## 2 Synthetic Approaches to Miktoarm Star Polymers
### Core-First Synthesis  
This methodology, as the name suggests, involves initiating polymerization from a core, and incorporates standard polymerization techniques that do not involve coupling or conjugation of separately synthesized polymeric chains. It includes methods, such as atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), anionic/cationic polymerization, and ring-opening polymerization (ROP). A notable early example demonstrating the flexibility and power of core-first methods was the synthesis of a trifunctional initiator molecule, with a (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) group for the stable free radical polymerization (SFRP) of styrene, a bromoisobutyrate for ATRP of \(t\)-butyl acrylate, and a free OH for ROP of caprolactone (Scheme 2) [101]. Each of these polymerization methods could be applied to a wide variety of different monomers. For example, substituting caprolactone with ethylene oxide for ROP would yield a PEG chain in place of PCL, thus making an amphiphilic miktoarm polymer suitable for aqueous self-assembly. Core-first synthesis has also been used to prepare amphiphilic miktoarm polymers for drug delivery. A series of A\({}_{2}\)B, A\({}_{2}\)B\({}_{2}\), AB\({}_{3}\), and A\({}_{3}\)B (A = polycaprolactone (PCL), B = poly(oligo(ethylene glycol)) monomethyl ether methacrylate) (POEGMA) miktoarm polymers, were synthesized from pentaeryth nitol and 1,1,1-tris(hydroxymethyl)ethane, tetrafunctional and trifunctional cores bearing 4 or 3 hydroxyls, respectively. Protecting/deprotecting strategies were used to functionalize these cores with \(\alpha\)-bromoisobutyryl bromide, an ATRP initiator. Unfunctionalized OH positions were then used to initiate the polymerization of caprolactone, after which ATRP of OEGMA was carried out at the remaining Br activated positions. The miktoarm polymers were found to have no significant cytotoxicities, could be self-assembled into micelles, and were able to efficiently encapsulate and deliver doxorubicin (DOX) to cells. Interestingly, the most hydrophobic PCL\({}_{3}\)POEGMA derived micelles had the lowest CMC, the highest DOX encapsulation efficiency, and the best therapeutic efficiency for DOX release [70]. Core-first methods are compatible with a variety of polymerization reactions. An AB\({}_{2}\) (A = poly(L-lactide) (PLLA), B = poly(N-acryloylmorpholine) (PNAM)) miktoarm polymer was prepared using a 1,1,1-tris(hydroxymethyl)ethane core, as in the above study. However, 2-bromopropolipid bromide and potassium ethyl xanthogene were sequentially coupled onto two of the core's hydroxy groups, to yield a multifunctional core containing two RAFT initiating groups and a free hydroxy group. The latter was employed to initiate the ROP of L-lactide, and subsequently the two RAFT initiating ends were used for the polymerization of N-acryloylmorpholine to give the final AB\({}_{2}\) polymer (Scheme 3) [72]. Since most amphiphilic polymers use PEG to confer aqueous solubility, PNAM is an interesting substitute noted for its low cytotoxicity, and promising properties, including its high aqueous solubility and low cytotoxicity [102; 103]. Figure 2: Core-first synthesis of an ABC miktoarm polymer (A = PCL, B = PS, C = PtBA) from a multifunctional core, using varied polymerization techniques. In another study, a \"macro-core\" was generated by conducting the anionic ROP of glycidol from methoxy PEG, leading to a highly branched polyglycerol (PG). The ROP of caprolactone was then performed using the PG terminal hydroxy ends as initiators to yield an AB\({}_{10}\) (A = PEG, B = PCL) mikoarm polymer, with good loading efficiency and sustained release of prednisone acetate, an adrenocortical hormone drug [37]. The methodology used in this procedure can be argued to be of a mixed type, and in fact, most miktoarm polymer syntheses in drug delivery do use a combination of core-first and arm-first methods.