Chemistry Reference
In-Depth Information
to industrial chemists, organic targets related to polymers or random assemblies of
specific monomers result. There are two different approaches to the creation of these
new macromolecules possessing a fractal design. One, in which the branching sites
are located at the termini, is generally comprised of industrially available monomers;
whereas, the other route utilizes a monomer or monomers possessing an internal
branching center as part of its composition. We undertook the later—that is, the
creation of new monomers that can be assembled to make 1
3 branching motifs
and have an inherent nonreversibility leading to an overall structural stability.
An iterative or repeating assembly is the second part of the molecular construction.
If traditional industrial monomers are used, retro-reactions and diverse side reactions
can occur resulting in a rather heterogeneous assembly composed of alternating
patterns. Easily accessible and readily assembled but uniform final families of
products resulted. The vast majority of all reported dendritic studies have been
conducted with only a few families of divergently generated dendrimers, of which
PAMAMs and PPIs dominate the older literature. In view of Mother Nature's forests,
whywould only one or two or three tree designs suffice? Thus, in general, convenience
or commercial availability leads to fractal-structured families possessing a uniform
exterior, which can be uniformly or randomly/combinatorially coated with little or no
control of the resultant product.
This chapter addresses the creation of branching monomers that can be assembled
in different ways, thus challenging chemists to devise interesting new patterns to
address specific objectives, targets, or needs. In our initial example [4] appearing
25 years ago, the core, the first branching layer and the surface—although divergently
(inside-to-outside) assembled—were comprised of different branching monomers;
this started to address molecular diversity. It was 6 years later, that Frechet and
Hawker reported [5] the convergent (outside-to-inside) approach to molecular growth
and introduced the use of the term “dendrons” for these wedges.
Since there is a litany of potential monomers or dendrons that can be devised, each
resultant synthetic target should start to address new and potentially utilitarian
composition. Herein, we briefly review families of designer monomers, then propose
other possibilities and, hopefully, challenge others to be adventuresome and create
new predendrons and dendrons to meet tomorrow's needs. Mother Nature has
demonstrated that there is a multitude of diverse structural patterns for the trees in
her forest; can they be mimicked within the chemist's domain?
Upon construction of the layers—or generations as they are commonly called—the
number of surface groups on a tetravalent core for a 1
!
!
3 branched dendrimer
increases (4, 12, 36, 108,
...
), which is faster than that of a 1
!
2 branched dendrimer
(4, 8, 16, 32,
), leading to increasing steric crowding at the periphery. This
construction makes use of preferred branched monomers or dendrons, allowing for
higher generations with fewer peripheral gaps and defects. Scheme 3.1 shows the
different modes of construction with a tetravalent core moiety. Route A illustrates the
divergent method of construction, building outward from a tetravalent core, generally
using industrial monomers. Transformation of the new termini at each generation
allows for further substitution and dendritic growth. Attaching preformed dendrons of
specific generation sizevia the convergent method (Route B) avoids the lack-of-control
...
