Side-Chain Liquid-Crystalline Polymer Tetrazoles : Synthesis and Characterization

The synthesis and phase behavior of three new side-chain liquid crystal polymers with short and long flexible alkyl spacers are described. These new materials show smectic C (SmC) mesophase. X-ray analysis for two samples has revealed that SmC mesophase displays partial interdigitation for the polymer with short flexible spacer, while for long flexible spacer polymer no interdigitation was observed.


Introduction
The synthesis and studies of the physical properties of thermotropic liquid crystals are the subjects of current interest as thermotropic liquid crystals have technological applications in electro-optic devices. 1,2From the viewpoint of the science basics, this interest is evident by numerous studies of both low-and high-molecular weight liquid crystals [3][4][5][6][7][8][9] indicating the structural dependence of liquid-crystalline properties.However, the relationship between structural aspects and mesophase behavior is not well understood.
Liquid crystals (LCs) are systems that can self-organize due to mesogenic groups that exhibit cooperative interactions among them.This is the origin of interesting properties of liquid crystals and extends to both types of high-molecular weight liquid crystals, i.e., side-and main-chain liquid crystalline polymers (SCLCP and MCLCP, respectively).It is well known that the nature of mesophases depends on the relationship established between polymer backbone, mesogenic core and the length of the flexible spacer group.][12] In order to investigate the correlation between flexible spacer length, molecular weight and the influence of tetrazole ring we have synthesized three polymer LCs containing N-alkylated tetrazole ring as part of mesogenic core connected to the polyacrylate backbone through flexible spacer with four and eleven carbon atoms.The tetrazole ring was built by 1,3-dipolar addition of azide anion (N 3 -) to an arylnitrile and regioselectively alkylation at position 2. The regioselectivity during the alkylation step has been recently established unequivocally by single crystal X-ray analysis for two final LCs. 13 prevent O-alkylation reaction.The tetrazole heterocycle was constructed by [3+2] 1,3-dipolar cycloaddition of azide anion to cyanophenol 1 followed by acetylation of phenol functionality to yield 2 in 51% yield over two steps.The alkylation of 2 with nonylbromide yielded the corresponding N-2 nonyl tetrazolylacetate, which was subsequently hydrolyzed in basic solution to furnish the free phenol 3 in 87% yield.
The synthesis of acids 6 and 8 carrying the polymerizable acrylate group is outlined in Scheme 2 and 3, respectively.The acrylate group was introduced using a large excess of acrylic acid in presence of p-toluene sulfonic acid (pTSA) and hydroquinone. 14The former was prepared according to a method described by Portugal et al.. 15 The only modification made in this method was in the first step, when 4-chlorobutyl acetate was employed instead of 4-bromobutyl acetate.4-Chlorobutyl acetate was obtained from ring opening reaction of tetrahydrofuran (THF) with acetylchloride and catalytic amount of zinc chloride as outlined in Scheme 2.
The intermediate 8 14 was prepared in three steps as depicted in Scheme 3. The flexible spacer has eleven methylene carbon atoms and the polymerizable acrylate group was introduced in the same way as described for compound 6.The target acrylates 6 and 8 were isolated and purified by recrystallization from isopropanol or aqueous ethanol.
M o n o m e r s 9 a n d 1 0 w e r e p r e p a r e d f r o m 4-[(4-propenoyloxy)butyloxy]benzoic acid 6 and 4-[(4-propenoyloxy)undecyloxy]benzoic acid 8, respectively, by reacting with tetrazolylphenol 3 using dicyclohexyl carbodiimide (DCC) in pyridine solution and pTSA as a catalyst. 15The yields after purification were in range of 30-60%.The monomers were subjected to free radical polymerization in toluene using 2,2'-azobisisobutyronitrile (AIBN) as radical initiator to give homopolymers 11 and 12 (Scheme 4).The polymers were purified by successive precipitation with a solution of dichloromethane in cold methanol.Finally, the amorphous solid was washed several times with cold ethyl acetate until no signal for ethylenic protons was observed in nuclear magnetic resonance (NMR) analysis.The comparative data of the homopolymers 11a,b and 12 are shown in Table 1.

