Rod-coil diblock copolymers

If indeed supramolecular dusters are formed spontaneously in bulk films of appropriately designed rod-coil systems, by inclusion of appropriate reactive units it should be possible to convert these into molecular objects by crosslinking, while maintaining the predse size and shape of the cluster. In order to test this hypothesis, rod-coil triblock copolymers, with structures similar to those described previously, but with a few modifications to enable crosslinking, were prepared. Stupp and coworkers replaced the poly(isoprene) block with a poly( butadiene) (PB) block (which contains both 1,2- and 1,4-linked repeat units), which is known to undergo thermal crosslinking at high temperatures [70]. Additionally, the phenolic OH 

One of the major motivations for the study of nano-aggregate formation, either in bulk or in solution, is because it provides an opportunity to construct macromolecular objects with dimensions significantly larger than molecular dimensions, and therefore would pave the way to further organize them well into the macroscopic domain. Thus, this serves as a step-wise approach to organizing macromolecules in the bulk phase, with a control over the molecular organization at each step along the way. The fixation of the mushroom-shaped supramolecular ag- 

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The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

Ober and Thomas et al. reported on rod—coil diblock copolymers consisting of poly(hexyl isocyanate) as the rod block and polystyrene as the coil block (Scheme 2).51 53 The polymers (2) were synthesized... 

Figure 8. Morphology diagram for rod—coil diblock copolymers (2).

Wu et al. reported on a rod—coil diblock copolymers based on mesogen-jacketed liquid crystalline polymer as the rod block and polystyrene as the coil block (Scheme 6).82 Styrene was polymerized by TEMPO mediated radical polymerization, followed by sequential polymerization of 2,5-bis[4-methoxyphenyl]oxy-carbonylstyrene (MPCS) to produce the rod—coil diblock copolymer (20) containing 520 styrene and 119 MPCS repeating units. The rod—coil copolymer was observed to self-assemble into a core—shell nanostructure in a selective solvent for polystyrene... 

The authors also reported on the supramolecular self-assembly from rod—coil—rod triblock copolymers prepared by copolymerization of 5-acetyl-2-aminob-ezophenone with diacetyl functionalized polystyrene with low polydispersity (Scheme 12).110 In contrast to the rod—coil diblock copolymers which exhibit multiple morphologies, the triblock copolymers were found to spontaneously form only microcapsules or spherical vesicles in solution as evidenced by optical polarized, fluorescence optical, and scanning electron microscopies (Figure 33). 

Wan et al. used TEMPO-mediated polymerizations to prepare liquid crystalline (LC) polymers [147,148]. pSt-TEMPO was chain extended with a mesogen-jacketed LC monomer, 2,5-bis [(4-methoxyphenyl)oxycarbonyl] styrene (MPCS, Fig. 9) to form a rod-coil diblock copolymer. The resulting copolymer had an Mn=19,500 with an Mw/Mn=1.48. There was tailing to lower molecular weights, indicating the presence of some unreacted macroinitiator, but after extraction with cyclohexane, the remaining macroinitiator was removed, leaving pure block copolymer. H and 13C HMR analysis indicated the presence of both blocks, as did DSC analysis, which showed two Tgs,one at 117.2 °C (pMPCS) and... 

Rod-Coil Diblock Copolymers Based on Perfectly Monodisperse Rods. . . 65... 

In contrast to the rod-coil diblock copolymer consisting of perfectly monodisperse rods, the liquid crystalline morphologies of rod-coil diblock copolymer containing polydisperse rods seem to be studied in less detail. In certain cases, the polydisperse nature of the rod-segments could hinder self-assembly into regularly ordered supramolecular structures. However, due to relatively simple synthetic procedures, liquid crystalline polymer can be of benefit for new materials with controlled internal dimensions ranging from the nanometer to macroscopic scale. 

Figure 2. Structure of liquid crystalline block copolymers (LC-BCPs) (A) rod-coil diblock copolymer (B) rod-coil diblock copolymer with flexible spacer in the rod block (C) side group liquid crystal-coil (SGLC- coil) diblock copolymers (D) coil -rod-coil ABC triblock copolymers (predicted to be novel ferroelectric fluid by R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett., 1987, 59(3), 343-346) (E) rod-rod diblock copolymer (one example of well-defined po-ly(n-hexyl isocyanate-fc-n-butyl isocyanate) rod-rod diblock copolymer was given by Novak et al. [68], however, no morphology studies were reported) (F) dendritic liquid crystal-coil (DLC-coil) diblock copolymer (not reported).

Liquid crystallinity and block microphase separation both compete during the minimization of free energy of the system. As we will show later in this review, in the case of a rod-coil diblock copolymer, liquid crystallinity plays a very important role in the microphase separation process and leads to morphologies distinctly different from the conventional spheres, cylinders and lamellar microstructures and include the arrow head, zig-zag, and wavy lamellae phases [40, 41], In the case of SGLC-coil... 

B Synthesis ol rod-coil diblock copolymers wilh a polypeptide (PBLG) rod block ... 

Scheme 2. Functionalization of living chain end (A) and synthesis of rod-coil diblock copolymer with a polypeptide rod block (B) [9].

Another type of polypeptide-containing block copolymer, amphiliphilic rod-coil diblock copolymers such as poly (/V-triflu-oroacetyl-L-lysine)-/>-sarcosine) (Kt - Sa), were also synthesized and characterized by Gallot and coworkers [47]. The hydrophobic rod block poly(A-trifluoroacetyl-L-ly-sine) (Kt) was prepared by polymerization of Kt-NCA using A-hexylamine as the initiator. After fractionation using DMF (good solvent)/water (nonsolvent), the narrowly dispersed polymer (Kt) was then used as macroinitiator to initiate polymerization of the second monomer (Sa-NCA) to afford the hydrophilic block. Final elimination of Kt and Sa homopolymers were performed by precipitation with acetone and water respectively. The synthesis of Kt-Sa diblock copolymer is shown in Scheme 3. 

Figure 3. Lamellar packing model for polypeptide containing rod-coil diblock copolymers (A) Model for polybuta-diene-b-PBLG diblock copolymer in which the PBLG chains fold in the lamellar layer and adopt hexagonal packing. This corresponds to a SmB mesophase [9]. (B) Model for amphiphilic polypeptide diblock copolymer. Note that the rods are tilted relative the lamella layer normal while maintaining hexagonal close packing with a constant domain D independent of tilt angle.

Table 1. Rod-coil diblock copolymers synthesized by Stupp and coworkers (adapted from [52]).

Scheme 6. Synthesis of poly(styrene-n-hexyl isocyanate) rod-coil diblock copolymers [40, 62].

Figure 4. TEM of poly(styrene- -hexyl isocyanate) rod -coil diblock copolymers. The orientation of rods in LC domain is indicated by arrows. (Reprinted from [41], with permission).

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