Star-shaped polymer

Star-shaped polymers consist of a multifunctional center from which at least three polymer chains (arms) radiate.[13] 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.[13]

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.[4][1][13] 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.[4][14] 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.[1] In addition, star-shaped polymers exhibit lower melting points, lower crystallization temperatures and lower degrees of crystallinity than comparable linear analogues.[13]

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.[1] 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.[1] 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.[1][15] 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.[1] 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.[13]

In the arm-first (also known as the "arm-in" or convergent approach[1]) method, monofunctional living polymers with known characteristics are used as precursors in the reaction. The active site at the end of their chain can be directly reacted with an appropriately reactive multifunctional polymer core (also known as a linking agent[1]) to produce a star-shaped polymer. In this approach the resulting star-polymer consists of homogeneous chain groups. The arm-first synthesis route is arguably the most efficient synthesis of star-shaped polymers.[1][16] This is because each step can be directly controlled and assessed; the arms and core can be isolated and characterized prior to a stoichiometric reaction, and the functionality of the final star-polymer can then be accurately and directly measured.

One common approach to the arm-first synthesis is through anionic polymerization methods. This involves using "arms" that are anionic and reacting them with a core containing deactivating groups for the arms to react with.[16] The deactivating groups on the core are often chlorosilanes, chlorine leaving groups, or deactivating alkenes. Chlorosilanes serve as especially reactive cores, and can react quantitatively (or very close to quantitatively) with carbanion living polymers; this reaction involves carbanions performing electrophilic substitution with the Si-Cl groups (as shown in the below figure). In a case like this, the resulting arms are all homogeneous and can be well characterized, and the core can also be well characterized, leading to a well-characterized star-shaped polymer. Since both the core and the arms are rather reactive, essentially all Si-Cl undergo electrophilic substitution, and the resulting star-shaped polymers thus have a rather narrow polydispersity index.[16]

In the core-first approach (also known as the "arm-out" or divergent approach[1]), a multifunctional core serves as the initiator simultaneously for several arms. This approach proves to be more complicated than the arm-first approach, in that finding an appropriate and stable core is difficult, and characterizing the synthesized star-polymer is challenging.[16]

The core-first route was first approached in 1988 through functionalizing DVB using potassium naphthalenide to create a multifunctional core.[17] The core can than be reacted with ethylene oxide to create a star-shaped polymer. As is typical of most core-first approaches, this scheme had issues with high viscosity and gelation. The star-shaped polymer was characterized by size-exclusion chromatography and light scattering techniques.