Liquid-crystalline properties
The mesomorphic properties of the polymers were studied using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC).The texture of the mesophase 16 was identified by microscopic studies on cooling from the isotropic to mesomorphic state.The liquid-crystalline properties of the polymers 11a,b and 12 are compiled in Table 1.Table 1 lists the observed layer spacing obtained from the Bragg equation and the calculated length of the monomer molecule obtained from the program ChemBio3D Ultra.
The synthesis of polyacrylate 11a was carried out using 3 wt.% of AIBN while polymerization reaction to give 11b and 12 were performed using 5 wt.% of AIBN.The molecular weight (Mn) and degree of polymerization (DP) for 11a is twice than 11b and 12, probably related to the content of initiator during the chemical polymerization process.Polymers 11b and 12 may be considered as oligomers considering the low value of DP for these polymers.All the polymers exhibited a glass transition temperature (Tg) and enantiotropic SmC phase.For instance, polymer 12 shows Tg at 70.5 °C and enters the smectic C phase with a range of 36.5 °C for the mesophase and goes to isotropic phase at 107 °C.The flexible spacers in this study have four and eleven methylene carbon atoms.The polymer backbone for 11a and 11b are closer to the mesogenic core while in 12 it is relatively away from the mesogenic core.However, the mesomorphic behavior does not drastically change with lengthening of the flexible spacer.Even the variation of the DP was not enough to change the nature of the mesophase or to significantly alter the mesophase range as is evident form the comparison of DP for 11a and 11b, where the first is almost twice as heavier as the second.Previously reported results have revealed that when the mesogenic core is derived from 1,4-phenylene unit, the mesophase is nematic in the case of a short flexible spacer and smectic A phase for long flexible spacer. 17Considering the polymeric nature of 11a,b and 12 and that they do not tend to crystallize on cooling a mesophase fixation when the samples were exposed to polymerization action.From POM analysis, both polymers show a glassy appearance.The glass transition temperature tends to decrease as the molecular weight decreases for polymers with the same flexible spacer, compare 11a and 11b.The Tg tends to be higher for a polymer that has longer flexible spacer as observed for 12.In general, a lower polymer glass transition temperature implies that the backbone is more flexible.
Data compiled in Table 1 shows that the shortening or lengthening of flexible spacer does not have any influence on the nature of the mesophase.However, as will be discussed in the X-ray section, the structure of SmC phase is dependent on the length of flexible spacer.Smectic C phase observed in this study probably is the result of folded-shape of the mesogenic core having a non-symmetrical tetrazole ring.The folded-shape of mesogenic core prints on the polymer material its structural feature, which overcomes the tendency of the main-chain to adopt a random coil conformation. 18he mesomorphic behavior of compounds 11a,b and 12 was investigated between crossed polarizers using optical microscopy.Figure 1a displays the normal texture for 11a,b and 12. Schieliren texture is barely seen in this picture due to the nature of the sample and the thickness.However, in other areas where the thickness is smaller, especially in the border zone of the samples, the Schlieren texture flourishes radiantly (Figure 1b).The Schlieren texture with four-point brushes characteristic of smectic C phase is seen for 11b (Figure 1b).The assignment to nematic phase was discarded considering that from isotropic phase no droplets were observed upon cooling.Also, the absence of optically extinct homeotropic and flashing areas on the mechanical disturbance corroborates to eliminate the nematic phase.
X-Ray diffraction studies X-Ray diffraction (XRD) studies were carried out to confirm the POM results and obtain more detailed information on the structures and types of liquid crystalline phases.Generally, a diffuse broad peak where 2q assumes values from 16 to 22° in the wide angle region of X-ray diffraction curves can be observed for polymers but not for polymer LCs with smectic, nematic, and N* phase structures due to the average distance between the mesogenic groups.Sharp and strong peaks at low angles in which 2q assumes values from 1 to 5° can be observed for a normal smectic structure which exhibits a layered packing arrangement, but not for N* and nematic structures.
Figure 2 shows the patterns of compounds 11b and 12 to investigate the structure of their smectic phases at 90 and 80 °C, respectively.In both cases the spectrum contains a sharp peak in the low-angle region corresponding to the Miller index (001), where by applying Bragg's law resulted in an interlayer spacing of 45.2 ± 0.1 Å for 11b and 36.6 ± 0.1 Å for 12.In addition, a broad diffuse band centered around 4.4 Å is present, which is related to the short-range correlations between neighboring mesogenic molecules inside the layers.Compound 11b also displayed secondary order peaks (002) and (003) corresponding to distances of 23.8 ± 0.1 and 16.1 ± 0.1 Å, respectively, following the relation 1:2:3 with respect to (001), as expected for smectic phases.However, only the (001) is observed for compound 12, which indicates that the smectic order is short range correlated.The molecular length of the monomer molecules 9 and 10 was calculated with the program ChemBio3D Ultra considering the most extended configuration, giving 32.8 ± 0.1 and 40.4 ± 0.1 Å, respectively.One can see that the calculated value for monomer 10 is around 3.8 Å larger than the interlayer distance measured for polymer 12, which is consistent with conformational disorder of the aliphatic chains and inclination of the molecules inside the layers expected for the SmC phase.On the other hand, monomer 9 is around 12.4 Å shorter than the interlayer distance measured for polymer 11b.The ratio between the interlayer distance and the molecular length is 1.38 and this observation is consistent with a partial interdigitation of the mesogens inside the layers.
If we consider that the backbone is confined by the smectic field and lies between the layers, 19 two arrangements for polymers 11b and 12 are possible.In one arrangement, the side chains are closer to the backbone than in the other, causing a partially interdigitated SmC phase.In the other arrangement, the side chains exhibit no interdigitation.In both cases, the motion of the backbone is constrained by the orientational and positional order of the smectic layer.The evidence for this classification comes from other data published in the literature. 20From Table 1, the higher glass transition temperature is observed for 12, which exhibits no interdigitated smectic phase.The side chains are more decoupled from the polymer backbone due to the longer flexible spacer.The anisotropic cores have more freedom to arrange themselves in an appropriate manner with more independence from the main chain orientation of backbone.For polymer 11b, with smaller flexible spacer, the behavior is similar to the nematic one.In this sense, it is possible to expect some interdigitation of the laterally anisotropic core.These structural features of the smectic phase have pronounced effects on packing density as well as transition properties.Mesophase structure that possesses a more compactness degree has a less specific free volume. 21The closer the mesogenic cores to the random coil conformation of polymer, the higher the influence of polymer backbone on the final structure of the mesophase.The main chain was no longer a random coil.A considerable degree of mesogenic order is transferred to the main chain by the flexible spacer. 18,22The behavior of 11b is in accordance with the general tendency that the increasing backbone flexibility for a given spacer length and mesogenic core tends to diminish the glass transition temperature while increasing the temperature of clearing (Tc) or the clearing point. 2,15,23

