Core-first star synthesis

The Chemistry of Radical Polymerization Tabic 9,29 Star Precursors for NMP 

I he method of polymerization needs to be chosen for compatibility with functionality in the cores and the monomers to be used. Star block copolymers have also been reported. Mulli(bromo-compounds) may be used directly as ATRP initiators or they can be converted to RAFT agents. One of the most common 

For ihe case of NMP and RAFT, there exist two basic ways of growing star copolymers (this discussion also applies to block and graft copolymer synthesis). 

In the arm-first approach the arms are prepared and then self-assembled to form the core. There are two main variants that will be considered. 

The arm-first synthesis of star microgels by initiating polymerization or copolymerizalion of a divinyl monomer such as diviiiylbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers. The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with -j-j p6 7-7oo 

The product contains dormant functionality in the core. Tliis can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP/° ATR ). 

The shcll-crosslinking of self assembled micelles based on block copolymers made by NMP or ATRP has been exploited extensively by Wooley and coworkersand others to make a variety of structures. RAFT has also 

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In this case, the chains are grown away from the core, and attached when undergoing transfer reactions hence, stars only contain branches in a dormant form. One advantage of this technique is that complications such as star-star couplings, as encountered in the core-first star synthesis, can be avoided. A potential problem, however, is the reduced accessibility to the RAFT moieties of the core by polymeric arms (shielding effect). Some typical multifunctional RAFT agents used in the Z-approach are shown in Figure 27.4. 

The first step for the core-first stars is the synthesis of multifunctional initiators. Since it is difficult to prepare initiators that tolerate the conditions of ionic polymerization, mostly the initiators are designed for controlled radical polymerization. Calixarenes [39, 58-61], sugars (glucose, saccharose, or cyclodextrins) [62-68], and silsesquioxane NPs [28, 69] have been employed as cores for various star polymers. For the growth of the arms, mostly controlled radical polymerizations were used. There are only very rare cases of stars made from nitroxide-mediated radical polymerization (NMRP) [70] or reversible addition-fragmentation chain transfer (RAFT) techniques [71,72], In the RAFT technique one has to differentiate between approaches where the chain transfer agent is attached by its R- or Z-function. ATRP is the most frequently used technique to build various star polymers [27, 28],... 

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]. 

The synthesis of core-first stars from multi-ionic precursors (XXVIII to XXXVII) may be complicated by the limited solubility of the multiple growing chains, giving rise to insoluble aggregates in organic solvents [38]. When using multifunctional precursors (XXXVIII to XLVI) in free-radical polymerization, the conditions must be identified that will avoid any of the growing arms undergoing irreversible terminations, that may result in a loss of control of the star functionality [46-48]. 

Although the core-first method is the simplest, success depends on initiator preparation and quantitative initiation under living conditions. This method is of limited use in anionic polymerization because of the generally poor solubility of multifunctional initiators in hydrocarbon solvents [12]. Solubilities of multifunctional initiators are less of an issue in cationic polymerizations, and tri- and tetrafunctional initiators have been used to prepare well-defined three- and four-arm star polymers by this method [7] Except for two reports on the synthesis of hexa-arm polystyrene [27] and hexa-arm polyoxazoHne [26], there is a dearth of information in regard to well-defined multifunctional initiators for the preparation of higher functionality stars. 

Synthesis of star-shaped copolymers by the core-first method. 868... 

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. 

Scheme 12 Synthesis of star-shaped polyelectrolytes by the a arm first and b core first methods...

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. 

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. 

Scheme 27.4 Core-first synthesis of star-like polymers from multifunctional initiators.

Scheme 30.14 Synthesis of a three-arm star PS-fc-PEO block copolymer by combining the core-first approach using ATRP with CuAAC click" coupling. Reproduced with permission from Ref. [109] 2007, john Wiley. Sons, Inc.

The most important method for the synthesis of star polymers by GTP involves the use of a diftmctional monomer either as an arm-first or as a core-first approach. Usually, EGDM is used as the difunctional monomer. However, other multifunctional monomers, such as tetramethylene glycol dimethacrylate, trimethylol propane trimethacrylate, and 1,4-butylene dimethacrylate have been used. 

The core-first approach was also adopted for the synthesis of star polymers. According to this procedure, the catalyst was reacted with the difunctional monomer to give the multifunctional initiator, followed by the addition of norbomene." Unfortunately, a multimodal product was obtained revealing the presence of linear chains, dimers, and star stmctures. 

Miktoarm polymers are essentially heteroarm star polymers where two or more arms of the star are chemically unique. Therefore, the same general approaches for the synthesis of star polymers also apply to miktoarms, with some additional constraints. Many research groups have sequentially performed orthogonal polymerization techniques to access a variety ABC and ABCD miktoarm polymers, in a core-first approach. 

Example 9 External Initiation using Ni(COD)2/PPli3 (Exemplified by the Core-First Synthesis of the Y-Shaped Three-Armed P3HT Star) (20.16)... 

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 ... 

As mentioned above, some difficulties arose in the synthesis of dendrimer-like star-branched polymers by the two aforementioned methodologies based on the core-first divergent and arm-first convergent approaches described in Sections 5.2.1 and 5.2.2. In fact, most of the polymers are limited to 4G stages and a few 10 g/mol orders in molecular weight except for two cases reported by Gnanou et al. (Tables 5.1 and 5.2). Moreover, the structures of the resulting polymers could not be well characterized and structural imperfections were indicated in several cases. 

Gao, H.F., Min, K., and Matyjaszewski, K. (2007) Synthesis of 3-arm star block copolymers by combination of core-first and coupUng-onto methods using ATRP and click reactions. Macromolecular Chemistry and Physics, 208,1370. 

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