Star-shaped polymers with functionalized arms

The core functionalization in 21 may lead to the higher accumulation of polar hydroxyl groups in a core region that should be smaller in size than the arm moiety of the star-shaped polymers with functionalized arms. Another important feature of 21 is that an outer hydrophilic shell of 21 can effectively surround the hydrophilic microgel core with many hydroxyl groups. Therefore, the core-functionalized star polymers are amphiphilic but are expected to possess properties that differ from those of the star block and the heteroarm star amphiphiles. 

The main topic of interest is the properties of molecules of finite size, having no large rings, and in general having trifunctional branch-points. These are typically produced by chain-transfer with polymer in free-radical polymerizations, though they can of course be made in other ways. Molecules with branch-points of higher functionality are also of interest, especially star-shaped molecules with several arms, as these are both easy to synthesize and relatively easy to discuss theoretically. 

The so-called core-first method has been extensively used for the synthesis of various kind of star-shaped polymers, water-soluble or not. The functionalizable outer end of the branches offers an original access to a large scope of macromolecular architecture star-shaped polymers with copolymeric branches, functional star-shaped polymer networks, etc. The in-out method combines the advantage of the arm-first method and the core-first method allowing good control of the structure and the presence of functionalizable outer end of the branches. Different unsaturated compounds have been used to generate the core, such as DVB and DPE, the latter compound giving access to star-shaped terpolymers. 

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. 

On the other hand, as soon as many stars are in the matrix, the entanglement points of the matrix are increased due to additional permanent crosslink points (i.e., centers of star polymers). Thus the average mesh size of the matrix decreases. In consequence, the cooperative diffusion becomes faster. But in the case of 12-arm star polystyrene as the test chain, the effect of star centers was not so obvious. According to the model of star shaped polymers by Daoud and Cotton,there is a region with size around the center of a star, inside which the chains of other polymers do not penetrate. The distance x is a function of the number of arms, i.e., / where f is the arm number of the star. So the unpenetrable distance of 12-arm star is larger than that of a 4-arm star, leading to an effective decrease in the number of entangled points. 

Scheme 20 Preparation of star-shaped polymer conjugates with cholic acid (CA) core and polyether arms functionalized with pendant COOH or NH2 groups [150] (reprinted with permission from American Chemical Society)...

A final arm-first method based on the preparation via anionic polymerization of star-shaped polymers will be mentioned. Here the anionic polymerization is only used to prepare a linear (D-fimctional polymer of controlled molar mass and functionality and not for the coupling reaction. In many cases, anionic polymerization is the only way to access these well-defined monofunctional precursor chains. In a second step, these functional polymers were reacted with plurifunctional compounds exhibiting antagonist functions. Some examples are given below. 

Star shaped macromolecules are polymers, where the one end of f > 2 (f functionality of the star) linear chains is chemically attached by covalent bonds to a small central linker unit, are the simplest form of branched polymers. Modern anionic polymerization techniques allow us to synthesize star systems with a large number of nearly monodisperse arms [133, 134],... 

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 technique is based on the use of well-defined soluble multifunctional initiators, which, in contrast to anionic multifunctional initiators, are readily available. From these multiple initiating sites a predetermined number of arms can grow simultaneously when the initiating functions are highly efficient independently of whether the other functions have reacted or not. Under these conditions the number of arms equals the number of initiating functions and living polymerization leads to well defined star polymers with controlled MW and narrow MWD. Subsequent end-functionalization and/or sequential monomer addition can also be performed leading to a variety of end-functionalized An or (AB)n star-shaped structures. 

A comprehensive list of the grafting reactions of (a) and (b) onto different polymer backbones is given in Ref 543. The methods summarized there include radiation techniques [623], grafting by radical transfer [624], and grafting initiated by functional groups in backbone polymers [625,626]. Macromonomers of (a) have been synthesized by means of anionic polymerization techniques and have been copolymerized with styrene [627,628]. Only a few examples are known in which polymers from (a) and (b) were used as backbone [629]. Star shaped block copolymers with four arms were prepared by coupling living styrene/(a) block copolymers with 1,2,4,5-tetrakis-bromomethyl-benzene [626]. 

More recently, the same photodimerization was exploited by Yagci et al, who functionalized linear or three-armed star-shaped poly(propylene oxide)s with two or three terminal benzoxazine coumarin functions, respectively [KIS 14], These products were used as multifunctional macromolecular precursors for the preparation of self-healing pol5mier films. Under irradiation at wavelengths over 300 nm, the formation of cyclobutane junctions from the chain-end chromophores of the macromonomers resulted in cross-linking. Regarding the recovery of mechanical properties, a healing efficiency up to 44% was obtained in the case of a cracked polymer sample studied in this work. 

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