Arm-first method

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... 

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. 

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]. 

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. 

In the case of star polymers (Scheme 2.28), these include symmetric, asymmetric, and miktoarm stars, which are prepared by reacting active chain ends with a core using the arm-first method or via the core-first method. Miktoarm stars have been reported using anionic, cationic, and ATRP methodologies and typically have AB o "... 

Another approach to synthesize multiarm star block copolymers is based on a combination of CuAAC and the arm-first method (Durmaz et al, 2010). Protected alkyne PS polymers were prepared via ATRP and subsequently crosslinked by a divinyl containing compound. The formed 27-arm star-shaped polymers containing a protected alkyne periphery, was deprotected and subsequently coupled with azide-end-functionalized PEG and PtBuA to form star block or mixed block copolymers. The CuAAC reactions occurred at room temperature for 24 h, surprisingly leading to a full click efficiency. The quantitative character of the latter click reaction at ambient temperatures for such dense polymer structures is in contrast to those obtained by other research groups, as mentioned in previous paragraphs. 

The convergent or "arm-first method This consists first in the synthesis of arms by a living/ controlled polymerization. The latter are finally linked to each other by adding a multifunctional agent to the polymerization system. Addition of a small amount of a bis-monomer... 

An arm-first method for the preparation of star-shaped block copolymers and peptide-conjugated polymer nanoparticles was reported by the Heise group. They used nitroxide-functionalized amines for NCA polymerization. The nitroxide... 

As an arm-first method, macromonomers (MM) with the styryl terminal moiety were synthesized by CGCP of 3-(alkylamino)benzoic acid esters 25 in the presence of phenyl 4-vinylbenzoate as an initiator, and copolymerization with N,N -mediylenebisacrylamide (MBAA) as a divinyl monomer in the presence of 2,2 -azobis(isobutyronitrile) (AIBN) at 60°C yielded the corresponding star polymers (Scheme 34) [75]. 

Star polymers exhibit interesting properties, especially their lower bulk and solution viscosities compared to linear analogues of the same molar mass. In addition, these architectures contain a higher amount of chain-end functionalities, which may be of high importance regarding many applications. Star polymers are usually prepared from CLRP by two different methods (1) the core-first method, and (2) the arm-first method. ... 

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. 

Following the excellent examples, the syntheses of amphiphilic heteroarm and core-functionalized star polymers were achieved using similar but modified arm-first methods. In the s)mthesis of heteroarm star polymers, polymer-linking... 

Scheme 6.16 Arm-first method for synthesis of core-crosslinked microgels.

In the first method, referred to as the arm-first method, monofimctional living chains are used as initiator for the polymerization of bifimctional monomers to generate the star core. The arm-first method produces homopolymeric... 

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