Star copolymer heteroarm

In miktoarm-star copolymers (or heteroarm star copolymers ), the unlike blocks are connected at one junction point, as shown in Fig. 34. As in linear-block copolymers, exactly two blocks are linked. Hence, it seems tempting to approximate those copolymers as a set of diblocks with the free ends of one block component joined together in a cluster [111]. 

Similar onion type micelles were obtained by the combination of PS-P2VP heteroarm-star copolymers with a P2VP-PEO diblock copolymer, as reported recently by Tsitsilianis et al. [269]. 

Fig. 19 Schematic representation of block-arm star copolymer of type (PS- -P2VP) (PS) and (PS) (P2VP) heteroarm star diblock copolymer complexed with DBSA. In the drawing n = 4, Reprinted with permission from [135,136]. 2005 and 2006 American Chemical Society...

Tsitsilianis et al. recently published [245] preliminary results on the micelliza-tion behavior of anionically synthesized amphiphilic heteroarm star copolymers with polystyrene and poly(ethylene oxide) branches in THF and water. The former solvent is not very selective for one of the segments whereas the latter is strongly selective for PEO. The apparent molecular weights found for the micelles in THF were two orders of magnitude larger than the ones measured for the unimers. By increasing concentration an increase in the depolarization ratio was observed supporting the conclusion that multimolecular micelles are formed by this kind of miktoarm star copolymer. 

VOU Voulgaris, D. and Tsitsilianis, C., Aggregation behavior of polystyrene/poly(acrylic acid) heteroarm star copolymers in 1,4-dioxane and aqueous media, Macromol. Chem. Phys., 202, 3284, 2001. 

For the synthesis of block copolymers the reader is refered to a previous article in this series (23) and some other works describing the application of various controlled polymerization techniques for their synthesis, such as anionic polymerization (24-26), cationic polsmierization (27,28), controlled radical polymerization (29,30), as well as combinations of various techniques (31-34). Synthetic procedures for star copolymers have been reviewed (35). The first synthesis of a heteroarm star terpolymer with three immiscible blocks has been described in Reference 36. 

Heteroarm or miktoarm star copolymers have attracted considerable attention in recent years due to the imique properties of these polymers, for example, they exhibit dramatic difference in morphology and solution properties. In comparison with the linear block and star-block copolymers, the s)mthesis of heteroarm or miktoarm star copolymers has been one of the more challenging projects available. A typical example is that the synthesis of the heteroarm H-shaped terpolymers, [(PLLA)(PS)]-PEO-[(PS)(PLLA)], in which PEO acts as a main chain and PS and PLLA as side arms (Fig. 4.11). The copolymers have been successfully prepared via combination of reversible addition-fragmentation transfer (RAFT) polymerization and ring-opening polymerization (ROP) by Han and Pan [166]. Another interesting example is that Pan et al. [167] successfully... 

Tsitsilianis, C., Voulgaris, D., Stepanek, M., Podhajecka, K., Prochazka, K., Tuzar, Z. and Brown, W. (2000) PS-P2VP heteroarm star copolymer micelles in aqueous media and onion type micelles stabilized by diblock copolymers. Langmuir, 16, 6868-6876. 

Havrankova J, Limpouchova Z, Prochazka K (2003) Monte Carlo study of heteroarm star copolymers in good and selective solvents. Macromol Theory Simul 12(7) 512-523. doi 10.1002/mats.200350012... 

Stepanek M, Matejicek P, Humpolickova J, Havrankova J, Podhajecka K, Spirkova M, Tuzar Z, Tsitsilianis C, Prochazka K (2005) New insights on the solution behavior and self-assembly of polystyrene/poly(2-vinylpyridine) hairy heteroarm star copolymers with highly asymmetric arms in polar organic and aqueous media. Polymer 46(23) 10493-10505. doi 10.1016/j.polymer.2005.08.031... 

