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

Dendrimers and Dendrimer-Polymer Hybrids 191
Frey and coworkers critically discuss MALDI-TOF spectra of carefully chromatographed
G2 and G3 poly(carbosilane) dendrimers with 36 and 108 terminal
allyl groups [42] (see Fig. 3a for related structure). They quoted these samples
to have apparent MW distributions in the range of 1.05 to 1.06 by SEC. The
G2 sample has two main peaks with MWs for dendrimers with 36 and 34 allyl
end groups, respectively. The peak intensities indicate that 80% of the parent
G1 dendrimer is completely hydrosilylated and that the remaining 20% is hydrosilylated
11 out of 12 times. This is equivalent to a 98.3% reaction yield. In
the case of the G3 dendrimer a wider range of dendritic species is observed.
The most abundant species has 106 allyl end groups (35 out of 36 allyl group
are hydrosilylated) but species with 100 to 108 allyl groups are also present. A
dendrimer with 100 allyl groups is the product of 32 out of 36 hydrosilylations
and it contains 4 allyl groups belonging to the G2 shell. Nevertheless, from a
polymer perspective, these samples are very monodisperse, Mw/Mn being of
the order of 1.01. The allyl groups in the carbosilane dendrimers have also
been converted to primary alcohols via hydroboration oxidation. In the MAL-
DI-TOF spectra of the hydroxylated G2 compound traces of lower MW material
together with the main 36 and 34 hydroxyl containing dendrimers are observed.
In the case of the hydroxylated G3 dendrimer the MALDI-TOF spectrum
almost completely reflects the MW pattern of the parent allylic G3 dendrimer.
Recently, MALDI-TOF results on poly(aryl ether) dendrimers allowed the detection
of a small impurity characterized by an extra -C 6H 4S- group (108 D) and
confirmed upon oxidation by a small extra species with a -C 6H 4SO 2- group
(140 D) in the sulfone form [43]. Two impurities are detected in the third generation
dendrimer with one and two extra -C 6 H 4 S- groups, respectively. However,
it was not possible to quantify the amount of these defects. Fourth generation
dendrimers (18,212 D) could not be analyzed by the MALDI-TOF method.
Finally, the MALDI technique was used for the characterization of poly(phenylacetylene)
[17]. These dendrimers were synthesized through a double growth
process (Scheme 7). The most advanced application involved the reaction of a
third generation dendrimer with 16 reactive terminal groups (G-3–16) with a
third generation dendron. The aim was to skip directly to the sixth generation
dendrimer (G-6–256) with 256 end groups. The reaction occurred in 86% yield
after optimization of the conditions. MALDI-TOF analysis revealed the major
compound to be the desired dendrimer contaminated only with a small amount
of a compound having 16 fewer end-groups. Chemical ionization, infra-red laser
desorption proved the superior method for generation 0 to 2 dendrimers with
MW up to 3631. MALDI-TOF is more successful with generation 3 and 4. In all
MALDI spectra dendrimer dimers and sometimes dendrimer trimers are observed,
despite very low sample to matrix ratios. The spectra are also somewhat
complicated by partial fragmentation of these dendrimers during the analytical
process [44, 45].
New reports indicate that MALDI-TOF is beginning to be used on a routine
basis (like NMR) to monitor the synthesis and modification of each batch of den-