Kinetics in polymerizations mediated by trithiocarbonates

The following chapter to a major part has already been published in refs.^{[27, 85]}

The stationary approach for determination of Keq in RAFT polymerization of butyl acrylate (SPRs at −40 °C) has also been exploited for benzyl propyl trithiocarbonate (BPT) as the RAFT agent (see chater 4.1). The UV stability of BPT has been checked under conditions identical to the ones used in the present study. It turned out that BPT degradation by continuous UV irradiation occurs to less than 2 % within the time interval required for recording an EPR spectrum from which the ratio of cINT• and _•

c is obtained. After addition P

of a radical species to BPT and formation of an initial INT0• species in this so-called pre-equilibrium period, fragmentation of INT0• yields a benzyl radical plus a RAFT species.

Even if fast fragmentation of INT0•[162] does not allow for a significant build-up of INT0•

concentration and thus points toward a negligible impact of INT₀^• on our experimental approach, RAFT agents, which bear leaving groups that have identical radical functionalites to the one of propagating radical species, are preferable for kinetic investigations. Apart from potential chain-length effects of addition and fragmentation steps, main-equilibrium conditions (see Scheme 6) are introduced by these RAFT agents from the very beginning of polymerization on. Therefore stationary experiments were additionally carried out using the RAFT agent EAPT which carries a secondary ethyl propionate leaving group (mimicking an SPR from ethyl acrylate) instead of a benzyl group in BPT. Since superposition of pre- and main-equilibrium conditions introduces significant complexity to the kinetic scheme required for fitting time resolved cINT• and _•

c after SP, SP-PLP-EPR on trithiocarbonates P

has only been carried out for EAPT.

The Keq values determined during BPT-mediated polymerization of BA at −40 °C by fitting the section of the EPR spectrum measured under stationary conditions (illustrated on the r.h.s. of Figure 39) which is depicted in on the l.h.s. of Figure 39. The K_eq = 1.0·10⁴ L·mol⁻¹

126 TERMINATION,ADDITION AND FRAGMENTATION IN RAFT-POLYMERIZATION

is obtained from the slope of the straight line fit to the cINT• / c data plotted vs. cP• RAFT (see eq. (28)).

TERMINATION,ADDITION AND FRAGMENTATION IN RAFT-POLYMERIZATION 127

Figure 39. L.h.s.: Dependence of

INT P

c • c • on the RAFT species concentration, cRAFT. The slope to the straight line fit yields the equilibrium constant, K_eq. R.h.s.: Fitting of a section of the full EPR spectrum which is representative for measuring the ratio of INT^• and P^• concentrations (right and left EPR line, respectively).

To check for the impact of the benzyl leaving group on Keq, further stationary experiments were carried out using the RAFT agent EAPT which carries a secondary ethyl propionate leaving group instead of benzyl. Keq is found to be 2.8·10⁴ L·mol⁻¹ (see Figure 40, data is compared to the Keq found in BPT), and thus is rather similar to the Keq found for BPT under otherwise identical conditions. The minor difference by a factor of 3 may be assigned to the different leaving groups in EAPT and BPT. The difference in Keq should arise from a slightly increased (apparent) k_β present during BPT mediated polymerization because (i) the energy of the INT^•s should not significantly be different for a benzyl or ethyl propionate leaving group, as stabilization of the radical functionality occurs via electron-donation from the α-sulfur atoms.^[163] (ii) Re-addition of a benzyl group to a RAFT species is unlikely due to the excess of monomer over RAFT species (c_RAFT / c_M ≤ 1.5·10⁻⁴). It appears likely that fragmentation of a benzyl group is slightly preferred over fragmentation of an ethyl propionate group due to stabilization of the radical functionality by spin-delocalization into the aromatic ring.

128 TERMINATION,ADDITION AND FRAGMENTATION IN RAFT-POLYMERIZATION

Figure 40. L.h.s.: comparison of K_eq found for BA polymerization at −40 °C mediated via trithiocarbonates. The slope of the straight lines represent K_eq = 2.8·10⁴ L· mol⁻¹ obtained for EAPT (black line and squares) and of 1.0·10⁴ L·mol⁻¹ obtained for BPT (gray line, underlying data is from Figure 39). R.h.s.: K_eq found for BA at −40 °C and pre-polymerized EAPT. The symbols refer to different (average) chain-lengths of the p(BA) leaving group:

unity (EAPT data from l.h.s., squares) 17 (triangles) and 37 (upside-down triangles).