Conclusions
In summary, we have synthesized three homopolymers having the tetrazole ring as the key structural brick for the mesomorphic phenomenon.They showed smectic C enantiotropic behavior with a large mesophase range.Flexibility of polymer backbone, Tg and Tc transition temperatures are dependent on the polymerization degree.These are important parameters to be considered in the design and synthesis of new materials.X-Ray analysis showed a partial interdigitation of the mesogens inside the layers of the polymer with short flexible spacer.

Experimental
Characterization 1 H and 13 C NMR spectra were recorded on a Bruker AC-300F spectrometer in CDCl 3, using tetramethylsilane (TMS) as the internal standard.Infrared (IR) spectra were recorded on a Perkin-Elmer 781 spectrometer in KBr disc or film.The thermal transitions and the mesomorphic textures of the compounds were observed by optical microscopy using a Leitz Ortholux polarizing microscope in conjunction with a Mettler FP 52 heating stage.The XRD experiments were carried out using X'PERT-PRO (PANalytical) diffractometer using the linear monochromatic Cu Kα radiation (λ = 1.5418Å) and applied power of 1.2 kVA.The samples consisted of an amount of powder on a glass plate placed in the diffractometer chamber on the TCU2000 temperature control unit (Anton Paar), allowing temperature control during the measurement.The scans were performed in continuous mode from 2 to 30° (2q angle) with the samples in the mesophase, obtained by cooling from the isotropic state.

Synthesis
All reagents were purchased from Aldrich Chemical Company (USA) and solvents were used as received from Merck.The synthesis of the title compounds were carried out according to standard methods outlined in Schemes 1, 2, 3 and 4.
4-[(4-Propenoyloxy)butyloxy]benzoic acid (6): 25 In a Dean-Stark apparatus were added 4-(w-hydroxybutoxy) benzoic acid (7.7 g, 0.037 mol), acrylic acid (23.0 g, 0.324 mol) chloroform (150 mL), pTSA (1.5 g, 0.008 mol) and hydroquinone (1.5 g, 0.014 mol).The mixture was refluxed until the end of the reaction.After a standard workup procedure the product was recrystallized from isopropanol.Yield 65%; m.p. 120 °C; IR (KBr) n max /cm - Monomers 9 and 10: General procedure.Acids 6 or 8 (0.005 mol), 4-(2-nonyl)-2H-tetrazol-5-ylphenol (3) (0.0055 mol), DCC (0.006 mol) and pTSA (0.0026 mol) were dissolved in freshly distilled pyridine (5 mL).The reaction was stirred for 3 days at room temperature.The mixture was filtered and washed with chloroform.The organic phase was washed with 10% cold aqueous hydrochloric acid (100 mL) and water and dried over sodium sulfate.The solvent was removed in a rotatory evaporator and the crude solid was purified by recrystallization from methanol.Polymerization reaction: In a Schlenk flask dry toluene was added under inert atmosphere followed by addition of 0.001 mol of the monomer (9 or 10).The mixture was stirred for 30 min at room temperature.To this mixture AIBN (3 or 5 wt.%) was added and the reaction mixture was further heated at 70 ºC for 72 h.After completion of the reaction the mixture was poured into cold methanol to precipitate the product.The solid was filtered and washed several times with ethyl acetate to separate the unreacted monomer, which left behind pure polymers 11a,b and 12.

Figure 2 .
Figure 2. (Top) X-Ray diffraction patterns for compound 11b (black line) and 12 (gray line), with indication of the Miller indices.SmC phase at 90 and 80 ºC for 11b and 12, respectively.(Bottom) Two types of smectic C structure exhibited by side-chain liquid crystal polymers 11b and 12.

Table 1 .
Data for polymers 11a,b and 12: GPC, transition temperature and X-ray diffraction a Transition temperatures were acquired upon heating by POM; b data collected at 90 °C; c data collected at 80 °C.Mn: g mol -1 ; DP: number average degrees of polymerization; GPC: gel permeation chromatography.Scheme 4. Synthesis of polymers 11 and 12.