to dendrimers, dendrigraft structures are less controlled since grafting may occur
along the entire length of each generational branch, and the exact branching
densities are somewhat arbitrary and difficult to control. More recently, both
Gnanou [76, 77] and Hedrick [78, 79] have developed approaches to dendrigrafts
that mimic dendrimer topologies by confining the graft sites to the branch termini
for each generation. These methods involve so-called “graft from” techniques and
allow better control of branching topologies and densities as a function of generation. Topologies produced by these methods are reminiscent of the dendrimer
architecture. Since the branch-cell arms are derived from oligomeric segments, the
products are referred to as polymeric dendrimers [22, 78, 79]. These more flexible
and extended structures exhibit unique and different properties compared to the
more compact traditional dendrimers. Fre ´chet, Hawker, and coworkers [80] have
utilized the techniques of living polymerization and a staged polymerization
process (in which latent polymerization sites are incorporated within growing
chains) to produce dendrigrafts of mixed composition and narrow polydispersity.
Another exciting development has been the emerging role that dendritic architecture is playing in the production of commodity polymers. A recent report by
Guan et al. [24] has shown that ethylene polymerizes to dendrigraft polyethylene
(dendri-polyethylene) at low pressures, in contrast to high-pressure conditions
which produce only simple branched topologies. This occurs when using latetransition metal or Brookhart catalysts. Furthermore, these authors also state that
small amounts of dendri-poly(ethylene) architecture may be expected from analogous early-transition-metal metallocene catalysts.
3.3.3 Dendrons and Dendrimers
Dendrons and dendrimers are the most intensely investigated subset of dendritic
polymers. In the past decade, over 6,000 literature references have appeared dealing
with this unique class of structure-controlled polymers. The word “dendrimer” is
derived from the Greek words dendri- (tree-branch-like) and meros (part of), and
was coined by Tomalia, et al. about 20 years ago in the first full paper on PAMAM
dendrimers [45, 46]. Since this early disclosure, over 125 dendrimer compositions
(families) and 1,100 dendrimer surface modifications have been reported. The two
most widely studied dendrimer families are the Fre ´chet-type polyether compositions and the Tomalia-type PAMAM dendrimers. PAMAM dendrimers constitute
the first dendrimer family to be commercialized, and represent the most extensively
characterized and best-understood series at this time [55].
Dendrimer Synthesis: Divergent and Convergent Methods
In contrast to traditional polymers, dendrimers are unique core–shell structures
possessing three basic architectural components (Fig. 10): a core, an interior of
shells (generations) consisting of repeating branch-cell units, and terminal
338
D.A. Tomalia
along the entire length of each generational branch, and the exact branching
densities are somewhat arbitrary and difficult to control. More recently, both
Gnanou [76, 77] and Hedrick [78, 79] have developed approaches to dendrigrafts
that mimic dendrimer topologies by confining the graft sites to the branch termini
for each generation. These methods involve so-called “graft from” techniques and
allow better control of branching topologies and densities as a function of generation. Topologies produced by these methods are reminiscent of the dendrimer
architecture. Since the branch-cell arms are derived from oligomeric segments, the
products are referred to as polymeric dendrimers [22, 78, 79]. These more flexible
and extended structures exhibit unique and different properties compared to the
more compact traditional dendrimers. Fre ´chet, Hawker, and coworkers [80] have
utilized the techniques of living polymerization and a staged polymerization
process (in which latent polymerization sites are incorporated within growing
chains) to produce dendrigrafts of mixed composition and narrow polydispersity.
Another exciting development has been the emerging role that dendritic architecture is playing in the production of commodity polymers. A recent report by
Guan et al. [24] has shown that ethylene polymerizes to dendrigraft polyethylene
(dendri-polyethylene) at low pressures, in contrast to high-pressure conditions
which produce only simple branched topologies. This occurs when using latetransition metal or Brookhart catalysts. Furthermore, these authors also state that
small amounts of dendri-poly(ethylene) architecture may be expected from analogous early-transition-metal metallocene catalysts.
3.3.3 Dendrons and Dendrimers
Dendrons and dendrimers are the most intensely investigated subset of dendritic
polymers. In the past decade, over 6,000 literature references have appeared dealing
with this unique class of structure-controlled polymers. The word “dendrimer” is
derived from the Greek words dendri- (tree-branch-like) and meros (part of), and
was coined by Tomalia, et al. about 20 years ago in the first full paper on PAMAM
dendrimers [45, 46]. Since this early disclosure, over 125 dendrimer compositions
(families) and 1,100 dendrimer surface modifications have been reported. The two
most widely studied dendrimer families are the Fre ´chet-type polyether compositions and the Tomalia-type PAMAM dendrimers. PAMAM dendrimers constitute
the first dendrimer family to be commercialized, and represent the most extensively
characterized and best-understood series at this time [55].
Dendrimer Synthesis: Divergent and Convergent Methods
In contrast to traditional polymers, dendrimers are unique core–shell structures
possessing three basic architectural components (Fig. 10): a core, an interior of
shells (generations) consisting of repeating branch-cell units, and terminal
338
D.A. Tomalia