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The Beauty of Branching in Polymer Science
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The Beauty of Branching in Polymer Science
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Macromolecules

Cite this: Macromolecules 2020, 53, 9, 3257–3261
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https://doi.org/10.1021/acs.macromol.0c00286
Published May 12, 2020

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Copyright © 2020 American Chemical Society

A number of major research themes have emerged during the 100 years following Staudinger’s landmark 1920 paper (1) “Über Polymerization” and the establishment of polymer science as a discipline. Concepts such as block copolymers, living polymerizations, biodegradable materials, and dendritic macromolecules are now central to the next macromolecular century. It is therefore valuable to use this anniversary to personally look back at the genesis of these scientific directions. For ourselves, two pivotal papers in Macromolecules—(i) Tomalia et al. (2) “Dendritic Macromolecules: Synthesis of Starburst Dendrimers” (1986) and (ii) Kim and Webster (3) “Hyperbranched Polyphenylenes” (1992)— illustrate the emergence of branching as an important and tunable structural feature for the control of polymer properties.

Branching can range from hard to detect side reactions during the growth of linear polymers to perfectly branched, fractal-like dendrimers. As synthetic materials, dendrimers come closest to resembling proteins in their three-dimensional structure and discrete molecular weight. In fact, branched/fractal structures are widely found in nature and have attracted significant attention throughout history for their beauty and geometric complexity. Today a multitude of molecular-based systems are known to have fractal features including the actin cytoskeleton, hyperbranched glycogen, or amylopectin. Building on these natural systems, Paul Flory calculated in the 1950s the molecular weight distribution of ABx polycondensation products and demonstrated that no cross-linking can occur in such systems, in contrast to A2 + B3 systems. (4) However, it was not until the late 1970s that synthetic chemists developed the first strategies for the deliberate preparation of well-defined, branched molecules. (5) In doing so, they significantly extended macromolecular architectures beyond traditional linear or cross-linked materials. These were given a variety of names, such as arborols, starburst polymers, or cascade molecules; however, it is now generally accepted that dendritic macromolecules broadly cover all highly branched systems with dendrimers encompassing regular, near monodisperse materials and hyperbranched macromolecules comprising less regular, polydisperse structures (Figure 1).

Figure 1

Figure 1. Four major classes of macromolecular architectures as illustrated by Tomalia et al. (5)

For the synthesis of high molecular weight dendrimers, a major move away from traditional one-step polymerization processes to repetitive, multistep strategies was required. More akin to small molecule synthesis, a key insight is the acceleration in molecular weight buildup that is provided through the symmetrical nature of dendrimers. This shift in synthetic design was followed by an increase in focus on the preparation of monodisperse macromolecules and introduction of concepts such as generation number, focal point group, and degree of branching—all of which had to be adapted to fit these new materials. This marriage of organic chemistry and polymer synthesis caused an explosion of interest which has resulted in more than 10000 scientific papers and patents published in the area of dendritic macromolecules over the past 35 years. The overall impact of this rapid evolution in thinking has given polymer researchers another “knob to turn” in their continual search for new and/or improved properties.

In this Editorial, we have the distinct honor of discussing these two seminal Macromolecules papers, (2,3) both instrumental in defining dendritic polymers and bringing the community’s attention to the unique properties and potential for highly branched systems. In turn, these studies focused awareness on the role of controlled branching in developing structure–function relationships across polymer science, inspiring the postulation of fascinating questions, such as “when does a branched polymer become a particle?”, (6) and building on the development of other macromolecular architectures including brush or cyclic polymers.

In helping to develop dendrimers as a new macromolecular architecture, Don Tomalia took advantage of the research culture at Dow Chemical coupled with his passion as an amateur horticulturist. Mimicking the branching structure of trees (dendra, the Greek word for tree), Tomalia and his team pursued the idea of directing molecular growth in a stepwise manner by adding branch after branch to a central core molecule. On the basis of the combination of efficient Michael addition chemistry with transamidation, Tomalia reported the preparation of high molecular weight dendrimers using a divergent approach where ammonia is initially reacted with methyl acrylate to give a first-generation dendrimer with three terminal ester groups followed by a second amidation step with an excess of ethylenediamine (Figure 2). This leads to the regeneration of reactive primary N–H units and doubling of the number of N–H units, from three for ammonia to six for the first-generation dendrimer. Repetition of this two-step process then allows the molecular weight of the dendrimer to essentially double for each generation and the number of reactive terminal groups to increase in a geometric fashion—3 to 6 to 12 to 24 and upward. By use of this strategy, PAMAM dendrimers have been grown up to generation 11 with molecular weights of over 1000000 Da. Because of the close packing of end groups in higher generation dendritic structures, simple geometrical considerations dictate incomplete growth with an increasing number of failure sequences and associated dispersity. (2,5,7) Additional challenges are associated with the very large excess of ethylenediamine employed at higher generation numbers leading to purification issues.

