Miktoarm polymer

Fig. 41 Theoretical phase diagram calculated by Matsen et al. [20] for diblock copolymers in intermediate segregation regime. PS-arm-P2VP miktoarm polymers PS- -P2VP linear polymer. From [119]. Copyright 2000 American Chemical Society... 

Block copolymers have peculiar characteristics due to the coexistence of two or several different parts of different chemical compositions within a chain. They can undergo microphase separation transitions from a homogenous phase to a variety of spatially periodic structures [176]. A distinction should be made between star copolymers, where each arm is composed by two or more blocks, and miktoarm polymers, formed by homopolymer arms of different chemical compositions. Floudas et al. [177] recently performed an extensive study of four-... 

Miktoarm polymers are essentially heteroarm star polymers where two or more arms of the star are chemically unique. Therefore, the same general approaches for the synthesis of star polymers also apply to miktoarms, with some additional constraints. Many research groups have sequentially performed orthogonal polymerization techniques to access a variety ABC and ABCD miktoarm polymers, in a core-first approach. 

Figure 6 Formation of miktoarm polymer using the hybrid approach by attaching an end-functional polymer to the juncture point of a preformed AB diblock copolymer. ...

The development of PPE synthetic chemistry makes the synthesis of PPEs with various structures possible. Recently, PPE-based polymers with different topological structures including linear random copolymers, block copolymers, star polymers, miktoarm polymers, and brush and hyperbranched polymers have been synthesized. Among them, linear homopolymers or random copolymers of PPEs are perhaps the most studied. Different block copolymers with AB, ABA, and ABC architectures have been synthesized by controlled ROP. By the combination of ROP of PPE with other controlled polymerization methods, such as living radical polymerization, or click chemistry, more complex architectures including miktoarm, comb, or graft copolymers can be synthesized. The richness of structures has allowed the convenient adjustment of material properties of PPE for biomedical applications. 

Sharma, A., Kakkar, A., 2015. Designing dendrimer and miktoarm polymer based multi-tasking nanocarriers for efficient medical therapy. Molecules (Basel, Switzerland) 20 (9), 16987-17015. http //www.mdpi.eom/1420-3049/20/9/16987/htm (accessed 06.12.15.). 

Chlorosilanes were used applying an identical reaction procedure to prepare either quaterarm pol)miers of type ABCD or 4- miktoarm polymers of type A2B2. A, B, C, D correspond to PS, PBd, PI, and poly(4-methylst5n ene), respectively [33]. That method was extended recently to well-defined poly(iso-prene)/poly(butadiene) A2B2 copolymers [34]. The presence on the same nodu-lus of chains exhibiting different chemical structures leads to original solution properties. Roovers et al. [35] have examined in detail the solution properties and compared the specific behavior of these miktoarm star-shaped polymers to linear diblock copolymers. That strategy was extended to the preparation of... 

Several excellent books and review articles have been published covering this particular area of polymer science [1-3]. Nevertheless, this review will highlight recent (2000-2004) advances and developments regarding the synthesis of block copolymers with both linear (AB diblocks, ABA and ABC triblocks, ABCD tetrablocks, (AB)n multiblocks etc.) and non-linear structures (star-block, graft, miktoarm star, H-shaped, dendrimer-like, and cyclic copolymers). Attention will be given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

Fig. 1. a Star polymer, b Comb polymer, c Brush, d Miktoarm star copolymer, e Star copolymer, f Star chain center-adsorbed in a plane, g Dendrimer... 

Miktoarm star (or p-star) polymers are star polymers with branches of different polymers. These polymers are also called heteroarm or mixed-arm star polymers. Their synthesis is more difficult but success has been achieved by sequential coupling [Hadjichristidis, et al., 1999]. To obtain a 3-arm star with one polyisoprenyl (PI) and two polystyryl (PS) branches, polyisoprenyllithium is reacted with an excess of CH3SiCl3 to form the one-arm polymer, the unreacted CH3SiCl3 is removed, and then polystyryllithium is added ... 

Among the two ionic polymerization techniques mentioned above, a living anionic polymerization should show the best possible control of polymer architecture and composition. Mono dispersed homopolymers, complex-block, graft, star, and miktoarm architectures have been accessible primarily by anionic polymerization methods [22]. They have been used to grow polymer brushes from various small particles such as silica gels graphite,carbon black, and flat surfaces [23-26]. Recent results have been reported on living anionic polymerizations on clay [27] and silica nanoparticles [28,29]. 

