## 2 Synthetic Approaches to Miktoarm Star Polymers
### Chlorosilane Based Synthesis  
[PERSON] and coworkers were the first to prepare polymers with asymmetric branching based on an arm-first method using chlorosilane cores [82; 83; 84; 85; 86; 87]. Dubbed \"miktoarm\" after the Greek word \(\mu_{\text{IKTO}\zeta}\) (miktos) or \"mixed\", these polymers had a variety of branches emanating from a single core. More specifically, the first example of such a polymer was a construct made of polyisoprene (PI), polystyrene (PS), and polybutadiene (PB). Due to the increasing reactivity of the polymer anion termini in the order of PB \(>\) PI \(>\) PS, and the steric hindrance that follows the opposite trend, the miktoarm synthetic methodology was designed such that PI (with intermediate reactivity) would first be linked to a large excess of a chlorosilane core. Then, relying on the inability of the second PS arm to undergo complete reaction with PI-linked chlorosilane, PS was attached second, followed by excess PB, which would undergo complete exchange with the remaining Cl on the chlorosilane core (Scheme 1) [88].  
Figure 2: Examples of core-first synthesis (**left**) and arm-first synthesis (**right**) with examples of functionalities, monomers, and polymers.  
While this PB-PI-PS star, as described above, is mostly recognized as the first synthesized miktoarm polymer, a few years prior to this, [PERSON] and [PERSON] had reported A\({}_{2}\)A\({}^{\prime}\) polymers based on PS and PB arms with differing molecular weights, synthesized using a chlorosilane core-based approach [86]. Then in 1990, an AB\({}_{2}\)-type miktoarm polymer was prepared by the single grafting of PS end-capped with chlorosilane, onto the middle of a PI chain [89].  
[PERSON] and [PERSON] further took advantage of relative polymer chain reactivities to synthesize a variety of branched polymers. Using a tetrachlorosilane core, they were able to sequentially add anionic PS, poly(4-methyl styrene) (P4 MS), PI, and PB lithium salts, following each step with a titration, and confirming the ABCD miktoarm star end product via size exclusion chromatography (SEC). By swapping the middle components in the sequential addition with PS and PB, respectively, an A\({}_{2}\)B\({}_{2}\) miktoarm polymer was prepared [90].  
Due to the possible manipulations of chlorosilane cores in developing miktoarm architectures, more methodologies soon followed. While many of these included alterations of polymer addition onto trichlorosilane or tetrachlorosilane cores [91; 92; 93; 94; 95; 96; 97; 98], one of the more interesting examples was the synthesis of an A\({}_{8}\)B\({}_{8}\) (A = PS, B = PI) miktoarm polymer based on a core with 16 active chlorosilane bonds (called Si-Cl\({}_{16}\) for simplicity). This core was synthesized by transforming a tetravinylsilane initial core with methyldichlorosilane using vinylmagnesium bromide. PS, which was made into a living polymer chain with BuLi, was carefully reacted with Si-Cl\({}_{16}\) to produce PS\({}_{8}\)-(Si-Cl\({}_{8}\)). After product confirmation by SEC, the remaining chlorosilane bonds were used to link living PI chains to yield a PS\({}_{8}\)PIs miktoarm polymer [99]. [PERSON] and coworkers later used a different chlorosilane linker containing 6 Si-Cl bonds, 1,2-bis(trichlorosil)ethane, as the core for an AB\({}_{5}\) (A = PS, B = poly(2-methyl-1,3-pentadiene) (P2 MP)) miktoarm polymer. To avoid multiple PS conjugations, a living PS chain was slowly added dropwise to the core in solution, after which P2 MP was used to completely react with the remaining chlorosilane bonds [100]. Chlorosilane based synthesis has found great success over the years, and it paved the way for other miktoarm polymers. This methodology is well suited for the introduction of non-polar polymeric chains, and has largely been used with chains that have living anionic ends.  
Figure 1: Development of an ABC miktoarm polymer based on polyisoprene (PI), polystyrene (PS), and polybutadiene (PB), using the chlorosilane method.