1 Introduction

The side chain liquid crystal polymers (SCLCPs) are of paramount importance and have attracted enormous attentions due to harmonization of their special anisotropy liquid crystalline properties with their outstanding mechano-performances [1]. To understand the behaviour of this class of compounds, the molecular architectures of their corresponding monomeric units must be studied [2].

Methyl methacrylate polymers outstand others with comparatively low glass transition temperature (Tg), viscosity, and good mechanical, thermal stabilities, high silicon–oxygen bond angles mobility [3]. These properties rendered them useful in liquid crystal display (LCD), smart soft materials [4,5,6], information storage, non-linear optics and other applications as liquid crystal polymers with low phase transition temperature.

Inspite the above advantages of SCLCPs, little or no attention is being paid to the LC properties and characterisation of the monomers from which they are formed (synthesised).

Here, we present the synthesis and characterisation of methyl methacrylic monomers, taken into consideration the simplicity and more efficient synthetic methods, availability of materials, easy reaction monitoring and characterisation. The detailed synthesis of the intermediates and the products are given in the supporting information.

2 Description of the syntheses

Three methyl methacrylic liquid crystal monomers were designed and systematically synthesised and characterised. The polymeric unit/end chosen was methyl methacrylic to induce flexibility and because of its high polymerization power/ability, while the liquid crystalline moiety used was 4′-Undecycloxybiphenyl-4-yl 4-octyloxy-2-(pent-4-en-1-yloxy) benzoate (Me) [7, 8] with phase transition of Cr 53.3 N 71.7 I. The methyl methacrylic unit, iii, was systematically synthesised by the reaction between 3-buten-1-ol, i, and methacryloyl chloride, ii, with yield of 84% (Scheme 1). In this work, various chemical reactions were used to link the methyl methacrylic unit to the previously reported mesogen, Me (Scheme 1).

Scheme 1
scheme 1

Synthesis of but-3-en-1-yl methacrylate. (a) Triethyl amine, THF, 0 °C, 98 h, 84%

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The first monomer M1, C55H86O8Si2 (MW = 931.45), was synthesised using hydrosilylation [9] of the mesogen, Me, using 1,1,3,3-tetramethyl disiloxane in the presence of platinum catalyst to achieve the first intermediate (iv) in quantitative yield, 92% (Scheme 2).

Scheme 2
scheme 2

Synthesis of M1. (b) 1,1,3,3-Tetramehyldisioxane, Toluene, Pt catalyst, 30 °C, 12 h, 92%. (c) Toluene, Pt catalyst, 30 °C, 12 h, 86%

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The targeted monomer M1 (86%) was finally synthesised by the second hydrosilylation reaction between the intermediates iii and iv.

For the synthesis of the second monomer, M2, C47H66O7 (MW = 742), the hydroboration–oxidation sequence [10,11,12,13,14,15], was carried out on mesogen, Me, to afford intermediate hydroxyl compound, v. The esterification [16, 17] of the resulted intermediated hydroxyl compound gave M2 in quantitative yield (Scheme 3).

Scheme 3
scheme 3

Synthesis of M2. (d)(A) 9-BBN, THF, 40 °C, 12 h; or BH3⋅THF, 0 °C, 2 h (B) NaoH/H2O2, 0 °C, 2 h, 44%; or NaBO3⋅4H2O, rt, 2 h, 54%. (e) ii, Triethylamine, rt, 9 h, 98%

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The lateral alkene end of the mesogen, Me, was subjected to hydroboration by 9-BBN and subsequently oxidised by sodium perborate to afford hydroxyl intermediate compound, v, 54 %. The hydroboration by BH3.THF followed by NaOH/H2O2 oxidation was also investigated and gave yield of 44%. The disappearance of the signals due to methylene protons at 4.9 and 5.7 ppm in 1H NMR spectrum of the compound, v, confirmed the reaction. The hydroxyl functional end of the intermediate v was modified by esterification with methacryloyl chloride, ii, to yield the targeted monomer, M2, quantitatively, 98 %.

The third monomer, M3, C51H74O8 (MW = 814), was systematic synthesised as shown in Scheme 4.

