Pre-1980s

Before the early 1980s, the possibility of synthesizing and isolating discrete tree-like, macromolecular structures was considered to be an impossible challenge [1]. However, such hypothetical, tree-like entities were often visualized and proposed as transient intermediates by Flory to explain his pioneering concepts in the area of gelation theory during the early 1940s [2–4]. Flory’s seminal work ultimately led to recognition of the second major macromolecular polymer (architecture) after Staudinger’s linear architecture, namely, cross-linked polymers. The traditional polymer world during this era was quite simple. All synthetic polymers at that time were classified into two major categories based on physico-chemical properties. They were referred to as either (I) thermoplastics or (II) thermosets as described earlier (Chapter 1, Section 1.1.2, Figure 1.3). The very first examples of simple branched polymer architectures were just beginning to emerge. The notion of polymeric architecture consisting of “branches upon branches” was not in the vocabulary of polymer scientists at that time. However, it is noteworthy that Flory occasionally made references to “tree branching” polymeric architecture. He often used this architectural term as a visual for describing transient hyperbranched species that he hypothesized were involved in pathways to the “crosslinked” or “gelation state.” These vague but prophetic concepts were soon demonstrated experimentally toward the end of the 1970s, and led to the fourth major class of macromolecular architecture, namely; “dendritic polymers.”

Metaphorically speaking, dendrons are now referred to as “nanoscale molecular trees.” Anatomically, the tree root is the focal point (i.e. apex) of the dendron, whereas the interior consists of amplified branching layers growing from the root, and these are terminated by the molecular tree leaves (Z) or surface moieties of the dendron. As described in Chapter 1, the term dendrimer, coined by Tomalia et al. [5], is now a widely accepted scientific term or descriptor [6] for multiples or clusters of these nanoscale trees. More specifically, dendrimers are discrete, soft matter nano-building blocks.