Methyl methacrylate solution polymerization in ionic liquids

The following section has already been published to major extent in ref.^[29]

6.3.1. Motivation

Ionic Liquids (ILs) have attracted enormous attention as promising solvents within the last decade. ILs consist of a bulky cation and a complex anion and mostly are liquids even at ambient temperature. They exhibit interesting physical properties such as high thermal and chemical stability as well as a negligible vapor pressure which makes them attractive as solvents for environmentally friendly processes. An excellent overview on syntheses and characteristic properties of ILs as well as on their application in transition metal catalysis has been given by Wasserscheid and Keim.^[97]

Whereas ILs are frequently used solvents in organic syntheses, applications in free-radical polymerization are scarce. It is known that polymerization rate and polymer molecular weight in IL solution are enhanced as compared to polymerizations in conventional organic solvents or in bulk.^[98-102] The good solvent power of ILs indicates further advantages for copolymerization of monomers which largely differ in polarity.^[103-105]

PLP-SEC studies into free-radical polymerization of methyl methacrylate (MMA) and glycidyl methacrylate (GMA) showed that one reason for the higher rate in ILs is the enhancement of the propagation rate coefficient, kp.[106, 107]

This effect has been reported to be essentially due to polar interactions which lower the activation energy upon replacing the molecular environment of the transition state for propagation from bulk methacrylate to one which primarily consists of IL species.[105, 107, 108]

In addition to an increase in kp, Haddleton et al. reported a decrease of kt, by up to one order of magnitude, in passing from bulk polymerization of MMA to the reaction in highly viscous solution containing 60 vol.-% of the IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] PF6).^[106] Both rate coefficients, kp and kt, thus contribute to an enhancement of polymerization rates in the presence of ILs. To investigate the effect of ILs on kt more quantitatively, in particular with respect to chain-length dependence, MMA polymerization was studied in the two ILs: 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim] BF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim] NTf2). These ILs differ in viscosity, which is extrapolated from ref.^[109] to be 55.9 cP and 171 cP at 10 °C for [emim] NTf2 and [bmim] BF4, respectively.

TERMINATION IN POLYMERIZATION OF METHACRYLATE MONOMERS 57 To increase the S/N of the EPR measurements, per-deuterated MMA-d8 was investigated instead of MMA and up to 200 single scans were co-added during EPR. SP-PLP-EPR was carried out on 15 vol.% MMA-d8 in ILs at 10 °C at different degrees of monomer – to-polymer conversion. The traces were calibrated as described in section 5.3.1. The cR(t) data referring to chain-lengths 1 ≤ i ≤ 100 were analyzed for parameter values of 〈k_t〉, k_t(1,1) and αs as described in chapters 5.3.2 and 5.3.3. Propagation rate coefficients implemented within the fitting procedure were k_p/(L·mol⁻¹·s⁻¹) = 870 and 571 for 15 vol.-% of MMA in [bmim] BF4 and in [emim] NTf2, respectively as extrapolated from a literature PLP-SEC study^[107] to 10 °C or were measured via PLP-SEC at 40 °C and extrapolated to 10 °C.^[29] It was carefully checked that the reaction mixture remains homogeneous towards increasing monomer-to-polymer conversion during SP-PLP-EPR. In case of polymerizations carried out in [bmim] BF₄, the reaction mixture after the experiment was found to be inhomogeneous sometimes. Traces measured from these samples were rejected from fitting procedures.

The effect of an IL solvent for MMA-d₈ polymerization is illustrated in Figure 12. The spectra during 20 Hz PLP of MMA-d₈ at 5 °C in bulk and 15 vol.% [emim] NTf₂ solution at 10 °C were measured under otherwise identical conditions. It becomes clear from Figure 12 that stationary radical concentration is increased from 6·10⁻⁷ mol· L⁻¹ and 4.5·10⁻⁶ mol· L⁻¹ by passing from bulk (gray line) to polymerization in ionic liquid solution (black line).

Figure 12. EPR spectra recorded during a 20 Hz pulsed laser induced pseudo-stationary polymerization of MMA-d8 in bulk at 5 °C (gray line) and in [emim] NTf2 solution (15 vol.% MMA-d8) at 10 °C (black line) under otherwise identical conditions.