The r.h.s. of Figure 40 indicates that main-equilibrium conditions apply in the early stage of EAPT-mediated RAFT polymerization. Identical K_eq values to the ones found for EAPT are obtained (square symbols in l.h.s. and r.h.s.) when pre-polymerized EAPT is used as the RAFT agent (normal and upside-down triangles on the r.h.s. of Figure 40). The results depicted in Figure 40 may however not exclude a chain-length dependence of Keq, because the degree of polymerization is not well controlled under the experimental conditions of low concentration of the RAFT agent.

Since main-equilibrium conditions are present in the initial stage of EAPT mediated polymerization, instationary SP-PLP-EPR experiments were carried out for this system. The concentrations of INT^• and P^• were followed after applying an SP and were fitted by the kinetic model given in Scheme 7 via PREDICI simulation as detailed in chapter 5.3.5.

Different assumptions for the rate coefficient for termination between an INT^• and a P^•, k_t^cross were implemented into the kinetic scheme. The best fits between model and experimental data are depicted in Figure 41 for BA at −40 °C and a concentration of EAPT = 5·10^−5M.

TERMINATION,ADDITION AND FRAGMENTATION IN RAFT-POLYMERIZATION 129

Figure 41. Fitting of experimental INT^• and P^• concentrations after applying an SP (thin black lines on the l.h.s. and r.h.s., respectively) by a kinetic model which implements different assumptions for the cross-termination rate coefficients (black lines: ktcross

(1,1) = 0.4·kts,s

(1,1) and gray lines: ktcross

(1,1) = 1.0· kts,s

(1,1)).

The fit quality of the experimental data shows no significant dependence on the rate of cross-termination within the examined range. Estimates for the rate coefficients kad and k_β obtained from the best fits of the kinetic scheme via PREDICI simulation and different assumption for rate coefficient k_t^cross(1,1) agree within the experimental error margin. The results from the fitting are given in Table 17.

Table 17. Estimates for the rate coefficients kad and k_β of trithiocarbonate-mediated RAFT polymerization of BA at −40 °C as obtained from fitting SP-PLP-EPR data to the kinetic scheme (Scheme 7) via Predici simulation. The equilibrium constant (Keq = kad / k_β) is identical to Keq obtained via the stationary approach (see text).

C_EAPT / mol·L⁻¹ c_R0 / mol·L⁻¹

k_ad /

L·mol⁻¹·s⁻¹ k_β / s⁻¹ K_eq / L·mol⁻¹ 5·10⁻⁵ 2.0·10⁻⁵ 3.6·10⁶ 1.28·10² 2.82·10⁴

4·10⁻⁶-2·10⁻⁵ Stationary 2.78·10⁴

In more detailed investigations into BA polymerization mediated by EAPT, Keq was also determined for higher temperatures, which provides a first estimate for a temperature dependence in Keq.^[85] These studies should however taken with care, since MCRs occur during acrylate polymerizations to a significant extent. Thus Keq determined from the

130 TERMINATION,ADDITION AND FRAGMENTATION IN RAFT-POLYMERIZATION

stationary approach refers to an apparent Keq that includes the sum of concentrations for MCRs and SPRs into c_P• and implements the total concentration of INT^•s (either bearing an SPR or an MCR leaving group) into c_INT• for estimation of Keq via eq. (28). The Keq values determined from this approach is not independent of experimental conditions, it refers to a certain variable fraction of MCRs. RAFT polymerizations of BA at higher temperatures should preferentially be studied via SP-PLP-EPR and the concentrations of MCRs, SPRs and the (total) INT^•s should be independently monitored after an SP and fitted by a suitable kinetic model (a combination of Scheme 3 and Scheme 7). The reactivity of MCRs towards addition, fragmentation and termination with a RAFT species may be investigated via model MCRs produced by macromonomer addition to an initiator fragment (see chapter 9.2). For studies into the temperature dependence of the RAFT equilibrium constant, K_eq, and of the individual rate coefficients kad and k_β, monomers should be preferred which polymerize at higher temperatures without significant transfer reactions, such as methacrylates and probably vinyl esters.

10.4. Kinetics of polymerizations mediated by dithiobenzoates