Core-first strategy

Generally, there are two strategies to prepare star polymers the core-first strategy [37-44], and the arm-first strategy [45-52], The arm-first strategy starts with the linear arms first. Since the arms are prepared separately, many living/controlled polymerization techniques can be employed. Thus, the linear arms can be synthesized in a defined manner. Then one of the chain ends will be functionalized for further crosslinking reactions. Based on the functionalities of the chain ends, the arm-first methods can be divided into macroinitiator (MI) method and macromonomer (MM) method. 

Scheme 3 Core-first strategy for the synthesis of star polymers...

Based on the findings of the arm-first strategy, it is possible to obtain stars with well-defined arm length. The main problem of this strategy is the arm number distribution. Moreover, purification may cause many difficulties in the synthesis. In contrast, the core-first strategy requires multifunctional initiators and further polymerization initiated from the core. This is shown in Scheme 3. The maximum arm numbers of the stars are determined by the number of functionalities in the core. In the ideal case, the initiating efficiency of the core is close to unity, which will produce well-defined stars with precise numbers of arms. However, due to the steric hindrance and the limit of the polymerization techniques, it can be difficult to obtain full initiating efficiency. 

Star copolymers can be prepared by two main synthetic routes the arm-first and core-first methodologies. In the first case, living, or end-reactive, polymer chains are coupled to a multifunctional core. In the second case, a multifunctional core molecule is used to initiate the polymerization of the arms. Most of the star polypeptides reported so far have been obtained following the core-first strategy and were prepared using conventional primary amine-initiated NCA polymerization. In this way, Daly successfully prepared a series of... 

Two general strategies are possible for the synthesis of star-shaped copolymers The arm-first method is based on the reaction of living chains with plurifunctional electrophiles carrying at least three reacting groups alternatively, polymerization can be initiated by a multifunctional initiator according to the core-first method. 

To build a nanoparticle in a core-shell strategy. The cores are synthesized first and then the functional molecules are added as a shell by simple adhesion (physical adsorption in an insolubilization strategy). Examples are given in the literature [71, 72]. Organic molecules forming the core of an ONP usually include functional groups to which other molecules are added, such as peptides/ proteins and nucleic acids. 

Controlled polymerization techniques have enabled the preparation of well-defined polyelectrolytes of different architectures. Polyelectrolyte stars and cylindrical brushes are two typical examples with isotropic and anisotropic nature, respectively. Different synthetic strategies have been developed for these polyelectrolytes. However, the core first and grafting from strategies have turned out to be the most suitable methods for the synthesis of polyelectrolyte stars and cylindrical brushes. 

Fig. 6.3 First-generation GLCs based on the core-pendant strategy accompanied by DSC data from second heating seans. In addition to the symbols defined in Fig. 6.2, Sa denotes smectic A mesophase. Used with permission [24]...

A fourth strategy involves the polymerization of a monomer out of multifunctional initiators, following a divergent ( core-firsf ) approach (Scheme 27.4) [6, 12-14]. All main CLP methods have been applied to the synthesis of core-first star polymers, including chain growth by LAP [43] and hving cationic polymerizations [44], ROMP [45], nitroxide-mediated polymerization (NMP) [46], ATRP [47], or RAFT polymerization via the so-called R-approach [48]. The conden-sative chain-growth polymerization method, as developed by Yokozawa etal, has also been applied to the synthesis of well-defined, core-first, star-shaped polyamides [49]. 

One of the most popular and effective approaches that has emerged is actually a hybrid between the core-first and the coupling-to routes (see Figure 6). In this strategy, a block copolymer is synthesized so that at the jimction point of the two blocks there is a chemical handle for REO coupling. This handle at the junction is usually an azide or an alkyne and is used to couple the third block, which is a preformed linear... 

A star polymer is composed of / linear arms each arm comprises N monomer units. As shown in Figure 3, the arms are grafted by one of the terminal segments onto a multiftinc-tional core. Two major strategies, that is, the core first and the arms first, are used in synthesis of star polymers "Hie arms first approach " enables one to prepare arms with well-defined length. However, the control over the number of arms successively attached to the core is poor, which leads to significant polydispersity of the samples. "Hie core first ... 

Our strategies to improve the cell permeabihty of our lead 9b targeted the low logD value of —0.16 [41], From a chemical point of view, the free carboxylate group and the pyrazolone core are ideal starting points for the introduction of lipophilicity by suited structural modifications. We decided to start with the pyrazolone core first, because variations of the carboxylic acids would only result in a typical prodrug approach. 

A simple sequential polymerization of a aoss-linker followed by polymerization of a monomer provides a broadly applicable approach to star copolymers. Scheme 26. This method belongs to the broader category of core-first methodology and presents an alternative strategy for star synthesis, when compared with the traditional arm-first method, in which monomer is polymerized first followed by formation of the core by (co)polymetization of a cross-linker. 

Historically, building up a library around a given core structure has been the first strategy to access natural product libraries. This approach is still the preferred way to prepare either focused libraries to study the SAR of certain natural products or to build libraries of very complex natural product scaffolds which cannot be constructed on solid phase yet. In Fig. 1 two notable structures of natural product libraries built by core structure modification are highlighted, such as an Indolactam library by Waldmann [72], and a Taxoid library by Xiao [73]. 

Further advances along that line include the development of anionic polymerization methods with the aim of attaining better control of the functionality in core-first methods and developing original strategies to access so-called heteroarm star-shaped polymers. 

Strategy First (1) find the total number of electrons (Z Co = 27). Then (2) find the electron configuration the first 18 electrons form the argon core, and the remaining electrons enter the 3d sublevel. Finally (3) apply Hund s rule to obtain the orbital diagram. 

---