Star polymers core-functionalized

Star polymers are a class of polymers with interesting rheological and physical properties. The tetra-functionalized adamantane cores (adamantyls) have been employed as initiators in the atom transfer radical polymerization (ATRP) method applied to styrene and various acrylate monomers (see Fig. 21). 

Connal LA, Vestberg R, Hawker CJ, Qiao GG (2007) Synthesis of dendron functionalized core cross-linked star polymers. Macromolecules 40 7855-7863... 

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. 

Although the core-first method is the simplest, success depends on initiator preparation and quantitative initiation under living conditions. This method is of limited use in anionic polymerization because of the generally poor solubility of multifunctional initiators in hydrocarbon solvents [12]. Solubilities of multifunctional initiators are less of an issue in cationic polymerizations, and tri- and tetrafunctional initiators have been used to prepare well-defined three- and four-arm star polymers by this method [7] Except for two reports on the synthesis of hexa-arm polystyrene [27] and hexa-arm polyoxazoHne [26], there is a dearth of information in regard to well-defined multifunctional initiators for the preparation of higher functionality stars. 

An-Type Star Polymers with a Functionalized Core Polyfvinyl ether)n... 

Three arm star polymers of IBVE were synthesized by living cationic polymerization using trifunctional initiators 8 and 9 with the same trifluoroacetate initiating functions but different cores [19, 20]. The experimental conditions were selected to obtain living polymerization. A series of acetic acid derivatives including trifluoroacetic acid and the IBVE-acid adduct were found to be efficient... 

Star poly(methylmethacrylates) were synthesized via atom transfer polymerization using a small carbosilane dendrimer functionalized with a tertiary bromide moiety as an initiator core (Figure 12)100,101. a convergent approach to star polymers with a carbosilane dendrimer core was described in a report by Allgaier and coworkers102, in which living poly(butadienyl-lithium) arms were coupled with various SiCl-terminated carbosilane den-drimers. Utilizing smaller dendrimers with lower functionality was found to yield nearly ideal results in terms of substitution and polydispersity. 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

Similar host-guest interactions are found not only with the amphiphilic star block copolymers [210,220] but also with the corresponding heteroarm [211] and core-functionalized [212] versions. Overall, these starshaped polymers induce the interaction more efficiently than their linear counterparts [220],... 

Functional groups can also be introduced in the spacer units. Bifunctional initiators with bipyridine units such as MI-17 and MI-18 induced the living radical polymerizations of styrene and MMA, respectively, with copper catalysts to give polymers that carry a coordination site at the middle of the chain.87,333 These polymers can be connected together into star polymers with a ruthenium cation at the core, where the arm numbers are varied among three, four, five, and six in combination with the polymers obtained from the monofunctional initiator with a bipyridine unit (FI-21 and FI-22 Figure 13).416 A... 

The star polymers obtained with the bromoester-type calixarene-based initiators were analyzed by SEC equipped with a multiangle laser light scattering (MALLS) detector. The arm numbers were well controlled (close to the initiator s functionality), although the octafunctional initiator MI-51 induced star—star coupling in the styrene-polymerization at conversions higher than 20%.421 A similar series of tetra-, hexa-, and octafunctionalized initiators with calixarene cores were synthesized for sulfonyl chloride versions (MI-41, MI-48, and MI-52) and employed for copper-catalyzed MMA polymerizations.343... 

This functionalization method can be applied for the synthesis of star polymers only when a multifunctional initiator is used. The living branches emanate from the core and therefore can be subjected to several terminating reactions with suitable electrophilic reagents. 

Multiarm star polymers have recently emerged as ideal model polymer-colloids, with properties interpolating between those of polymers and hard spheres [62-64]. They are representatives of a large class of soft colloids encompassing grafted particles and block copolymer micelles. Star polymers consist of f polymer chains attached to a solid core, which plays the role of a topological constraint (Fig. Ic). When fire functionality f is large, stars are virtually spherical objects, and for f = oo the hard sphere limit is recovered. A considerable literature describes the synthesis, structure, and dynamics of star polymers both in melt and in solution (for a review see [2]). 

Poly (amidoamine) dendrimers (PAMAM) are new interesting class of star polymers. They have mono dispersive, well defined and developed three-dimensional structures comprising functional groups at high concentration. In its core they have amidic groups. Structurally modified PAMAM dendrimers with 1,8-naphthalimide derivatives are fluorescent dendrimers and they can be applied as effective and selective sensors for different metal cations and protons in organic solvents. This property can be nansferred to other polymer matrixes (for example textiles). 

Hyperbranched polymers have also been used as cores for star polymers. Up to 80 arms of pBA were grown from the core of pBPEA with DP-80. Since number of functional groups is equal to the degree of polymerization, incorporation of just one unit per chain end brings the content of another comonomer to -50% [89]. Hyperbranched polymers prepared by ATRP were modified in just such a way using monomers that were non-polymerizable by ATRP but carried useful functionalities such as epoxies or alkenes, as shown in Scheme 59. 

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. 

The MI method utilizes initiator-functionalized linear chains, which initiate the polymerization and crosslinking of a difunctional monomer (e.g., divinylbenzene). The active chain ends also attack the neighboring linear chains ends, and a core with crosslinked microgel is formed. In the meanwhile, certain numbers of linear chains are attached to the core. However, it is always difficult to obtain star polymers with narrow distribution of arm numbers. Quite often, many linear polymers are not attached to the core, which leads to problems in the course of the purification and for finally applications. By using multifunctional coupling agents, it is possible to get stars with uniform arm numbers. But the purification process is always unavoidable and difficult. 

The first step for the core-first stars is the synthesis of multifunctional initiators. Since it is difficult to prepare initiators that tolerate the conditions of ionic polymerization, mostly the initiators are designed for controlled radical polymerization. Calixarenes [39, 58-61], sugars (glucose, saccharose, or cyclodextrins) [62-68], and silsesquioxane NPs [28, 69] have been employed as cores for various star polymers. For the growth of the arms, mostly controlled radical polymerizations were used. There are only very rare cases of stars made from nitroxide-mediated radical polymerization (NMRP) [70] or reversible addition-fragmentation chain transfer (RAFT) techniques [71,72], In the RAFT technique one has to differentiate between approaches where the chain transfer agent is attached by its R- or Z-function. ATRP is the most frequently used technique to build various star polymers [27, 28],... 

Multiarm polymers (11) can be prepared that still have the reactive functional groups (Z) close to the core. As these are still active, they can be used as sites to initiate the growth of more arms by adding either the same monomer used to prepare (11) or a second monomer to prodnce mikto-arm star polymers, in which the arms have different chemical structures. Thus, an active ended poly(t-butyl acrylate), prepared by ATRP, can be coupled with divinyl benzene to form a multiann star polymer. This structure can be converted to a mikto-arm star polymer by reacting the living ends still present with n-butyl acrylate, and so propagate poly(n-butyl acrylate) chains from the core outward. 

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