Miktoarm star architectures

In order to probe the effect of junction point functionality on chain conformation and morphology of miktoarm star block copolymer architectures, a series of PI PS (n = 2, 4, 16) was synthesized [166]. A single batch of both living PS and PI arms have been used, in order to ensure that all chemically identical arms (either A or B) have the same molecular weights. The living A and B chains were reacted with the appropriate chlorosilane, under appropriate experimental conditions, to produce the corresponding //-stars, as shown in Scheme 88. 

The influence of the architecture on the phase behaviour of symmetric miktoarm stars AnBn (Fig. 34c) was investigated by Grayer et al. [119]. Symmetric miktoarm-star copolymers PS-arm-P2VP having a mean func-... 

Despite the fact that Milner s theory was originally developed for miktoarm-star copolymers, it can also be adopted for more complex branched structures. This empirical concept termed constituting-block copolymers approximates the architecture of branched molecules to be composed of an array of A2B and A2B2 miktoarms. This approach is capable of predicting the morphology of architectures as complex as centipedes or barbed wires, as shown in a very recent publication [125]. 

Higashimura et al. [70] tried to elucidate the interactions between amphiphilic miktoarm star molecules produced by cationic polymerization and small molecules using NMR techniques. In their comparison between two different architectures no distinct differences were observed for star-blocks and miktoarm stars, both species being sufficiently capable of accommodating hydrophilic molecules within their hydrodynamic volume. 

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. 

Teyssie and coworkers [86] studied the effect of macromolecular architecture on the lamellar structure of the poly(ethylene oxide) crystallizable arms in (poly tert-butyl styrene)(poly(ethylene oxide))2 [PtBuS(PEO)2] miktoarm stars by using SAXS and differential scanning calorimetry (DSC). The results were compared with the ones obtained on poly(tBuS-fe-EO) materials. At the same total molecular weight and composition the melting temperature, the degree of crystallinity and the number of folds of PEO chains were found to be lower for the branched samples. 

Aside from the linear architecture, BCs can be prepared with advanced architectures such as miktoarm star structures, i.e., BCs where arms of different chemical nature are linked to the same branch point [20]. Unique segregation properties are expected of these polymers [21, 22]. 

Anionic polymerization has proven to be a very powerful tool for the synthesis of well-defined macromolecules with complex architectures. Although, until now, only a relatively limited number of such structures with two or thee different components (star block, miktoarm star, graft, a,to-branched, cyclic, hyperbranched, etc. (co)polymers) have been synthesized, the potential of anionic polymerization is unlimited. Fantasy, nature, and other disciplines (i.e., polymer physics, materials science, molecular biology) will direct polymer chemists to novel structures, which will help polymer science to achieve its ultimate goal to design and synthesize polymeric materials with predetermined properties. 

Higashimura et al. [299] used NMR to probe the interactions between amphiphilic star molecules (star-blocks and miktoarm stars) and small molecules and tried to evaluate the influence of macromolecular architecture on these interactions. No distinct differences were observed between star-blocks and miktoarms, both being efficient enough for accommodating hydrophilic molecules within their structure. 

A different approach was used by Milner [326] in order to predict the phase diagram for asymmetric copolymer architectures (for example A2B, A3B etc. types of miktoarm stars). The free energy of the system can be calculated by summing the free energies of the polymer brushes existing on the two sides of the interphase. Milner described the effects of both chain architecture (i.e., number of arms) and elastic (conformational) asymmetry of the dissimilar chains, in the strong segregation limit, by the parameter... 

The above are examples of the influence of architecture on the microphase morphology of miktoarm stars where, due to the crowding of the chains on one side of the interface, the interface becomes curved resulting in different morphological structures to those expected for linear diblocks of the same composition. 

McLeish and coworkers have published results on the rheological behavior of S2I2 miktoarm star copolymers [359]. For the temperature range between 100 and 150°C it was evident that the rheology of a polymer with 20wt%. PS was independent of temperature, implying a particular molecular mechanism for stress relaxation for this architecture. For the sample having 35 wt% PS, a failure of the superposition principle was observed, a fact that was attributed to the temperature sensitive effective modulus of the polymer. 

PIS Pispas, S., Hadjichristidis, N., Potemkin, L, and Khokhloy, A., Effect of architecture on the micellization properties of block copolymers A2B miktoarm stars ys. AB diblocks, A/ocranzo/ecM/ex, 33, 1741, 2000. 

Recent advances in polymer chemistry, in particular, in controlled radical polymerization, have enabled the synthesis of complex macromolecular architectures with controlled topology, which comprise chemically different (functional) blocks of controlled length in well-defined positions. Block co- and terpolymers, molecular and colloidal polymer brushes, and star-like polymers present just a few typical examples. Furthermore, miktoarm stars, core-shell stars and molecular brushes, etc. exemplify structures where chemical and topological complexity are combined in one macromolecule. 

A series of star DHBCs were evaluated for their ability to transfect hmnan cervical HeLa cancer cells with the modified plasmid pRLSV40, bearing the enhanced green fluorescent protein as the reporter gene [55]. The copolymers utilized were composed of PDMAEMA and PHEGMA blocks (where PDMAEMA is an ionizable block, while PHEGMA is a non-ionic water soluble block). The experimental data indicate a decreased toxicity for the star copolymer, compared to a reference PDMAEMA star homopolymer, for the same amounts of star polymer tested. Moreover, it has been found that the architecture of the star copolymer, i.e. star block, miktoarm star etc, plays a decisive role on the transfection efficiency. The best performance, for all star copolymers tested, was observed for a star block copolymer with... 

In this case, chlorosilane reagents (XVIII, XIX), halomethyl benzene reagents (XX), and 1,1-diphenylethylene (DPE) derivatives (XXI) carrying aUsyUialide functions are typical coupling agents used to deactivate anionically derived polymers, mostly PS, PB, and PI. A variety of star-like polymers of precise functionality, including multiarm stars, star-block copolymers, asymmetric and miktoarm stars, and other branched architectures, are accessible in this way [1, 12-14, 35]. This... 

Amphiphilic (PsCL)i4(PAA)7 miktoarm star copolymers having a p-cyclodextrine core were prepared by ROP and ATRP techniques. Thermodynamically stable micelles were obtained in aqueous solutions. A variety of stmaures, such as spherical micelles, clusters, and wormlike aggregates, were observed by dynamic LS and AFM measurements. Both the copolymer architecture and the composition affea the morphology and the dimensions of the aggregates. 

Miktoaim stars consisting of one thermoresponsive PNIPAAM arm and four pH-responsive PDMAEMA arms were synthesized and their micellization behavior in aqueous solutions was compared with the corresponding linear PNIPAAM-b-PDMAEMA block copolymers.PNIPAAM-core micelles were obtained in acidic solutions at elevated temperatures, whereas PDMAEMA-core micelles were formed in slightly alkaline solutions at room temperature. Furthermore, the kinetics of pH-induced micellization of the AB4 miktoarm stars and the linear block copolymers was studied by the stopped-flow LS technique upon a pH jump from 4 to 10. The data of both types of copolymers could be fitted with double-exponential functions yielding a fast (xj) and a slow (T2) relaxation process. For both copolymers xj decreased with increasing polymer concentration. However, xj was independent of polymer concentration for the AB4 stars, whereas it decreased with increasing polymer concentration for the linear block copolymer. This result indicates that the macromolecular architecture may greatly influence the kinetics of micellization. 

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