Arm-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. 

There are only a few cases in which polyelectrolyte stars have been prepared by the arm-first strategy. Qiao et al. prepared pH responsive poly(acrylic acid) stars by the MI method using atom transfer radical polymerization (ATRP), which was used to form layer-by-layer (LBL) polyelectrolyte multilayers with linear cationic polyelectrolytes [54], Matyjaszewski et al. obtained cationic poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) stars and anionic PAA stars also using the MI method, which formed all-star LBL layers [55], Ishizu et al. obtained... 

Scheme 2 MI and MM methods for the arm-first strategy [53], Reprinted by permission of ACS...

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. 

In another arm-first strategy, it is possible to use a precursor bearing multiple complementary functions (X) to those (Y) of the living chains (Scheme 27.3) [6, 12-14]. Representative multifunctional agents employed in such an arm-first star polymer synthesis by the different controlled living polymerization (CLP) techniques are shown in Figure 27.4. 

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. 

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 ... 

The two models with their strategies make every intervention pair reachable. By modeling that Vb < ki - m, the braking triggered by the first strategy is no longer concurrent with interventions of the second invariant. In our example, inhib arm is compatible with brake and inhibit opening. 

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... 

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. 

The first (phenoxyl)zinc(II) complexes have been prepared by using a similar strategy (143-145). Ligands containing a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms form very stable (phenolato)zinc(II) precursor complexes (Table II). Accordingly, their cyclic voltammograms display one, two or three reversible one-electron oxidation waves that in all cases have been shown by spectroelectrochemistry and/or EPR spectroscopy to be ligand based. 

Finally, Lecomte and coworkers reported the synthesis of mikto-arm star-shaped aliphatic polyesters by implementing a strategy based on click chemistry (Fig. 36) [162]. Firstly, the polymerization of sCL was initiated by a diol bearing an alkyne function. The chain-ends were protected from any further undesired reaction by the esterification reaction with acetyl chloride. The alkyne was then reacted with 3-azidopropan-l-ol. The hydroxyl function located at the middle of the chain was then used to initiate the ROP of sCL and y-bromo-s-caprolactone. Finally, pendant bromides were reacted successfully with sodium azide and then with N, N-dimethylprop-2-yn-l-amine to obtain pendant amines. Under acidic conditions, pendant amines were protonated and the polymer turned out to exhibit amphiphilic properties. 

This section concerns the first synthesis and characterization of novel octa-arm PSt-h-PIB star-block copolymers [(PSt-h-PIB)g-C8]. The octafunctional initiator 1 was used to prepare PIB stars [61,62] of desired molecular weight, then St was added sequentially to obtain the sought star blocks. Scheme 6 outlines the synthetic strategy. 

The starting point for this study of British defence policy between 1904 and 1969 is the tendency for the costs of new weapons systems to rise more rapidly than the national income/ Three main insights are offered. First, British defence policy was based upon technological innovation. Second, reductions in the size of the armed forces to accommodate new weapons systems in defence budgets were not evidence of a decline in power. Third, British grand strategy, incorporating economic as well as military responses to external threats, was much more ambitious than is commonly believed. 

Both economic decline and military technology feature in Correlli Barnett s four-volume account of the collapse ofBritish power between the First World War and the Suez crisis of 1956. Barnett used a concept of total strategy which encompassed all factors that he believed to be relevant to a nation s ability to preserve or extend its power education, literature, religion and national myths, for example, as well as armed forces and economic and technological resources. His work may... 

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