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

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190 J. Roovers, B. Comanita Fig. 3a,b. Other classes of dendrimers: a carbosilane; b poly(a,e-L-lysine) height of about 100 D is observed together with an impurity (20%) at about 13,100 D possibly due to acid hydrolysis of a benzyl ether linkage. MALDI-TOF spectra provide clear proof of the superior quality of dendrimers made by the convergent method. The MALDI-TOF spectrum of [G-3] poly(benzyl ether) dendrimer-poly(ethylene glycol) triblock copolymer shows a broad band of peaks between 4300 and 6100 D with resolution of the individual ethyleneoxide (44 D) units. The MALDI-TOF spectrum of a [G-3] dendrimer with two polystyrene blocks (molecular peak=8073 D) shows material with 6000–11,000 D and a broad band corresponding to material with 2 M+Ag + . SEC can be used to prove that the latter species is indeed an artifact of the mass spectroscopic method. The authors claim almost exact agreement between the polydispersities derived from MALDI-TOF and SEC [40]. This does, however, not leave any room for the unavoidable column spreading in the latter method. Furthermore, anionically prepared low MW polymers have a minimum polydispersity given by (1+1/DP) [41].
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-
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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 241 Microphases 11z,
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