The synthesis of A2B miktoarm star polymers has been discussed and exemplified using PIB as a component. The synthesis involves a quasi living cationic polymerization of isobutylene from a monofunctional cationic initiator. This initiator also contains a blocked hydroxyl group. Eventually, the blocked hydroxyl group of the initiator is deblocked, and functionalized with a branching agent. This activated reagent is then used for an atom transfer radical polymerization process of /erf-butyl acrylate (18). 

L.K. Breland and R.F. Storey, Polyisobutylene-based miktoarm star polymers via a combination of carbocationic and atom transfer radical polymerizations, Polymer, 49(5) 1154-1163, March 2008. 

Star polymers of chemically different arms are usually called miktoarm stars. Although there are several individual methods for the synthesis of miktoarm stars four general methodologies have been developed. Three of them are based on anionic polymerization and the fourth on cationic polymerization. In all of them the use of appropriate linking agents is necessary. 

The recent development of living cationic polymerization systems has opened the way to the preparation of rather well defined star homopolymers and miktoarm star polymers [19 and see the chapter in this volume]. Divinyl ether compounds were used as linking agents in a manner similar to the DVB method for anionic polymerization. Typically the method involves the reaction of living polymer chains with a small amount of the divinyl compound. A star polymer is formed carrying at the core active sites capable of initiating the polymerization of a new monomer. Consequently a miktoarm star copolymer of the type AnBn is produced. 

Individual methods have also been devised for the preparation of miktoarm stars. One of these approaches involves the preparation of macromonomers possessing either central or end vinyl groups which can be used to produce miktoarm stars either by copolymerization of the double bonds or by reacting the double bonds with living polymer chains, thus creating active centers able to initiate the polymerization of another monomer. All these methods are limited to specific synthetic problems and cannot be used for the preparation of a wide range of different structures. 

A special technique was employed by Naka et al. for the preparation of A2B stars, A being PEO and B polyoxazoline (POX) [26] according to Scheme 8. Ru(III) complexes with bipyridyl terminated polymers were utilized in this method. Characterization data were not provided for these miktoarm star copolymers. 

A different approach but still in the frame of the chlorosilane method was adopted by Tsiang for the synthesis of (A-b-B)B3 miktoarm star copolymers, where A is PS and B is PB [29]. Living PB chains were reacted with SiCl4 in a molar ratio 3 1, followed by the addition of the living diblock PS-b-PBLi. The key step of the method is the succesfull synthesis of the (PB)3SiCl intermediate product. The reduced steric hindrance of the PBLi chain end poses questions about the purity of this polymer, since several byproducts, such as (PB)2SiCl2, (PB)4Si, PBSiCl3 can be formed in the first step of the synthesis. SEC analysis was performed to monitor the reaction sequence. 

The most widely used method for the preparation of miktoarm stars of the type AnBn is the DVB method, which has already been mentioned. The polymers prepared by this method have PS as A arms and PtBuMA, PtBuA, PBuMA, PEO or P2VP as B arms [41-43]. SEC was used to monitor the reaction steps and the molecular characterization data showed that the products were not of the same degree of homogeneity as those prepared by the chlorosilane method, due to the disadvantages inherent of the method. 

Relatively few theoretical studies have been devoted to the conformational characteristics of asymmetric star polymers in solution. Vlahos et al. [63] studied the conformational properties of AnBm miktoarm copolymers in different solvents. Analytical expressions of various conformational averages were obtained from renormalization group calculations at the critical dimensionality d=4 up to the first order of the interaction parameters uA> uB> and uAB between segments of the same or different kind, among them the radii of gyration of the two homopolymer parts < S > (k=An or Bm) and the whole miktoarm chain < /im > > the mean square distance between the centers of mass of the two homopolymer parts A and B < > and the mean square distance between the center of... 

Much experimental work has appeared in the literature concerning the microphase separation of miktoarm star polymers. The issue of interest is the influence of the branched architectures on the microdomain morphology and on the static and dynamic characteristics of the order-disorder transition, the ultimate goal being the understanding of the structure-properties relation for these complex materials in order to design polymers for special applications. 

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