New advances beyond the traditional AB 2 Flory-type, branch cell monomers
have been reported by Fre ´chet and coworkers [69, 70]. They introduced the concept
of latent AB 2 monomers, referred to as self-condensing vinyl polymerizations
(SCVP). These monomers, which possess both initiation and propagation properties, may follow two modes of polymerization: polymerization of the double bond
(i.e., chain growth) and condensation of the initiating group with the double bond
(i.e., step growth). Recent progress involving the derivative process of selfcondensing, ring-opening polymerizations (SCROP) has been reviewed by Sunder
et al. [71]. In addition, the use of enhanced processing techniques such as pseudo
chain growth by slow monomer addition [72], allow somewhat better control of
hyperbranched structures [71].
3.3.2 Dendrigraft Polymers
Dendrigraft polymers are the most recently discovered and currently the least
understood subset of dendritic polymers. The first examples were reported in
1991 independently by Tomalia et al. [73] and Gauthier et al. [74]. Whereas traditional monomers are generally employed in constructing dendrimers, reactive
oligomers or polymers are used in protect–deprotect or activation schemes to
produce dendrigrafts. Consequently, dendrigraft polymers are generally larger
structures than dendrimers, grow much faster, and amplify surface groups more
dramatically as a function of generational development. Both hydrophilic
[e.g., poly(oxazolines) and poly(ethyleneimines)] and hydrophobic (e.g., polystyrenes) dendrigrafts were reported in these early works. These first methodologies
involved the iterative grafting of oligomeric reagents derived from living polymerization processes in various iterative “graft-on-graft” strategies. By analogy to
dendrimers, each iterative grafting step is referred to as a generation. An important
feature of this approach is that branch densities, as well as the size of the grafted
branches, can be varied independently for each generation. Furthermore, by initiating these iterative grafting steps from either a point-like core or a linear core it is
possible to produce spheroidal and cylindrical dendrigrafts, respectively.
Depending on the graft densities and molecular weights of the grafted branches,
ultrahigh molecular weight dendrigrafts (e.g., M w > 104 kDa) can be obtained at
very low generation levels (e.g., G ¼ 3). Dramatic molecular weight enhancements
vis-a `-vis other dendrimer propagation methodologies are possible using dendrigraft
techniques [75]. Further elaboration of these dendrigraft principles allowed the
synthesis of a variety of core–shell-type dendrigrafts, in which elemental composition as well as the hydrophobic or hydrophilic character of the core were
controlled independently [74].
In general, the above methodologies have involved convergent-type grafting
principles whereby preformed, reactive oligomers are grafted onto successive
branched precursors to produce semicontrolled structures. Compared
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have been reported by Fre ´chet and coworkers [69, 70]. They introduced the concept
of latent AB 2 monomers, referred to as self-condensing vinyl polymerizations
(SCVP). These monomers, which possess both initiation and propagation properties, may follow two modes of polymerization: polymerization of the double bond
(i.e., chain growth) and condensation of the initiating group with the double bond
(i.e., step growth). Recent progress involving the derivative process of selfcondensing, ring-opening polymerizations (SCROP) has been reviewed by Sunder
et al. [71]. In addition, the use of enhanced processing techniques such as pseudo
chain growth by slow monomer addition [72], allow somewhat better control of
hyperbranched structures [71].
3.3.2 Dendrigraft Polymers
Dendrigraft polymers are the most recently discovered and currently the least
understood subset of dendritic polymers. The first examples were reported in
1991 independently by Tomalia et al. [73] and Gauthier et al. [74]. Whereas traditional monomers are generally employed in constructing dendrimers, reactive
oligomers or polymers are used in protect–deprotect or activation schemes to
produce dendrigrafts. Consequently, dendrigraft polymers are generally larger
structures than dendrimers, grow much faster, and amplify surface groups more
dramatically as a function of generational development. Both hydrophilic
[e.g., poly(oxazolines) and poly(ethyleneimines)] and hydrophobic (e.g., polystyrenes) dendrigrafts were reported in these early works. These first methodologies
involved the iterative grafting of oligomeric reagents derived from living polymerization processes in various iterative “graft-on-graft” strategies. By analogy to
dendrimers, each iterative grafting step is referred to as a generation. An important
feature of this approach is that branch densities, as well as the size of the grafted
branches, can be varied independently for each generation. Furthermore, by initiating these iterative grafting steps from either a point-like core or a linear core it is
possible to produce spheroidal and cylindrical dendrigrafts, respectively.
Depending on the graft densities and molecular weights of the grafted branches,
ultrahigh molecular weight dendrigrafts (e.g., M w > 104 kDa) can be obtained at
very low generation levels (e.g., G ¼ 3). Dramatic molecular weight enhancements
vis-a `-vis other dendrimer propagation methodologies are possible using dendrigraft
techniques [75]. Further elaboration of these dendrigraft principles allowed the
synthesis of a variety of core–shell-type dendrigrafts, in which elemental composition as well as the hydrophobic or hydrophilic character of the core were
controlled independently [74].
In general, the above methodologies have involved convergent-type grafting
principles whereby preformed, reactive oligomers are grafted onto successive
branched precursors to produce semicontrolled structures. Compared
Twenty-First Century Polymer Science After Staudinger: The Emergence of. . .
337