58 TERMINATION IN POLYMERIZATION OF METHACRYLATE MONOMERS

6.3.2. Composite model parameters for MMA in ionic liquid solution

Chain-length averaged termination rate coefficients, referring to i > 100 are given in Table 3 together with 〈kt〉 from an SP-PLP-NIR study on MMA bulk polymerization.^[56]

Table 3. Chain-length averaged termination rate coefficients for MMA polymerization in ILs at 10 °C referring to the regime of short chains (i ≤100) and 〈k_t〉 from SP-PLP-NIR of MMA in bulk which was estimated from data associated with i <1000 is also listed.^[56]

Solvent

〈k_t〉_1;100 / L·mol⁻¹·s⁻¹ for ILs

〈k_t〉_1;1000 / L·mol⁻¹·s⁻¹ for bulk

Reference

[bmim] BF₄ (2.4 ± 0.1)·10^{6 [29]}

[emim] NTf₂ (7.2 ± 0.3)·10^{6 [29]}

Bulk 2·10^{7 [56]}

The rate coefficients 〈kt〉1;100 for MMA-d8 in both ILs are well below the associated 〈kt〉 value measured for MMA bulk polymerization via the SP–PLP–NIR technique.^[56] The direction of change with 〈kt〉1;100 agrees with the one of the inverse of the bulk viscosities at 10 °C:

0.67 cP for MMA,^[96] 55.9 cP for dry [emim] NTf2 and 171 cP for dry [bmim] BF4 (with the numbers for the two ILs extrapolated from ref.^[109]). As the viscosity of MMA–IL solutions is strongly affected by moisture^[109] and by adding small amounts of organic solvents (here MMA)ⁱ and as the variation of bulk viscosity with the degree of monomer conversion is not known, no attempt is made to quantitatively correlate 〈k_t〉_1;100 with inverse bulk viscosity.

Another reason for not focusing too much on 〈k_t〉_1;100 is that this quantity is not well defined (i.e. refers to eq. (16) ) and depends on the chain-length range and on the radical concentration profile of the underlying experiment.

Composite model parameters αs and kt(1,1) for MMA-d8 polymerization in 15 vol.%

solution with ILs [bmim] BF₄ and [emim] NTf₂ are depicted in Figure 13.

TERMINATION IN POLYMERIZATION OF METHACRYLATE MONOMERS 59

Figure 13. Composite model parameters αs and k_t(1,1) for MMA-d₈ polymerization in 15 vol.% solution with ILs at 10 °C and different degrees of monomer-to-polymer conversion, X.

Within the limits of uncertainty, the power-law exponent is insensitive toward the final monomer conversion reached in a particular experiment. It should, however, be noted that X is below 20 % in each individual experiment. The exponent appears to be slightly higher, αs

≈ 0.72 ± 0.05, for MMA polymerization (15 vol-%) in [emim] NTf₂ than in [bmim] BF₄, where αs ≈ 0.61 ± 0.10 is obtained as the mean value. Averaging the entire set of αs values measured the two ILs, yields αs ≈ 0.66 ± 0.15.

The fitting procedure via eq. (26) has been refined by adopting the associated power-law exponent, αs, represented by the dashed lines in Figure 13 (l.h.s), i.e. mean values 0.61 and 0.72, as fixed parameters. The so-obtained kt(1,1) values deduced for solution polymerization in the two ILs at 10 °C are plotted in Figure 13 (r.h.s.). In spite of the significant scatter of the individual kt1,1

, the numbers for [bmim] BF4 appear to be slightly lower, as is also indicated by the arithmetic mean values: kt1,1

= (2.0 ± 0.5)·10⁷ L·mol⁻¹·s⁻¹ and k_t^1,1 = (1.5 ± 0.5)·10⁷ L·mol⁻¹·s⁻¹ for MMA solution polymerization in [emim] NTf₂ and [bmim] BF4, respectively.

The composite model parameters are discussed in detail in section 7.

60 TERMINATION IN POLYMERIZATION OF METHACRYLATE MONOMERS

6.4. Tridecafluorooctyl methacrylate bulk polymerization