Scheme 4
scheme 4

Synthesis of M3. (f) Methacrylic acid, hydroquinone, H2SO4, reflux, 3 h, 58%. (g) Chlorodimethylsilane, Toluene, Pt catalyst, rt, 12 h, 86%. (h) Imidazole, DCM, rt, 24 h, 67%

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One hydroxyl group of 1,4-butanediol, vi, was esterified with methacrylic acid, iii, to obtain the first hydroxyl methacrylate intermediate, vii, in 58% yield [18]. 1H NMR confirm the conversion in which there was emergence of signals due to methylene of methyl methacrylic end at 5.5 and 6.0 ppm. The second intermediate, viii, was synthesised by silylation of the mesogen, Me, by chlorodimethylsilane in the presence of platinum catalyst. From the 1H NMR spectrum, the disappearance of the signals due to methylene protons at 4.9 and 5.7 ppm found in the compound Me, confirm the product, viii. The condensation of compounds vii and viii using imidazole yielded the targeted monomer, M3, in 67 % yield.

3 Results and discussion

3.1 Liquid crystals properties

The liquid crystal properties were obtained using polarizing optical microscope (POM) and differential scanning calorimeter (DSC). The textures were obtained from POM while the transition properties were confirmed by DSC. Table 1 shows the liquid crystalline properties of the intermediates and monomer.

Table 1 Liquid crystalline properties of the intermediates and monomer
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The textures of the intermediates and monomers are as show in Fig. 1.

Fig. 1
figure 1

Textures of compounds as observed under POM (× 400). a Compound Me at 70.9 °C. b Compound v at 53 °C. c Compound M2 while cooling at 65 °C. d Compound iv when cooled with cardice. e Compound iv at rt. f M1 when cooled. g M3 just before isotropy

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The mesogen Me and the corresponding intermediates v, shows nematic texture (Fig. 1a, b) from the melting point (53 and 51 °C respectively) to the clearing point (~72 °C). The monomer M2 also shows nematic texture (Fig. 1c). The schlieren texture characteristic of nematic phase was seen for intermediate compound iv (d). An oil streaks texture of nematic is observed for compound iv at room temperature (Fig. 1e) in which lines show up and swim with thermal motion. Monomer M1 showed marble sheared nematic texture (f), while monomer M3 also showed nematic texture at low temperature, 10 °C (g).

3.2 Thermal properties

The thermal behaviour [19] of the intermediates and the monomers were investigated using DSC and the heating and cooling curves as obtained are shown in Fig. 2.

Fig. 2
figure 2

DSC curves of a Compound iv at 20 °C/min, b Compound M1 at 5 °C/min, c Compound M1 at 20 °C/min, d Compound v at 10 °C/min, e Monomer M2 at 10 °C /min, f Monomer M3 at 10 °C /min (Red curve = first heating; Green curve = second heating; Brown curve = third heating; Blue curve = first cooling; Pink curve = second cooling)

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Me melted to nematic phase at 53.3 °C with enthalpy of 71.92 J/g and turning isotropy at 71.7 °C with enthalpy of 1.5 J/g. Compounds M1 and M2 melted to nematic phase at −8.1 and −12.5 °C respectively with enthalpy of 0.2 and 1.93 J/g. M1 turned isotropic at 18.6 °C with energy of 0.009 J/g while M2 turned isotropy at 68.2 °C with energy of 0.44 J/g.

For M3, there was no crystallization phase and the clearing point from nematic to isotropy was 10.3 °C with energy of 0.15 J/g.

The range of nematic phase for the mesogen was 18.4 °C. Comparing the compounds, it is clear that the introduction of the methyl methacrylic unit increases the nematic phase range (Table 1). For compound M1, the nematic phase range was 26.7 and 81.3 °C for the compound M2. The range for the compound M3 could not be determined since there was no crystallization observed.

Also, worthy of note is the reduction in the clearing point of the compounds with the introduction of methyl methacrylic unit. This is expected for the compounds M1 and M3 with Si–O– and Si–O–Si linkages respectively [20, 21]. The compound M2, though without silicon-oxygen bond (linkage), still exhibited lower clearing point when compare to the mesogen, Me.

4 Conclusion

Three methyl methacrylic mesogenic monomers, with different linking units between the mesogenic unit and the polymerizable methyl methacrylate end, were successfully synthesised and characterised. The monomers were liquid crystalline with nematic mesophase below the room temperature. The introduction of the methyl methacrylic unit through siloxane, ester and disiloxane linker units causes reduction in the clearing points of the nematic phase of the mesogenic compound and also increment in the stability and temperature range of the nematic phase. The thermal behaviour of the monomers reveals that those with siloxane (Si–O– and Si–O–Si) linker exhibits very low clearing, disiloxane having the lower value.

Note: The experimental procedures are as contained in the additional information and are abstracted from reference [22].