Polymerization SCLCP

ROMP Ring-opening metathesis polymerization ADMET Acyclic diene metathesis polymerization ALTMET Alternating diene metathesis polycondensation MCLCP Main chain liquid crystalline polymer SCLCP Side chain liquid crystalline polymer mru molecular repeating unit... 

Ring opening metathesis polymerization (ROMP) can be used to build up SCLCPs using various mesogenic units. ROMP-derived SCLCPs exhibit a num-... 

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

Ring opening metathesis polymerization (ROMP) has proven to be a convenient technique that allows us to synthesize a wide variety of SCLCPs [8]. 

Cho et al. described the synthesis and polymerization of 4,8-cyclododeca-dien-l-yl-(4 -methoxy-4-biphenyl) terephthalate VIII [54,55]. Polymerization was carried out with WCl4(OAr)2/PbEt4. The double bonds in the polymer backbone were subsequently hydrogenated with H2/Pd(C), leading to a SCLCP with a fully saturated hydrocarbon backbone. This polymer system had a very flexible polymer backbone but a stiff connection between the main chain and the mesogenic unit. The distance between two adjacent side chains was about 12 methylene units. This very flexible main chain allowed the polymer to organize into a LC mesophase. Both polymers - the unsaturated and the saturated -showed smectic liquid crystalline mesophases with almost the same transition temperatures (see Table 5). 

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. 

Discotic SCLCPs were synthesized by the group of Grubbs [82]. For this purpose, norbornenes XXVIII-n (n=5, 10) and cyclobutenes XXIX-n (n=5, 10) with alkoxy-substituted triphenylenes as mesogenic units (see Fig. 18) were prepared. Polymerization was carried out with initiator 6. The resulting polymers had a narrow PDI between 1.09 and 1.17. Physico-chemical data for poly-XXVIII and poly-XXIX are listed in Table 18. 

Norbornene-based and oxa-norbornene-based monomers bearing dendritic side chains, XXX and XXXI (Fig. 19), were synthesized and polymerized via ROMP with initiator 6 [83]. Based on size exclusion chromatography data, the polymerization shows hving-like character up to DP=70. H- and C-NMR-spectroscopy revealed 35% cis and 65% tram sequences. These polymers displayed enantiotropic nematic and smectic mesophases, except for DP=5. In contrast to other classes of SCLCPs, the dependence of the DP on the transition temperatirre of the polymer was very weak. Glass transition and isotropization temperatures became independent of molecular weight above a degree of polymerization of about 10. 

In contrast to ROMP, ADMET offers the possibility of synthesizing both side-chain and main-chain liquid crystalline polymers. The scope and limitations of ADMET are discussed in detail by Wagener et al. in this issue. We herein focus on a few contributions that used step growth polymerization methods to prepare MCLCPs and SCLCPs. 

Unil recently, most side chain liquid crystalline polymers (SCLCPs) were prepared by either hydrosilations of mesogenic olefins with poly(methylsiloxane)s or by free radical polymerizations of acrylates, methacrylates and chloroacrylates [1,2]. Both routes involve chain polymerizations, either directly or prior to a polymer analogous reaction. Chain polymerizations involve the four elementary reactions shown in Scheme 1 [3], In contrast to step polymerizations,... 

The elementary synthetic and structural principles of SCLCPs are now being elucidated using living polymerizations in which the effect of a single structural feature can be isolated, while other structural variables remain constant. In particular, living polymerizations are used to determine the molecular weight at which the thermotropic be-... 

SCLCPs have also been prepared by group transfer polymerization (GTP) of mesogenic methacrylates are room temperature [47, 48]. The livingness of the nucleophilic catalyzed GTP was originally attributed to a new mechanism (Eq. 12), in-... 

Although many controlled cationic polymerizations that have been developed [120], only mesogenic vinyl ethers have been used in an attempt to prepare well-defined SCLCPs by a cationic addition mechanism [121]. Nevertheless, these polymerizations (and copolymerizations) provide the most complete series of SCLCPs with the widest range of structural variables. As shown in Eq. (22), most of these monomers, includ-... 

The CF3S03H/SM62 initiating system has also been used to cyclopolymerize ll-[(4 -cyanophenyl-4"-phenoxy)alkyl]undecanyl-3,4-bis(ethenyloxyethoxy)benzoate to form SCLCPs with crown ethers in the polymer backbone, although the polymerization is accompanied by termination [143]. Polymerization of the chiral vinyl ethers shown in Scheme 10 were reported to give only oligomers with DPn = 5 under the same condi-... 

In order to synthesize homopolymers by polymer analogous reactions, the reaction must go to 100% conversion. Hydrosila-tions of mesogenic olefins are the most common polymer analogous reactions used to prepare SCLCPs [199]. However, the precursor poly(methyl siloxane)s are generally not prepared by living polymerizations, nor do the hydrosilations readily go to comple-... 

SCLCPs have also been prepared by alkylation and/or quatemization of linear poly (ethylene imine)s [215, 216] and po-ly(4-vinylpyridine)s [217-223] with meso-genic alkyl halides or carboxylic acids. Although these precursor polymers can be prepared by controlled cationic [224] and anionic [225, 226] polymerizations, respectively, living polymerizations were either not used to prepare the SCLCPs, or the mo-... 

In contrast, the transitions of most well-defined SCLCPs prepared by controlled polymerizations are relatively narrow. The effect of polydispersity was therefore investigated by blending well-defined (pdi < 1.28) poly 5- [6 -[4"-(4 "-methoxy-phenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene)s of varying molecular weights (DP =5, 10, 15, 20, 50, 100) to create poly disperse samples (pdi = 2.50-4.78) [22]. In this case, both monodisperse samples and multimodal blends underwent the nematic-isotropic transition over a narrow temperature range. Polydispersity also had no effect on the temperatures of transi-... 

Free radical polymerization of a vinyl monomer, e.g., acrylate, methacrylate, or styrene derivative, has been widely used to produce NLO single-component SCLCPs. This type of monomer is suitable as it is relatively easy to prepare and the conditions for free radical polymerization of acrylates, methacrylates, and styrene derivatives have been well documented. There are, however. 

Several additional interesting effects were noted in this study. First, it was shown that the NLO SCLCPs investigated exhibited enhanced stability over isotropic polymers. Second, it was found that the temperature at which the polymeric films were stored below was important in determining the rate of degradation of. Excellent thermal stability was observed for 46 at room temperature, while a decrease in J33 was evidenced for 45 (jc=0.8), and, to a greater extent, 45 (.r=0.55). Sub-7g mobil-... 

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