Miktoarm stars

Abstract This review highlights 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 is given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

Miktoarm stars of the A(BC)2 type, where A is PS, B is poly(f-bulyl acrylate) (PtBA), and C is PMMA [161] have been synthesized, by using the trifunctional initiator 2-phenyl-2-[(2,2,6,6-tetramethyl)-l-piperidinyloxy] ethyl 2,2-bis[methyl(2-bromopropionato)] propionate (NMP, ATRP) (Scheme 86). In the first step, a PS macroinitiator with dual < -bromo functionality was obtained by NMP of styrene in bulk at 125 °C. This precursor was subsequently used as the macroinitiator for the ATRP of ferf-bulyl acry-... 

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. 

By using a combination of RAFT and ring opening polymerization (ROP), (polyethylene oxide) methyl ether)(polystyrene)(poly(L-lactide) 3-miktoarm star terpolymers have been successfully synthesized [182]. The synthetic approach involved the reaction of the cw-functionalized - OH group of the poly(ethylene oxide) methyl ether with maleic anhydride under conditions where only one hydroxyl group can be esterified (MPEO). The double bond... 

By utilizing a combination of RAFT and cationic ROP, the synthesis of [poly(methyl methacrylate)][poly(l,3-dioxepane)][polystyrene] miktoarm star terpolymers was achieved [182], The approach involved the synthesis of PS functionalized with a dithiobenzoate group by RAFT polymerization and subsequent reaction with hydroxyethylene cinnamate (Scheme 98). The newly created hydroxyl group was then used for the cationic ring opening polymerization of 1,3-dioxepane (DOP). The remaining dithiobenzoate group was used for the RAFT polymerization of methyl methacrylate. 

A third example combines cationic ROP and ATRP for the synthesis of (polytetrahydrofurane)(poly-l,3-dioxepane)(PS) miktoarm stars (Scheme 99). The initiating sites for the above polymerization were created step-by-step from amino-succinic acid (Scheme 99). 

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]. 

However, the central junction of miktoarm-star copolymers exhibits more crowding compared to linear diblocks. The penalty for blocks bending at the interface will be higher (Fig. 34a,c), resulting in a shift of the phase boundaries and different spacings compared to their linear analogues of similar composition. 

Recently, Grason and Kamien calculated the phase diagrams in the weak and strong segregation limit for AB miktoarm-star copolymers using both... 

Fig. 34 Illustration of A B miktoarm star-block copolymer, a AB4 diblock copolymer with A arm depicted by dashed line and B arms depicted by solid lines. b AB interfaces for A15 phase shown in Pm 3 n unit cell (extracted from SCFT results for n = 5 at yN = 40 and tfi = 0.349 [along Hex-Al5 phase boundary] vide infra). From [112]. Copyright 2004 American Chemical Society, c A B miktoarm-star block copolymer with n = 4. From [121]. Copyright 2003 American Chemical Society... 

Fig. 35 Phase diagrams AB miktoarm-star copolymers for n = 2, n = 3, n = 4 and n = 5. mean-field critical point through which system can transition from disordered state to Lam phase via continuous, second-order phase transition. All other phase transitions are first-order. From [112]. Copyright 2004 American Chemical Society...

Investigations on a series of miktoarm-star AB copolymers of PS and poly(2-methyl-l,3-pentadiene), PS(P2MP)3, indicate a different morphological behaviour [113] (Fig. 37), as predicted by Milner s theory [111] (Fig. 38). The discrepancies near the boundaries between different morphologies when compared with corresponding PS/PI systems on the one hand and to theoret-... 

Fig. 36 SCFT results for AB miktoarm stars at strong segregation limit /W = 100. Phase transitions (A) Dis bcc-, (o) bcc Hex-, (0) Hex Lam. All boundaries are computed at /N = 100 with exception of low-0 bcc - Hex and Hex Lam ones for n = 3, 4 and 5. For n = 3 these were computed at /AT = 80, and for n = 4 and 5 these boundaries are computed at /N = 60. Equilibrium results from experiments on Pl-arm-PS melts [219]. From [112]. Copyright 2004 American Chemical Society...

Fig. 37 Morphology of 4-miktoarm-star copolymers of PS(P2MP)3 type, a TEM images, b SAXS data. From [113]. Copyright 2003 American Chemical Society...

Fig. 38 Milner s diagram for morphologies of A-b-B copolymers as reported in literature [111]. Observed morphologies of linear and miktoarm-star copolymers of BS-b-P2MP system are represented. Lamellar structure, cylinders of P2MP in PS matrix spheres of P2MP in PS matrix, biphasic structure of lamellar/double gyroid microdomains. From [113]. Copyright 2003 American Chemical Society...

In the same study the coexistence of two morphologies (L and G) exhibited by a four-miktoarm-star copolymer was reported. This biphasic structure... 

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-... 

Fig. 39 TEM micrograph of lamellar region of PS-arm-PB-arm-P2VP miktoarm-star block copolymer stained with OsC>4 and CH3I. From [115]. Copyright 2000 American Chemical Society...

Fig. 40 a TEM micrograph of PS-arm-PB-ar n-P2VP miktoarm-star block copolymer stained with OSO4. b Simulation of [112] projection (dark matrix, translation 0, thickness 0.33). c Simulation of [112] projection (dark matrix, translation 0.45, thickness 0.33). d Simulation of [1.08 0.84 2.04] projection (dark matrix, translation 0.48, thickness 0.35). From [115]. Copyright 2000 American Chemical Society... 

Fig. 42 Double-logarithmic dependence of first-order reflection peak, q, on total degree of polymerization, N. o AB diblocks A2B2 miktoarm stars. Solid lines fitting results for power law scaling of q N a, with a = 0.73 0.04 and 0.70 0.04 for linear and stars respectively. From [120]. Copyright 2000 American Chemical Society...

The dependence of the lamellar thickness and the number of arms (n = 1, 2, 4 and 16) for symmetric PSn-arm-PIn miktoarm stars shows an increase in the spacing with n (Fig. 43). This indicates an additional chain stretching induced by the spatial confinements close to the junction point. However, the exactness of the results may be influenced by non-separable impurities. As these contamination species are resistant to detection via standard SEC and other separation techniques, it can be reasoned that previous results reported in the literature might suffer from the same shortcomings [121]. 

In conclusion, it can be suggested that the lamellar spacing of miktoarm-star copolymers is controlled by two parameters (i) the molecular weight of the corresponding AB diblock, which also controls the segregation strength, and (ii) the functionality of the central core. 

Fig. 43 Plot of normalized lamellar long periods, Dn/Rg,m of (PS) -arm-(PI)M miktoarm-star copolymers (n = 1, 2, 4 and 16) divided by corresponding diblocks of same series, Di/Rg,u against respective star functionality, n. Normalization factor PgiM represents unperturbed radius of gyration of diblock consisting of one PS block, one PI block and average number of bonds linking these two arms through core. From [121]. Copyright 2003 American Chemical Society...

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]. 

Blending of ABC Miktoarm-Star Terpolymers with AB-Diblock Copolymers... 

Besides linear-block copolymers, also blends with miktoarm-star terpolymers have been reported [193]. Blending a PS-arm-PB-arm-P2VP miktoarm-star terpolymer showing hexagonal symmetry with a PS- -P2VP diblock... 

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 ... 

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