usually 1.0) that does not allow
accurate quantification of structural imperfections [7, 10, 31] In some cases,
it has been possible to observe distinct NMR signals for similar atoms belonging
to different generations. For example, four different 29 Si resonances are observed
in some carbosilane dendrimers [32] and five distinct 15 N resonances are
reported in DAB (CN) 32 . See Scheme 1 for related structure. In the latter case, intensities
of the resonances qualitatively match the theoretical ratio of the
number of nitrogen atoms in each shell [33].
2.2
Mass Spectrometry
Various modern mass spectrometric methods have been applied to the analysis
of dendrimers. The earliest publication describes fast ion bombardment mass
spectrometry (FAB-MS) on polyether dendrimers derived from pentaerythrytol
[31] (Scheme 5). The first generation dendrimer containing 12 hydroxyl groups
yields the expected molecular ion (M+H + =608 D) and a peak at 490 D identified
as the parent dendrimer minus one -CH 2 C(CH 2 OH) 3 group. The impurity could
not be quantified. The second generation dendrimer with 36 hydroxyl groups
shows the 2025 D parent peak with no low MW impurities observable in the
noise. However, SEC of this generation shows a high molecular weight shoulder