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

188 J. Roovers, B. Comanita
Scheme 5
that might not be detected by FAB-MS. The third generation dendrimer could
not be analyzed by this method.
A PAMAM dendrimer of generation four was analyzed by electron spray ionization
mass spectrometry (ESI-MS) with detection in the 600–1600 D range
[34]. Multiple charged ions with +7 to +11 are observed. After deconvolution
two groups of species are recognized. The first group consists of the expected
parent peak at 10632 D and six species with MW fitting the relation 10,632–
n·114
with n=1 to 6. These are recognized as compounds missing from 1 to 6
CH2 =CHCONHCH2CH2NH2 groups out of a total 48 possible terminal groups
due to incomplete Michael addition (see Scheme 6). Another series of compounds
with MW=10,632–n·60
is due to the formation of cyclic structure in the
amidation step leading to 1 to 6 cyclic groups in the outer shell. If it is assumed
that the mass spectrometric intensities are proportional to the number of molecules
then this fourth generation dendrimer sample contains only about 8% perfect
dendrimers and 92% of the dendrimers are deficient in from one to ten terminal
amine functional groups. The authors calculated that this molecular mass
distribution is representative of an overall yield of 97.5% in the combined twostep
reaction to form each generation. A more detailed study of the side reactions
leading to these defects was made on lower generation dendrimers by
chemical ionization mass spectrometry [7]. Schwartz et al. have advanced the
analysis of PAMAM dendrimers by ESI-MS to the tenth generation [35]. Spectra
up to generation four have clearly resolved multiple ion bands. For higher generation
dendrimers species with different molecular weight and charge number
form one envelope which cannot be deconvoluted. A comparison of m/z values
with the theoretical MW indicates that the charge (z) on the dendrimer increases
with the size of the dendrimer.