Commercially available nitroxides : TEMPO and TEMPO–derivatives

TEMPO and TEMPO–derived substances were the most common nitroxides used to control the styrene polymerization before several newly designed nitroxides have been developed. Controlled ethene polymerizations were also reported in the presence of TEMPO and its derivatives ²⁰. In this work, ethene polymerizations with TEMPO in combination with DTBP, or with an alkoxyamine, e.g. hexyl–TEMPO, were investigated over a wide range of temperatures. Polymerization rates and molecular weights have been studied from 180 up to 220°C at 2000 bar and at different ratios [TEMPO]/[DTBP], from 1 up to 10.

0.0

Fig. 5.28. ln([M]₀/[M]) vs time plot for TEMPO–mediated ethene polymerizations at 2000 bar and at different temperatures in the presence of 50 ppm DTBP.

[TEMPO]/[DTBP] = 2.

The plot of the conversion index vs polymerization time for ethene polymerization ([TEMPO]/[DTBP] = 2) at 2000 bar and between 180 and 220°C is shown in Fig. 5.28. The DTBP efficiency is assumed to be lower than 1. Thus there is an excess of the persistent radical over the growing radicals from the beginning of the polymerization. The rate of ethene polymerization in the presence of TEMPO is very low and linear at 180°C (10 % conversion in 20 hours) and reaches 70 % conversion in 12 hours at 220°C. Under these conditions, the ln([M]0/[M]) vs time plot exhibits a slight downward curvature, indicating that termination occurs.

The influence of the ratio [TEMPO]/[DTBP] on the conversion index vs time plot is described in Fig. 5.29. Ethene polymerizations were performed at 200°C and 2000 bar for a ratio R = [TEMPO]/[DTBP] between 0 and 10. In the absence of TEMPO, R = 0, the polymerization starts very fast and reaches a limiting upper conversion around 25 % after few minutes. Thereafter a very small conversion due to spontaneous ethene polymerization may be detected. Increasing the TEMPO concentration in the system leads to a decrease of the concentration of free radicals produced by initiator decomposition and the extend of polymerization reached in the first stage is reduced. At R = 2, no inhibition is observed and

the ln([M]0/[M]) vs time correlation exhibits a slight downward curvature due to termination.

In the presence of an excess of TEMPO, an inhibition period is observed, the duration of which depends on the TEMPO concentration. As in DTBN–mediated ethene polymerization, the inhibition period increases with the size of R. The rate of polymerization is lower at higher ratio of [TEMPO]/[DTBP], probably slowed down by high persistent radical concentrations.

0.0 0.2 0.4 0.6

ln ([ ] /[ ]) M M

0 20 000 40 000 60 000 80 000 100 000

t / s

R = 0 R = 1 R = 2 R = 5 R = 10

Fig. 5.29. ln([M]0/[M]) vs time plot for ethene polymerizations at 200°C and 2000 bar for different ratios [TEMPO]/[DTBP]. [DTBP]/[styrene] = 50 ppm.

The molar mass and the polydispersity index as a function of monomer conversion for the TEMPO–mediated polymerization at 200°C and 2000 bar and at a ratio [TEMPO]/[DTBP] = 2 are plotted in Fig. 5.30. Polymerization rate for this experiment has been reported in Fig. 5.28. Very low and conversion–independent number average molecular weights, MN around 15 000 g⋅mol^–1, are observed. MN = 56 000 g⋅mol^–1 was expected for 20 % ethene conversion. The polydispersity index increases with monomer conversion from 4 up to 8 at 14 and 50 % ethene conversion, respectively, that is far off the theoretical Ip which should be below 1.5. Polyethylene samples, obtained at different ratios R and at several T and P conditions, all show the same characteristics, i.e. low conversion–independent MN and high polydispersity indices. All experimental data are reported in Appendix A2.

0

Fig. 5.30. Number average molecular weight and polydispersity index as a function of monomer conversion for ethene polymerizations at 200°C and 2000 bar in the presence of 50 ppm DTBP. [TEMPO]/[DTBP] = 2.

Further ethene polymerizations were carried out in the presence of DTBP and CXA, a TEMPO–derived nitroxide with two nitroxyl radical functions and with a better thermal stability than the one of TEMPO. Polymerization rates observed are similar to TEMPO–

mediated ethene polymerization under the same conditions. Conversion, molar masses and polydispersities are reported in Table 5.31 for the ethene polymerization in the presence of CXA ([CXA]/[DTBP] = 1.1) at 220°C and 2000 bar.

t / s conversion / % M_N / g⋅mol^–1 Ip

3600 3.3 28 000 5.3

7200 8.7 23 200 6.7

10800 14.3 24 800 7.2

Table 5.31. Number average molecular weight and polydispersity index as a function of monomer conversion for CXA–mediated ethene polymerizations at 220°C and 2000 bar in the presence of 50 ppm DTBP. [CXA]/[DTBP] = 1.1.

The presented results do not show any characteristics of controlled radical polymerization, probably because of a too low stability of the nitroxides used and of a too high stability of the dormant species.

Recent studies ²¹ showed that oxygen–centered radicals (such as the ones produced by DTBP decomposition) may induce decomposition of certain nitroxides such as TEMPO. To avoid such reactions, polymerization experiments have been performed using a TEMPO–

derived alkoxyamine (hexyl–TEMPO) which has a good thermal stability as was checked by ESR ³.

In the presence of hexyl–TEMPO, the reproducibility of the conversion vs time correlations of ethene polymerization is very good between 190 and 250°C. Characteristics of ethene polymerizations performed in the presence of 100 ppm hexyl–TEMPO at 2000 bar and between 190 and 250°C are reported in Table 5.32.

210°C 230°C 250°C

Tab. 5.32. Number average molecular weight and polydispersity index as a function of monomer conversion for ethene polymerizations in the presence of 100 ppm hexyl–TEMPO at 2000 bar and temperatures between 210°C and 250°C.

ln([M]₀/[M]) at different temperatures increases with time and exhibits a slight downward curvature at temperatures above 230°C. No inhibition period has been observed.

The spontaneous ethene polymerization was also monitored at 2000 bar and several temperatures for comparison. Polymerization rates observed are approximately twofold higher than the spontaneous ethene polymerization (Fig. 5.33). Nevertheless, molecular weights do not linearly increase with monomer conversion and are far from the theoretical molecular weights estimated from the initiator concentration. Polydispersities are very high, typically between 6 and 14 at high conversion.

Fig. 5.33. ln([M]0/[M]) vs time plot for spontaneous thermal ethene polymerizations at 2000 bar and three temperatures.

Fig. 5.33 shows that the conversion index vs time plot for spontaneous thermal ethene polymerization is quite linear because of the low and constant initiation rate for spontaneous ethene polymerization. Polymerization rates are much lower than the ones observed in nitroxide–mediated ethene polymerization, so that it is assumed that dormant species or the nitroxide will undergo a bond dissociation and will enhance polymerization rate.