Core-functionalized star-branched polymer

Basically, intermediate polymeric anions were reacted with l-(3-bromopropyl)-2,2,5,5-tetramethyl-aza-2,5-disilacyclopentane, followed by spontaneous deprotection during workup to prepare well-defined in-chain- or core-functionalized star-branched polymers with primary amine group(s). These functionalized polymers could be used as macroinitiators for the living polymerization of NCAs. The success of those star-polymer syntheses may be derived from the 100% introduction of primary amino groups at the precise position and extremely clean media for NCA polymerization in completely sealed reactors based on the high-vacuum technique. 

The synthesis was initiated to prepare a chain-end-(DPE)2-functionalized polymer (A) by a reaction of a living anionic polymer (A) with 4. In the first iteration, a linking reaction of the DPE-functionalized polymer (A) with a living polymer (B), followed by reacting either 1 or 4, afforded two kinds of 3-arm AB2 star-branched polymers. The number of core-functionalized DPE functions was the same (two) as that of the starting polymer in the former reaction, while it doubled to four in the latter reaction. The polymers abbreviated as 3-arm AB2 (DPE)2 and 3-arm AB2 (DPE)4, respectively. In the second iteration by repeating the same reaction... 

Finally, an attempt to synthesize a 64-arm A32B32 star was made by a coupling reaction of the 16-arm AgBg star-branched polymer anion with a core bearing four BnBr functions. The coupling reaction proceeded insufficiently and the target 64-arm A32B32 star was obtained in only a few per cent. 

Hirao et al. successfully synthesized a 3-arm ABC, followed by a 4-arm ABCD p-star polymer using the DPE functionalized agent, 7 (Higashihara and Hirao, 2004). As illustrated in Scheme 4.15, a chain-end-functionalized PI with both benzyl chloride and DPE moieties was prepared in 62% yield by the slow addition of PILi, after end capping with DPE, with a 8.7-fold excess of 7. After the isolation of the chain-end-functionalized PI by fractional precipitation, DPE-end-capped PMOSLi was added to react selectively with the benzyl chloride functionality, followed by reacting PSLi with the residual DPE functionality, resulting in a 3-arm ABC star-branched polymer having an anion at the core (Af = 31 800 g/mol, M /Mn = 1 03). 

Higashihara, T., Inoue, K., and Hirao, A., Successive synthesis of well-defined star-branched polymers by convergent iterative methodology using core-functionalized 3-arm star-branched polymer and a speciaUy designed 1,1-diphenylethylene derivative. Macromolecular Symposia, 296,53-62. 

As seen in Scheme 5.1, preparation of the IG polymer in this synthesis involved the ROP of cCL and subsequent chain-end modification. Conversion of the terminal hydroxyl group to two hydroxyl functions enabled further ROP to the 2G polymer. The IG polymer synthesized by this procedure was a 6-arm star-branched PcCL. The target dendrimer-like star-branched polymer was obtained as a 2G polymer by the second iteration and possessed a minimum architectural unit. One more repetition of the synthetic sequence involving the two reaction steps resulted in a 3G dendrimer-like star-branched PaCL. The 3G polymer possessed six branches at the core and two branches at the junctions in both the 2G- and 3G-based layers, composed of 42 arm segments (6 (IG) + 12 (2G) + 24 (3G) = 42). The observed M value was 96 000 g/mol, close to the theoretical value, and the molecular-weight distribution was not narrow, but an acceptable value of 1.14. 

This method has been extended by Rempp and co-workers [39,40] as a general core-first method to prepare star-branched polymers, as shown in Scheme 4. The plurifimctional metalorganic initiator (A) was prepared by potassium naphthalene-initiated polymerization of DVB in tetrahydrofuran (THF) at -40°C with [DVB]/[K ] ratios of 0.5-3. Microgel formation was reported outside of this stoichiometric range or when m-DVB was used instead of either / -DVB or the commercial mixture. Within the prescribed stoichiometric ratios, star polymers (B) with arm functionalities varying from 8 to 42 were reported. As expected for this type of process, the polydispersities were described as being quite broad and were attributed primarily to a random distribution of core sizes and functionalities. 

In the second method, the core-first method, a polyfunctional core is used to initiate the polymerization of the branches of the star. This method allows for easy access to chain end functionalization by simple deactivation of the active sites. The extension of the core-first method to the preparation of functional star-shaped polymers in nonpolar solvents will be discussed. 

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. 

The synthesis and properties of star polymers and dendrimers functionalized with ferrocene units has attracted a great deal of attention. The synthesis of high-generation dendrimers functionalized with chiral ferrocenyl units in their structures has been reported. The chiroptical properties of this class of dendrimer was dependent on the number of ferrocenyl groups and their chemical environment, but not on their position within the dendrimer. Deschenaux has reported the synthesis of hquid crystalline ferrocene-based polymers prossessing an enantiotropic smectic A phase. Ferrocene-functionahzed cyclic siloxane (29) and silsesquioxane branched polymers have also been reported. A hyperbranched polymer with a cubic silsesquioxane core was used to mediate the electrocatalytic oxidation of ascorbic acid. 

Star-shaped polymers have gained increasing interest because of their compact structure and high segment density, and because very efficient synthetic methods have made possible the functionalization of the outer branch ends. Until recently, anionic polymerization was one of the best methods to obtain well-defined star-shaped polymers of predetermined branch molar mass. This technique provided the long lifetime for the active sites necessary to allow the formation of star-shaped macromolecules. Anionic polymerization also limited the polymolecularity of the samples. Given the appropriate reaction conditions, the functionality of the core can be controlled in advance. 

Star polymers generated using the core-first method have a large distribution in functionalities. To minimize the distribution in functionalities of the stars, the in-out technique was developed. The in-out method is a combination of the two techniques mentioned above and first generates a small arm-first star with living active sites, then uses this core to initiate the polymerization of the star branches. The resulting stars can be functionalized, and the control over the distribution in functionalities is greatly improved. 

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