PerspectiveChemistry

Architecturally Complex Polymers with Controlled Heterogeneity

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Science  26 Aug 2011:
Vol. 333, Issue 6046, pp. 1104-1105
DOI: 10.1126/science.1209660

The properties of polymers depend not only on their composition—the types of monomers used to synthesize them—but also on their topology. Differences in how polymer chains are connected within the molecule can lead to materials properties that vary for polymers made from the same monomer. For example, high-density polyethylene made with mostly linear chains and few branches is stiff and strong, and can be used for pipes. When made with many branching chains, it is more flexible and can be used in shopping bags. More complex structures can be created with copolymers containing two or more monomers that allow variations in both composition and chemical functionality. In this way, advanced materials used in health and beauty products, optoelectronic and microelectronic materials, and structural applications have been developed (1, 2). The latest challenge is to combine all of these elements—composition, topology, and functionality—into one material, and to do so in ways that reduce the complexity and cost of synthesis.

There are many ways in which two or more different types of monomer units, which can be chosen from thousands of available monomers (3, 4), can be distributed along a polymer chain. Monomers can be distributed statistically, grouped into segments or blocks, repeated periodically, or changed progressively in composition to create a gradient. Topologies of polymers include (but are not limited to) linear, branched, hyperbranched, and even cyclic chains, as well as structures that resemble combs, stars, and lattice networks (see the figure, panel A). Various functional groups can be distributed randomly along the chain, or they can be placed in specific locations; for example, in telechelic polymers, reactive groups are placed at the chain ends. The functional groups can allow further reactions to occur—they can be polymerizable groups, so the chain acts as a “macro” monomer, or they can react with small molecules. When multiple architectural elements are combined into one material (see the figure, panel B), complex macromolecules can form, such as molecular brushes with grafted side chains in the form of gradient or block copolymers, pom-pom–like structures, or stars with mixed (“mikto”) arms with different functionalities.

A recently reported example combined block and gradient copolymers in a circular shape (5). The synthesis of cyclic polymers is among the most difficult tasks for polymer chemists, as the ends of long chains must somehow be brought together before they react with other chains (6). Reactions of various telechelic materials can proceed intramolecularly to form a cyclic product but also compete with intermolecular reactions that lead to longer polymer chains. To prepare sufficiently pure cyclic polymers, it is necessary to work under extreme dilution conditions or to use special templating processes to avoid intermolecular reactions (7). Alternatively, ring expansion can also lead to cyclic polymers, and such processes can also proceed catalytically (8).

Macromolecular architectural guide.

The three main aspects of polymer architecture—composition, topology, and function—are shown separately in (A) and in combination in (B).

The composition of the reported cyclic copolymer illustrates the challenges of creating both block and gradient copolymers (5). Pure block copolymers can be prepared in several ways, including coupling of two already synthesized blocks (a quite difficult process), using initiators that can grow two segments by different mechanisms (9), and switching from one monomer to another monomer during chain growth. Such reactions require very high selectivities for adding the correct monomer. Synthesis of a typical symmetrical diblock copolymer with a molar mass of ∼200,000 g/mol consisting of ∼1000 units of each monomer must proceed with selectivity of a chain growth over any chain-breaking reactions exceeding 99.9% for each block, and requires clever chemistry to switch efficiently from one monomer to the other.

However, these efforts are worthwhile in that they can lead to new nanostructured materials. Most polymers cannot mix with each other but instead separate like oil and water, so block copolymers often adopt morphologies in which one polymer forms nanoscale components within the other polymer, which acts as a matrix. Different morphologies can result by changing how the blocks are connected, as well as the volume fraction and the relative solubility of the different blocks. Typically, diblock copolymers separate into spherical, cylindrical, and lamellar morphology, but triblock copolymers can lead to more than 30 different and more complex structures (10). Block copolymers are used as thermoplastic elastomers, dispersants, surfactants, additives, and many other advanced-technology products (1).

Gradient copolymers are in a sense easier to prepare than pure block copolymers because the synthesis can be switched gradually from one monomer to the other. They can be considered to be “fuzzy” block copolymers; the phase boundary in nanostructured morphologies is more diffuse than that found in pure block copolymers (11). However, they have many new exciting properties, including an unusually broad temperature range for vitrification and several energy-relaxation processes that confer excellent vibration- and noise-damping properties. Gradient copolymers have much higher critical micelle concentration (the maximum concentration of free surfactants in solution), which leads to very good surfactant properties. Thus, these gradient copolymers, which can be viewed as a “poor man's” block copolymer, may be even more interesting than pure block copolymers.

Precisely controlled size distribution of polymer chains—that is, a low-molecular weight distribution (MWD)—is usually the target for most polymer synthetic efforts, but gradient copolymers can sometimes outperform pure block copolymers despite a broader MWD. For example, they can lead to materials with new morphologies and much larger nanodomains than block copolymers with very low dispersity (1214). Thus, control of MWD and designed shape of MWD—which is accessible by using exchange reactions between macromolecular growing species of different activity (15), some deliberately chosen chain-breaking reactions, or even slow initiation—can open new avenues to nanostructured materials.

The cyclic polymer with both blocky and gradient structures and relatively broad MWD reported in (5) illustrates how several architectural features can be incorporated into one macromolecule. It will be interesting to precisely characterize such polymers and correlate their properties with molecular structural parameters and to compare cyclic and linear analogs. New polymers with complex architecture and synergistically combined controlled heterogeneity, including MWD, topology, functionality, and composition, can lead to new advanced nanostructured functional materials.

References and Notes

  1. I thank A. M. Elsen for helpful discussion. Supported by NSF grant DMR 09-69301.

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