In polymer science, star-shaped polymers are the simplest class of branched polymers with a general structure consisting of several (at least three) linear chains connected to a central core. The core, or the center, of the polymer can be an atom, molecule, or macromolecule; the chains, or "arms", consist of variable-length organic chains. Star-shaped polymers in which the arms are all equivalent in length and structure are considered homogeneous, and ones with variable lengths and structures are considered heterogeneous. Star-shaped polymers' unique shape and associated properties, such as their compact structure, high arm density, efficient synthetic routes, and unique rheological properties make them promising tools for use in drug delivery, other biomedical applications, thermoplastics, and nanoelectronics among other applications.
History Star-shaped polymers were first reported by John Schaefgen and Paul Flory in 1948 while studying multichain polymers; they synthesized star-shaped polyamides. The next major publication regarding star-shaped polymers was in 1962 by Maurice Morton et al. Their research presented the first study demonstrating a method to create well-defined star-shaped polymers; this route was through living anionic polymerization. Many studies on the characteristics, syntheses, and applications of star-shaped polymers have since been undertaken and remain an active area of study.
Nomenclature Recommendations on nomenclatures still differ widely across different regulatory bodies (IUPAC, CAS, MDL). According to IUPAC star-shaped polymers are designated by a star prefix which can be further specified as f-star when the number of arms f is known. An example would be star-(polyA; polyB; polyC) for a variegated (heteroarm) star polymer with three arm species, but an undefined number of arms and distribution of arms. When the number of arms and its distribution is known this can be designated as for example 6-star-(polyA(f3); polyB(f3)) where 6 arms exist in total whereof 3 consist of polyA polymer. Stars containing only one species (same chemistry and molar mass) of arms are called regular stars (also called homo-arm). Stars with more than one arm species are designated as variegated stars (hetero-arm).
Properties
Structure Star-shaped polymers consist of a multifunctional center from which at least three polymer chains (arms) radiate. These arms can be chemically identical (homostars) or different (heteroarm stars). Additionally, individual arms may be composed of multiple polymers, resulting in star-block polymers or star copolymers. The unique properties of star-shaped polymers come from their chemical structure as well as the length and number of their arms.
Dynamic and rheological properties Some of the most interesting characteristics exhibited by star-shaped polymers are their unique rheological and dynamic properties compared to linear analogues of identical molecular weight and monomer composition. Generally, they have smaller hydrodynamic radii, radii of gyration and lower internal viscosities than linear analogues of the same molecular weight. Internal viscosity increases with increased functionality and molecular weight of branches with the effects of functionality eventually saturating, leaving viscosity dependent only on molecular weight of the arms. Heteroarm stars have observed viscosities and hydrodynamic radii higher than homostars. This is due to the increased repulsive interactions that occur as a result of a greater number of heterocontacts between the different arms. In addition, star-shaped polymers exhibit lower melting points, lower crystallization temperatures and lower degrees of crystallinity than comparable linear analogues.
Self-assembly The unique self-assembly properties of star shaped polymers make them a promising field of research for use in applications such as drug delivery and multiphase processes such as separation of organic/inorganic materials. Generally, star-shaped polymers have higher critical micelle concentrations, and so lower aggregation numbers, than their analogous, similar molecular weight linear chains. The addition of functional groups to the arms of star-shaped polymers as well as selective solvent choice can affect their aggregation properties. Increasing the number of functional groups while retaining the same molecular weight decreases aggregation numbers. Heteroarm polymers have been shown to aggregate into particularly interesting supramolecular formations such as stars, segmented ribbons, and core-shell-corona micellar assemblies depending on their arms' solubility in solution, which can be affected by changes in temperature, pH, solvent, etc. These self-assembly properties have implications for solubility of the whole star polymers themselves and for other solutes in solution. For Heteroarm polymers, increasing the molecular weight of soluble chains increases the overall solubility of the star. Certain Heteroarm star-block polymers have been shown to stabilize water-organic solvent emulsions, while others have demonstrated the ability to increase the solubility of inorganic salts in organic solutions.
Syntheses
Star-shaped polymers can be synthesized through various approaches. The most common syntheses include an arm-first approach, in which the living chains are used as the initiators, and a core-first approach, in which the core is used as the initiator. Other synthetic routes include: controlled sol-gel processes, group transfer polymerization, transition metal catalysis, living anionic polymerization, living cationic polymerization, ring opening polymerization, ring-opening metathesis polymerization (ROMP), and controlled radical polymerization.
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