dendrimers according to divergent or convergent dendritic branching principles has
been well demonstrated. Such systematic filling of space around cores with branch
cells, as a function of generational growth stages (branch cell shells), to give
discrete, quantized bundles of mass has been shown to be mathematically predictable (Fig. 11) [10, 11, 26]. Predicted molecular weights have been confirmed by
mass spectroscopy [87–89] and other analytical methods [9, 85, 91, 92, 104].
Predicted numbers of branch cells, numbers of terminal groups, and molecular
weights as a function of generation for an ethylenediamine-core (N c ¼ 4) PAMAM
dendrimer are shown in Fig. 12. It should be noted that the molecular weights
approximately double as one progresses to the next generation. The number of
surface groups and branch cells amplify mathematically according to a power
function, thus producing discrete, monodispersed structures with precise molecular
weights and nanoscale diameter enhancement, as described in Fig. 11. These
predicted values are routinely verified by mass spectroscopy for the earlier generations (i.e., G ¼ 4–5); however, with divergent dendrimers, minor mass defects are
often observed for higher generations as congestion-induced de Gennes dense
packing begins to take affect (Fig. 12).
4.1 Dendrimer Shape Change: A Nanoscale Molecular
Morphogenesis
As illustrated in Fig. 12, dendrimers undergo congestion-induced molecular shape
changes from flat, floppy conformations to robust spheroids, as first predicted by
Goddard and coworkers [84]. Shape change transitions were subsequently confirmed by extensive photo-physical measurements, pioneered by Turro and
coworkers [105–108] and solvatochromic measurements by Hawker
et al. [109]. Depending upon the accumulative core and branch cell multiplicities
of the dendrimer family under consideration, these transitions were found to occur
between G ¼ 3 and G ¼ 5. Ammonia-core, PAMAM dendrimers (N c ¼ 3,
N b ¼ 2) exhibited a molecular morphogenesis break at G ¼ 4.5, whereas the
ethylenediamine-core PAMAM dendrimer family (N c ¼ 4, N b ¼ 2) manifested a
shape change break at around G ¼ 3–4 [84] and the Fre ´chet-type convergent
dendrons (N b ¼ 2) at around G ¼ 4 [109]. It is readily apparent that increasing
the core multiplicity from N c ¼ 3 to N c ¼ 4 accelerates congestion and forces a
shape change at least one generation earlier. Beyond these generational transitions,
one can visualize these dendrimeric shapes as nearly spheroidal or slightly ellipsoidal core–shell architectures. Studies by Tomalia and colleagues [110] as well as
Schluter and colleagues [111] have shown that the cylindrical or rod-shaped
dendrimers are routinely formed by dendronizing traditional linear polymers.
These new constructs derived from linear polymer backbones are pendant dendrons
and are referred to as “architectural copolymers” [52].
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D.A. Tomalia
been well demonstrated. Such systematic filling of space around cores with branch
cells, as a function of generational growth stages (branch cell shells), to give
discrete, quantized bundles of mass has been shown to be mathematically predictable (Fig. 11) [10, 11, 26]. Predicted molecular weights have been confirmed by
mass spectroscopy [87–89] and other analytical methods [9, 85, 91, 92, 104].
Predicted numbers of branch cells, numbers of terminal groups, and molecular
weights as a function of generation for an ethylenediamine-core (N c ¼ 4) PAMAM
dendrimer are shown in Fig. 12. It should be noted that the molecular weights
approximately double as one progresses to the next generation. The number of
surface groups and branch cells amplify mathematically according to a power
function, thus producing discrete, monodispersed structures with precise molecular
weights and nanoscale diameter enhancement, as described in Fig. 11. These
predicted values are routinely verified by mass spectroscopy for the earlier generations (i.e., G ¼ 4–5); however, with divergent dendrimers, minor mass defects are
often observed for higher generations as congestion-induced de Gennes dense
packing begins to take affect (Fig. 12).
4.1 Dendrimer Shape Change: A Nanoscale Molecular
Morphogenesis
As illustrated in Fig. 12, dendrimers undergo congestion-induced molecular shape
changes from flat, floppy conformations to robust spheroids, as first predicted by
Goddard and coworkers [84]. Shape change transitions were subsequently confirmed by extensive photo-physical measurements, pioneered by Turro and
coworkers [105–108] and solvatochromic measurements by Hawker
et al. [109]. Depending upon the accumulative core and branch cell multiplicities
of the dendrimer family under consideration, these transitions were found to occur
between G ¼ 3 and G ¼ 5. Ammonia-core, PAMAM dendrimers (N c ¼ 3,
N b ¼ 2) exhibited a molecular morphogenesis break at G ¼ 4.5, whereas the
ethylenediamine-core PAMAM dendrimer family (N c ¼ 4, N b ¼ 2) manifested a
shape change break at around G ¼ 3–4 [84] and the Fre ´chet-type convergent
dendrons (N b ¼ 2) at around G ¼ 4 [109]. It is readily apparent that increasing
the core multiplicity from N c ¼ 3 to N c ¼ 4 accelerates congestion and forces a
shape change at least one generation earlier. Beyond these generational transitions,
one can visualize these dendrimeric shapes as nearly spheroidal or slightly ellipsoidal core–shell architectures. Studies by Tomalia and colleagues [110] as well as
Schluter and colleagues [111] have shown that the cylindrical or rod-shaped
dendrimers are routinely formed by dendronizing traditional linear polymers.
These new constructs derived from linear polymer backbones are pendant dendrons
and are referred to as “architectural copolymers” [52].
344
D.A. Tomalia