Star-shaped polymer
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