Star synthesis

The two strategies for star synthesis each have advantages and limitations. Star-star coupling only occurs with strategy method (a). The propagating radicals remain attached to the core as shown in Scheme 9.68 for the case of a RAFT polymerization and an example is shown in Figure 9,12a.626... 

This category comprises those methods which combine different polymerization methods to produce //-stars. The initiating sites for the different polymerizations are created, step-by-step, during the pu-star synthesis. 

Mechanism of Star Synthesis by BCi3-TiCi4 Coinitiators... 

The experimental observations may be explained by the mechanism shown in Scheme 5 together with the following speculations. The initial event of star synthesis is the formation of 2 from 1 (under the influence of BCI3 or TiC [7]) which in the presence of excess BCI3 gives 3. The existence of an equilibrium between dormant and active species such as 2 and 3, respectively, in living IB polymerization has been discussed in detail [7]. The ionicity of the active form 3 and its position in the Winstein spectrum [7] depend on experimental conditions (i.e., nature and concentration of the coinitiator, additives (e.g., organic bases), temperature, solvent polarity, etc.). 

Keywords. Asymmetry, Miktoarm stars, Synthesis, Morphology, Aggregation, Chain conformation... 

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 arm-first approach was also adopted by linking properly functionalized macromolecular ligands to the metal center and by termination or coupling of living polymers with suitable metal complexes bearing functional groups. Steric constraints play an important role in this case. There is usually an upper molecular weight, above which the star synthesis is not efficient. 

A real synthetic strength of the TEC reaction is its versatility with respect to ene substrates that can be used and include both activated and nonactivaed species such as norbomenes, (meth)acryl-ates, maleimides, and allyl ethers to name but a few (Yu et al., 2009). Recent examples describing the application of the thiol-ene reaction in the polymer/materials eld include the use in thioether dendrimer synthesis (Killops et al., 2008), convergent star synthesis (Chan et al., 2008), synthesis of multifunctional branched organosilanes (Rissing and Son, 2008), and modi -cation/functionalization of wide range of polymers (Justynska and Schlaad, 2004 Killops et al., 2008). 

Chan, J.W., Yu, B., Hoyle, C.E., and Lowe, A.B. (2009b) The nucleophilic, phosphine-catalyzed thiol-ene click reaction and convergent star synthesis with RAFT-prepared homopolymers. Polymer, 50,3158. 

This lower average branch number could be explained either by incomplete initiation of the CEVE polymerization by the 4 acetal ends of the precursor and/or by some termination or transfer during the propagation reaction. It is difficult at this stage, using SEC or other conventional analytical techniques, to further identify the nature of the side reactions that predominantly occur during the star synthesis. 

A wide range of dithioester RAFT agents has been reported. Common examples of mono-RAFT agents and their application are provided in Tables 11 (Z = aryl) and 13 (Z = alkyl or aralkyl). RAFT agents can contain various unprotected functionality on the R fragment of dithiobenzoate including hydroxy, carboxylic acid/carboxylate, sulfonic acid/sulfonate, olefin, and siloxane. Examples of bis- and multi-dithioester RAFT agents (Z=aryl) that may be used for triblock or star synthesis are shown in Tables 12 and 22, respectively. Bis-dithioesters can be used to synthesize triblock copolymers in a two-step process. 

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. 

In many cases, the well-defined end groups of polymers can be obtained by any of the numerous controlled polymerization techniques and they can be directly used in a star synthesis. A common example is the use of living anionic polymers (Scheme 26). Thereby, the negatively charged polymer chain end reacts with the olefinic double bond of 1,1 -diphenylethylene (DPE) to give a more stabilized anion. This resulting 1,1-diphenylalkyl anion can be subsequently reacted with a halide-modified DPE derivative to introduce a new DPE functionality. To introduce... 

A detailed study of the methacrylate star synthesis by GTP has been presented by Simms [35]. The purpose of the study was to define experimental conditions to minimize termination process of the living chain ends and to ensure that the maximum level of living arms could be carried into the core-... 

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