Figure 2

Figure 2. Original “starburst” dendrimer synthesis using the divergent growth approach by Tomalia and co-workers. (2,5,7)

A consequence of this stepwise approach is that dendrimers have a unique set of features making them distinct from conventional polymers. This includes precise control over shape, size, and molecular weight as well as three distinguishing architectural features: a core, internal layers defined as generations, and a multitude of chain end or terminal functionalities. Significantly, each of these can be finely tuned to give a myriad of possible structures which allow more rigorous characterization when compared to traditional polymers. Various analytical methods such as 1H and 13C NMR spectroscopies have proven to be especially useful, in combination with size exclusion chromatography and mass spectroscopy, for illustrating the near-monodisperse and discrete nature of dendritic structures.

Other notable contributions around this time, including the synthesis of cascade polyols (arborols) by Newkome and co-workers (8) at Louisiana State University and the synthesis of dendritic poly(lysine) derivatives by Denkewalter at AlliedSignal, (9) clearly demonstrated the wide variety of building blocks and associated chemistries that could be used for the preparation of dendrimers. An explosion of other dendritic structures based on alternative building blocks and growth chemistries was then subsequently published in Macromolecules, (10−14) illustrating the ability to tune the reactivity and stability. Stability is an important consideration for these highly functional materials with stability studies being facilitated by the monodisperse nature of dendrimers. (15)

These initial studies also foresaw the potential of these branched systems as unique, three-dimensional microenvironments having a well-defined outer surface. A pivotal example is the development of the “dendritic box” by Bert Meijer and co-workers in 1994. (16) This seminal study was a powerful illustration of the potential of dendrimers, taking advantage of their highly branched nature to achieve a dense packed outer shell that acts as a molecular barrier to diffusion. Notably, this phenomenon was predicted by Pierre de Gennes as a fundamental property of dendrimers driven by their highly branched structure with this surface congestion now referred to as “de Gennes dense packing”. (17) In the Meijer system, modification of a fifth-generation poly(propyleneimine) dendrimer with N-BOC-l-phenylalanine groups leads to supramolecular encapsulation of guest molecules in the internal cavities of the dendrimer driven by preferred interactions of guests with the inner atoms of the dendrimer (Figure 3). Remarkably, the diffusion of guest molecules out of the “dendritic box” into solution was unmeasurably slow because of the close packing of the shell, even though the overall diameter of the dendrimer was ∼4.5 nm, with the shell being <1 nm thick. Theory and simulations of these systems by Ballauff (18) and Goddard (19) have provided key insights into the question of the dynamic nature of chain end back-folding and how the shape and inner structure of dendrimers depend on the generation number as well as the effective interactions that exist between dendrimers in solution. Molecular encapsulation has subsequently evolved to include a number of other polymer architectures including single-chain nanoparticles. (20)

Figure 3

Figure 3. Rose Bengal molecules encapsulated within a “dendritic box” as initially proposed by Meijer et al. (16)

In expanding the impact of dendrimers prepared by the divergent growth approach, a number of disadvantages, driven primarily by the increasing number of reactions required for full functionalization, became apparent. In addressing this challenge, Hawker and Fréchet introduced the convergent growth approach to dendrimers. (21,22) Instead of growing the dendrimer from a central core through an ever-increasing number of peripheral coupling steps, the convergent growth approach starts at what will become the periphery of the molecule and requires only a limited number of coupling steps for all generations, resulting in increased purity for higher generation materials. Larger and larger dendritic fragments can therefore be prepared in a stepwise fashion with the final reaction being coupling with a central core or focal point group. In a similar fashion to divergently grown dendrimers, a wide selection of building blocks and associated chemistries is available with the convergent growth approach allowing unparalleled control over functionality at specified locations within the dendrimeric framework. This strategy also provides easy access to numerous novel architectures such as hybrid dendritic–linear diblock copolymers where a single, linear polymer chain is attached to the focal point of a dendritic fragment (Figure 4). (23)

