SCLCPs block copolymers

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

Block copolymers with well-defined segments often show microphase-separated morphologies (such as lamellar layers, hexagonal ordered cylinders, and micelle formation). If we use SCLCP blocks together with non-liquid crystalline segments, the mesophases are formed within one of the separated microdomains. If the non-SCLCP block has a higher Tg than the phase transition temperature of the mesophase, the amorphous block should physically support the SCLCP microdomains, forming a self-supported SCLCP system. 

Komiya et al. described the living ROMP synthesis of AB-type block copolymers that contain side chain liquid crystalline polymer blocks and amorphous blocks [62]. Norbornene (NBE), 5-cyano-2-norbornene (NBCN) and methyl-tetracyclododecene (MTD) were used for the amorphous polymer blocks, while I-n (n=3,6) were used for the SCLCP block (see Fig. 9). Initiator 1 was used for the ROMP. Block copolymers with monomer ratios from 75/25 to 20/80 (amor-... 

Block copolymers consisting of a smectic SCLCP-block and a partially crystalline apolar block were synthesized via ROMP of IV-n with cyclooctene and initiator 1 or 2 [63]. The block copolymers also formed smectic liquid crystalline mesophases and showed lamellar phase-separation. 

A block copolymer consisting of a SCLCP-block of monomer XXVII with a laterally-attached mesogenic unit, and butyl-acrylate, was synthesized using a combination of ROMP and atom transfer radical polymerization (ATRP) (Fig. 16) [81]. 

ROMP of the SCLCP block was carried out with the Grubbs -initiator 6 in a monomer to initiator ratio of about 25. The polymerization was terminated by reaction with 4-(2-bromopropionyloxy)-but-2-enyl 2-bromopropionate, leading to a macroinitiator for ATRP. The radical polymerization was carried out with CuCl, 4,4 -di(n-nonyl)-2,2-bipyridine, and butylacrylate (BA), giving a poly-XXVII-fo-BA copolymer. The homopolymer of poly-XXVII had a narrow PDI of 1.06, while the diblock copolymer showed a PDI of 1.32. 

The homopolymer showed an enantiotropic nematic mesophase, whereas the diblock copolymer generated microphase-separated lamellae, in which the SCLCP block possessed a nematic-isotropization transition similar to the homopolymer (Table 17). Upon heating, the nematic microphase decreased continuously in the nematic phase from 38.5 nm to 27 nm and showed a constant value of about 26 nm after the nematic-isotropization transition. Therefore, materials in which these block copolymers are macroscopically aligned are expected to show reversible contraction in one dimension, making this polymer system an interesting candidates for an artificial muscle or actuator. 

Polybutadiene was the first well-defined polymer used in a polymer analogous reaction to synthesize SCLCPs [201 -203], primarily for comparison to the corresponding block copolymers discussed in Sec. 4.1.1 of... 

A controlled free radical polymerization technique was also used, namely, atom transfer radical polymerization (ATRP), to synthesize ATE on the basis of ABA triblock copolymers (Fig. 11.3b) (Cui et al., 2004). The triblock copolymer was designed to have a rubbery midblock of poly(n-butyl acrylate) (PnBA) and two end blocks of poly 6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) (PAzoMA) that is azo-SCLCP. For synthesis, a dibromo initiator, namely, l,l -biphenyl-4,4 -bis(2-bromoisobutyrate), can first be used to prepare the dibromo PnBA macroinitiator, which is then used to polymerize the azobenzene methacrylate monomer to yield the two end blocks of PAzoMA. This ATE is different from azo-SCLCP-grafted SBS. It is a thermoplastic elastomer, in which... 

---