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

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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.
Dendrimers and Dendrimer-Polymer Hybrids 189 Scheme 6 A complete ESI-MS analysis of generations 1–5 has allowed one to determine quantitatively the extent of the two possible defect introducing reactions in DAB dendrimers [36] (see Scheme 1). The Michael addition is on average 99% complete. During hydrogenation 0.5% of end groups form rings. As a consequence, the fifth generation DAB(NH 2 ) 64 contains 23% pure compound. The remainder are known impurities lacking one or more end groups. It should be noted that all these impurities do not increase the polydispersity of the dendrimer beyond Mw/Mn=1.001. An ESI-MS has also been coupled to the outlet of a capillary electrophoresis instrument allowing the mass identification of different elution peaks. When applied to DAB(CN) 8 different isomers of defective compounds can be identified [37]. ESI-MS was also used to study a polystyrene-DAB (NH 2 ) 8 hybrid. The series of peaks corresponding to charge z=+4 displayed a spacing equal to 26, characteristic of the styrene unit. Other smaller peaks may be due to dendrimer imperfections or fragmentation [38]. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry MALDI-TOF appears the newest technique particularly suitable for the study of oligomers and dendrimers because, under appropriate conditions, the parent peak is obtained uncontaminated by fragmentation species. However, in some cases supramolecular clusters have been observed which could be misinterpretated as dimers and higher multiplets [13]. The molecular weight range available reaches 50,000 D with a potential resolution between 0.01 and 0.05%. Polyesters dendrimers obtained by the divergent method by means of 1,3-dicyclohexylcarbodiimide esterification and catalytic hydrogenation to remove the benzylic protecting group gave essentially the parent molecule peak with a width at half height of 4–8 D [39]. The highest MW observed is 5147 D. SEC data corroborated the MALDI-TOF results with Mw/Mn values between 1.005 and 1.007 for dendrimers with benzyl terminal groups and 1.007 to 1.017 for dendrimers with hydroxyl terminal groups. MALDI-TOF data have been obtained on a sixth generation poly(benzyl ether) dendrimer containing 64 perdeuterobenzyl terminal groups [40] (see Scheme 3). The correct molecular species (M+K + =13,965 D) with a width at half
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142 Advances in Polymer Science Edi
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Branched Polymers I Volume Editor:
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Volume Editor Dr. Jacques Roovers I
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Preface While books have been writt
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Contents Synthesis of Branched Poly
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Synthesis of Branched Polymers by C
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72 N. Hadjichristidis, S. Pispas, M
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100 N. Hadjichristidis, S. Pispas,
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Poly(macromonomers): Homo- and Copo
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Poly(macromonomers), Homo- and Copo
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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