Figure 4

Figure 4. Hybrid dendritic–linear diblock copolymer based on 10K linear PEO. (23)

The excitement surrounding dendrimers drove research in many different directions from fundamental studies concerning shape and chain end location (24) to applications as drug delivery agents. (25) However, an underlying issue was, and still is, synthetic availability. Obtaining a large amount of a fifth-generation dendrimer is challenging. Recognizing this as an opportunity to bridge the gap between discrete, highly branched dendrimers and traditional linear polymers, DuPont scientists Young Kim and Owen Webster reported the one-step preparation of soluble, branched polyphenylene derivatives from AB2 monomers, coining the term “hyperbranched macromolecules” in 1990 (Figure 5). (26) This was a surprising and unexpected result since linear polyphenylenes are known for their insolubility at even low molecular weights. The impact on the community of a procedure for preparing high molecular weight polyphenylenes that were not only soluble but also soluble in water cannot be overstated.

Figure 5

Figure 5. Hyperbranched polyphenylenes. (3)

In 1992, Kim and Webster published a key follow-up paper in Macromolecules, “Hyperbranched Polyphenylenes”, which detailed the characterization and postsynthetic modification of these hyperbranched polyphenylenes to alter mechanical and solubility characteristics. (3) In a single step and on a large scale, they were able to prepare soluble derivatives with molecular weights in the range 5000–35000 and dispersities <1.5 while still maintaining thermal stability up to 550 °C.

The commercial potential of this work can be seen in the range of scalable systems that have been subsequently developed in academic and industrial laboratories from readily available ABx monomers through both condensation and addition chemistries. (27,28) This includes hyperbranched poly(ethylenimines), first commercialized by BASF in the 1970s as Lupasol, (29) polyethers, (30) Boltorn hyperbranched polyesters which are available with hydroxyl, amino, fatty acid, and nonionic peripheral functionality, (31) and Hybrane hyperbranched polyesteramide developed by DSM. (32) A distinguishing feature of these systems is the concept of degree of branching, (33) where the percentage of linear, dendritic, and terminal units within the structure is mathematically analyzed. For the majority of hyperbranched polymers, this value is ∼0.5, which places them between linear polymers and dendrimers in terms of branching. This level of branching is borne out in the physical and mechanical properties of hyperbranched polymers which are intermediate between entangled linear polymers and nonentangled dendrimers.

In 30+ years since the first experimental manifestations of dendrimers and hyperbranched polymers, the impact on broader fields within polymer science is readily apparent. The concepts have permeated neighboring scientific disciplines and driven new research in exploring different branched architectures and the powerful interplay between structure, properties, and function. While the acceptance of dendritic structures in polymer science was initially slow, Paul Flory in the 1980s summed up the potential succinctly: “architecture is a consequence of special atom relationships and just as observed for small molecules, different properties should be expected for new polymeric architectures”. (5) The availability of branched architectures opened up new possibilities in polymer research, and their applications in fields ranging from viral vectors to rheology modifiers and porogens have continued to expand. (34,35) We owe much to the pioneering Macromolecules publications from Tomalia and co-workers in 1986 and Kim and Webster in 1992.

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Acknowledgments

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Primary support by the National Science Foundation (NSF) through the Materials Research Science and Engineering Center at UC Santa Barbara (DMR-1720256) is gratefully acknowledged.

References

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Macromolecules

Cite this: Macromolecules 2020, 53, 9, 3257–3261
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https://doi.org/10.1021/acs.macromol.0c00286
Published May 12, 2020

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  • Figure 1

    Figure 1. Four major classes of macromolecular architectures as illustrated by Tomalia et al. (5)

    Figure 2

    Figure 2. Original “starburst” dendrimer synthesis using the divergent growth approach by Tomalia and co-workers. (2,5,7)

    Figure 3

    Figure 3. Rose Bengal molecules encapsulated within a “dendritic box” as initially proposed by Meijer et al. (16)

    Figure 4

    Figure 4. Hybrid dendritic–linear diblock copolymer based on 10K linear PEO. (23)

    Figure 5

    Figure 5. Hyperbranched polyphenylenes. (3)

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