Arm-first

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 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). 

The most convenient order of carrying out the various steps is to prepare the side arm first, bend the main tube and blow out the hole for the T-joint, and then make the joint before the glass has time to cool. 

LCB polymers can be formed by chemically linking preformed polymers (arm first or polymer first method) or by growing polymer chains from a multifunctional initiatior (core first method). In both cases living polymerization techniques are preferred because they provide better control over MW, MW distribution and the final branching architecture. However, highly selective coupling reactions e.g. with multifunctional isocyanates, or dicyclohexylcar-bodiimide (DCC) coupling, have also been successful. 

The reaction scheme is very general, but control over the extent of the intermolecular reactions and the distribution of the number of arms in the star is limited. The arm first method includes the polymerization (to form star polymers) or copolymerization (to form comb or graft copolymers) of macromonomers. The technique provides a handy simplification if the arm MW need not be very high and the MW control of the branched polymers is not very important. 

Fig. 33 Synthesis of a three-arm star-shaped PCL by the arm-first method...

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. 

Star polymers may be synthesized in several ways. The arm-first method joins preformed arms together using a linking agent, and the core-first method utilizes a multifunctional initiator to grow the... 

I unlocked the door, but then I hung my keys back on my belt and turned to face him, folding my arms. First you must give me your oath that you will not remove anything. ... 

On an eight-arm radial maze containing seven black arms and one white arm, rats rapidly learned to enter the white arm first when it was the only arm containing food. 

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. 

There are two basic synthetic routes for star polymers (Scheme 12)-the core first method (polymerization from multifunctional initiators or microgels) and the arm first method (where growing polymer chain ends are reacted with a multifunctional terminating agent or a divinyl compound). Whereas the use of multifunctional initiators leads to stars with a well-known (but often low) number of arms, the use of microgels or divinyl compounds leads to a rather broad arm number distribution, where the average arm number can be quite high. 

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

The formation of PAA star polymers using the core first method has been demonstrated in the ATRP process by use of multifunctional initiators [111, 112]. In this method, the number of arms in the star polymer can be determined by the number of initiating sites on the initiator. Star-shaped PtBuA was prepared by the arm first method via ATRP, using divinylbenzene, 1,4-butanediol diacrylate, and EGDMA as coupling reagents [113]. 

The product may be of more complex structure if more than two monomers are utilized. For example, if nonhomopolymerizable monomers as described above402 are included at the beginning of the polymerization, the final product would have them on the periphery of the branched structure. Feeding the polymerization with different monomers would provide additional flexibility to this arms-first approach of making hyperbranched polymers. 

All of the aforementioned reports showed star polymer formation originating from a core. The so-called arm-first approach has also been demonstrated. 

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. 

The word dendrimer is derived from the Greek words for tree- or branchlike (dendron) and part (meros). The strictly controlled structure of ideal dendrimers results from the layered assembly of BC surrounding the core, which is attained through sequential reaction cycles. This can be achieved in two different ways, namely, by divergent (core-first) or convergent (arm-first/core-last) methods. 

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