Core-first star synthesis polymerization

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. 

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. 

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. 

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. 

Zhang, K.J., Ye, Z.B., and Subramanian, R. (2009) A trinuclear Pd-diimine catalyst for core-first synthesis of three-arm star polyethylenes via ethylene living polymerization. Macromolecules, 42,2313-2316. 

To address this issue, Hadjichristidis developed the synthesis of 3- and 4-arm star copolypeptides by high-vacuum polymerization using the core-first method. They prepared 3-arm stars containing poly(Z-Lys) and poly(Bn-Glu) block copolymers that were simultaneously grown off of a 2(aminomethyl)-2-methyl-1,3-propanediamine initiator core (Scheme 2). This method produced well-defined star copolymers with narrow molecular weight distributions. Attempts to prepare 4-arm star copolymers... 

Taton D, Saule M, Logan J, Duran R, Hou S, Chaikof EL, Gnanou Y (2003) Polymerization of ethylene oxide with a calixarene-based precursor synthesis of eight-arm poly(ethylene oxide) stars by the core-first methodology. J Polym Sci A Polym Chem 41 1669-1676... 

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. 

Two examples of the core-first approach for the synthesis of star polymers by GTP have been reported. Trimethylolpropane triacrylate is converted to a silyl enol ether that is used to initiate the polymerization of ethyl acrylate (Scheme 6). A pol)aner with a = 2190 and MJM = 1.39 was obtained [9]. A cyclic tetramer of methyl hydrogen siloxane was converted to a core containing four initiating groups using a Pt-catalyzed hydrosilylation reaction. The tetrafiinctional initiator was used to initiate the polymerization of MMA to form a four-arm star PMMA (Scheme 7), with about 20 to 150 MMA repeat... 

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

Stars with high arm numbers are commonly prepared by the arm-first method. This procedure involves the synthesis of living precursor arms which are then used to initiate the polymerization of a small amount of a difunctional monomer, i.e., for linking. The difunctional monomer produces a crosslinked microgel (nodule), the core for the arms. The number of arms is a complex function of reaction variables. The arm-first method has been widely used in anionic [3-6,32-34], cationic [35-40], and group transfer polymerizations [41] to prepare star polymers having varying arm numbers and compositions. 

The first synthesis of star polymers with a microgel core was reported by Sa-wamoto et al. for poly(isobutyl vinyl ether) (poly(IBVE)) [3,4]. In the first step, living cationic polymerization of IBVE was carried out with the HI/ZnI2 initiating system in toluene at -40 °C. Subsequent coupling of the living ends was performed with the various divinyl ethers 1-4. 

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