FCS is not only restricted to assembly studies of block copolymers and homopolymers, but also more complex aggregation systems can be analyzed. As an example, Stepanek et al. investigated the solution behavior and self-assembly of a heteroarm star copolymer consisting of ca. 20 short PS and 20 long P2VP arms [167]. 

Tsitsilianis C, Voulgas D, Kosmas D (1998) Heteroarm star copolymers as emulsifying agents in polymer blends. Polymer 39 3571-3575... 

Fernyhough, C.M., Young, R.N., and Tack, R.D. (1999) Synthesis and characterization of polyisoprene-poly(methyl methacrylate) AB diblock and A2B2 heteroarm star copolymers. Macromolecules, 32,5760-5764. 

Star polymers consist of several linear polymer chains connected at one point. Prior to the development of CRP, star molecules prepared by anionic polymerization had heen examined. However, due to the scope of ionic polymerization, the composition and functionality of the materials were limited. The compact structure and globular shape of stars provide them with low solution viscosity and the core-shell architecture facilitates entry into several applications spanning a range from thermoplastic elastomers (TPEs) to dmg carriers. Based on the chemical compositions of the arm species, star polymers can be classified into two categories homoarm star polymers and miktoarm (or heteroarm) star copolymers... 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

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],... 

An interesting example of macromolecular co-assemblies derived from starshaped polyionic species was reported by Ge et al. [81]. The authors found that a star-shaped double hydrophilic poly(methacrylic acid)-poly(ethylene oxide) heteroarm copolymer [(PMAA)x-PDVB-(PEO)x, with PDVB being poly(divinylbenzene) and X denoting the number of PMAA and PEG arms] can interact in alkaline media with a double hydrophilic poly(ethylene oxide)-block-quaternized poly[2-(dimethylamino)ethyl methacrylate] (PEO- -PDMAEMAQ) diblock copolymer. At Z = [PDMAEMAQ]/[PMAA] = 1, well-defined water-soluble onion-like (core-shell-corona) macromolecular co-assemblies are formed, with a hydrophobic core consisting of a PDVB microgel. The interaction of the PMA arms of the hybrid coronas of such copolymer stars with the PDMAEMAQ+ blocks of the diblock copolymer generates an insoluble inner layer (shell) around a PDVB core. Meanwhile, PEG blocks from both PEG- -PDMAEMAQ and (PMAA)x-PDVB-(PEG)x build up a hydrophilic nonionic corona that stabilizes the whole complex in aqueous media. 

The so-called palm tree-like copolymers consist in an arrangement of nB blocks with one A block of much larger size in a heteroarmed star architecture [8]. These architectures were obtained by sequential copolymerization of norbornenyl-PS macromonomers with a molecular comonomer, namely cyclooctadiene (COD), Scheme 6. Dumbbell-shaped copolymers can be viewed as double stars that are linked through a linear block. They are obtained upon using palm tree-like copolymers to trigger the polymerization of a new amount of PS macromonomers. Scheme 6. 

The nomenclature of the block copolsrmers is as follows A ByCz is a block copolymer composed of the blocks A, B, and C, where subscripts denote the weight fraction (%) and M is the number-averaged overall molecular weight (kg/mol). Heteroarm star terpolymers are indicated by an asterisk (A ByCz ). The morphological schemes are presented in such a way that the typical colors found in tern images of correspondingly stained samples are used (Table 1). 

In this section the question of how the morphology is influenced by changing the block sequence for a given overall composition in linear ternary block copolsuners is discussed. At the end of this section linear block copolymers are compared with their heteroarm star terpolymer analogues. 

Finally the morphologies of ternary linear block copolymers and heteroarm star terpolymers are compared. Here systems containing I or B are also compared, since these two polymers show rather similar interactions toward the other blocks (see Table 2 for solubility parameters). Figure 22 shows in the left column symmetrically composed SIM heteroarm star terpol5uners (134) and linear BSM and SBM triblock copolymers (76). In the right column corresponding SBV (135) and SIV (144) heteroarm star terpolymers are compared with their linear compositional... 

---