Cyclohexyl Methacrylate: Aqueous Polymerization Complexes and Dynamic Fragility

Cyclohexyl Methacrylate (CHMA) is a monofunctional monomer with a characteristic high reactivity of methacrylates and a cyclic hydrophobic moiety. Copolymers of which can be prepared with (meth)acrylic acid and its salts, amides and esters, and with (meth)acrylates, acrylonitrile, maleic acid esters, vinyl acetate, vinyl chloride, vinylidene chloride, styrene, butadiene, unsaturated polyesters and drying oils, etc. Cyclohexyl Methacrylateis also a very useful feedstock for chemical syntheses, because it readily undergoes addition reactions with a wide variety of organic and inorganic compounds. In order to prevent polymerization, Cyclohexyl Methacrylate must always be stored under air, and never under inert gases. The presence of oxygen is required for the stabilizer to function effectively. It has to contain a stabilizer and the storage temperature must not exceed 35 °C. Under these conditions, a storage stability of one year can be expected upon delivery. In order to minimize the likelihood of overstorage, the storage procedure should strictly follow the “first-in-first-out” principle. For extended storage periods over 4 weeks it is advisable to replenish the dissolved oxygen content.

Carbohydrate/monomer complexes in aqueous polymerizations

Hydrophobic methacrylic monomers were polymerized in aqueous media using methylated (1.8)-beta-cyclodextrin (MeCD) additives. Hydrophobic monomers tert-butyl methacrylate (tBuMA), cyclohexyl methacrylate (CMA), and 2-ethylhexyl methacrylate (2EHMA) were each dissolved in chloroform with MeCD. Chloroform was then evaporated to yield solid monomer/cyclodextrin complexes. Complexes were shown by 1H NMR and thermogravimetric analysis (TGA) to have molar ratios of monomer to MeCD as high as 0.72/1.00. The water-soluble complexes were readily polymerized in aqueous media using free radical initiation. During polymerization, hydrophobic methacrylic polymers precipitated and the majority of MeCD remained in solution. Poly(alkyl methacrylates) synthesized via this method exhibited number-average molecular weights ranging from 50,000 to 150,000 with polydispersities from 3.2 to 5.5 depending on monomer structure, and isolated yields were as high as 86%. Additionally, corresponding methacrylic/carbohydrate films were prepared and examined. High molecular weight poly(tBuMA), poly(Cyclohexyl methacrylate), and poly(2EHMA) were blended with MeCD to produce optically clear films with as high as 20 wt % MeCD. Differential scanning calorimetry (DSC) characterization indicated that the glass transition temperatures of these novel carbohydrate blends were controllable over a 20 degrees C range depending on the relative concentration of each component.[1]

Dynamic fragility in poly(cyclohexyl methacrylate)

The glass transition is an important characteristic involving a transformation from the supercooled liquid into a glass upon cooling for amorphous materials.1 Bulk properties and behaviors, for example, mechanical, volume, and thermodynamic features of polymer materials, experience significant changes approaching the glass transition temperature (Tg), which results in huge effects on the applications of glassy materials. Poly(cyclohexyl methacrylate) (PCHMA) is a unique acrylic polymer with a bulky cyclohexyl ring at the side group; it possesses low fragility and weak mass dependence, as already revealed by dielectric analysis. Moreover, Poly(cyclohexyl methacrylate) owns typical hierarchical relaxation processes, in addition to the conventional α-relaxation; a hindered β-relaxation with an activation energy of 73 ± 5 kJ/mol appears as a weak peak in the dielectric loss spectra. It is worth noting that a well-defined γ-relaxation with an activation energy of 47 ± 0.8 kJ/mol can also be found at low temperatures, which originates from the chair-to-chair inversion of the cyclohexyl ring. In this work, PCHMA was chosen as a model polymer, and various phenolic small-molecules with different loadings and inter-HB strengths (Δυi) were mixed with Poly(cyclohexyl methacrylate) to mediate the hierarchical dielectric processes. Enthalpy relaxation is also applied to further clarify the effects of inter-HB strength and conformational transition on the fragility of the ring-containing polymer via analysis of the heat capacity jump (ΔCp) at Tg and the related enthalpy hysteresis (ΔHR).[2]

In this article, hierarchical relaxation processes of the cyclohexyl ring containing PCHMA and its blends with hindered phenols were systematically investigated by dielectric and enthalpy relaxation. Notably, neat Poly(cyclohexyl methacrylate) belongs to the strong liquid, while the introduction of hindered phenol leads to an increased m and Tg simultaneously regardless of the concentration or inter-HB strength. Moreover, the activation energy Eγ of γ-relaxation associated with the chair-to-chair conversion of the cyclohexyl ring increases significantly. These results are opposite to the situation in PnBMA/AO300 mixtures without cyclohexyl ring structures. The negative deviation of ΔHR from the linear additivity rule with AO300 loadings indicates the reduction in configurational states, which originates from the hindered chair-to-chair conversion. These results demonstrate that the chair-to-chair conversion has a good correlation to the fragility in Poly(cyclohexyl methacrylate).

References

[1]Madison, P H, and T E Long. “Carbohydrate/monomer complexes in aqueous polymerizations: methylated-beta-cyclodextrin mediated aqueous polymerization of hydrophobic methacrylic monomers.” Biomacromolecules vol. 1,4 (2000): 615-21. doi:10.1021/bm0055706

[2]Shi, Gaopeng et al. “Experimental evidences for the correlation between chair-to-chair conversion and dynamic fragility in poly(cyclohexyl methacrylate)/hindered phenol blends.” The Journal of chemical physics vol. 162,21 (2025): 214501. doi:10.1063/5.0268112