3 Materials, Experimental Procedures and Data Evaluation
3.13 High-Temperature Polymerizations
Figure 3-8 The setup for high temperature experiments is shown schematically. It could be operated in two modes.
Figure 3-8 depicts the setup schematically.XIII The reaction mixture started from the reservoir container located on a balance (Omnilab, OL 3100-P), which was connected to a computer to calculate mass flow in 1 s intervals. The reaction mixture flowed via a PTFE tube (inner diameter 1 mm) to the degasser (Ercatech AG, ERC-3215α) and further to an HPLC pump (built by the mechanical workshop of the Institut für Physikalische Chemie, Georg-August-Universität Göttingen). The pump gave a pressure of 200 bar and from here on connection was by a stainless steel capillary (AD: 1/16´´) with an inner diameter of 0.5 mm. Then it entered a double line tube (1 m) to get a temperature of 50 °C., which was not enough to induce significant monomer conversion but reduced temperature difference to reaction temperature. In the next step the reaction mixture flowed into the tubular reactor located in a heating bath (Haake, N3) filled with silicone oil. The stainless steel, tubular reactor consisted of two 510.1156 (Nova Swiss, AD: 1/16´´, ID: 0.5 mm). It was spiral-shaped, had a length of 1050 cm, an inner diameter of 0.5 mm ± 0.05 mm, and thus a geometric volume of 2.06 mL ± 0.41 mL. Temperature was measured by CIA S250 Chromel/Alumel thermocouple and pressure by a P3MB (HBM). Afterwards the reaction mixture was cooled to 50 °C in a double line tube (1 m). Then it flowed into a high-pressure cell (described in more detail elsewhere[145,148]) heated electrically to 50 °C and located in the FTIR spectrometer IFS 88 (Bruker Optik). A controlling
XIII The high-temperature polymerizations were all carried out by Daniel Weiß.[148] under cosupervision of Dr. H.-P. Voegele.
valve followed to set the velocity of the flow. The last part of the setup was a collecting vessel.
The setup allowed for two different modes of operation described in what follows.
3.13.1 Stopped-Flow experiments in High-Pressure Cell
The tubular reactor was bypassed and polymerization took place in the high pressure cell. The reaction mixture flowed without prior warming through the cell until stationary condition was reached. Then the valves of the high-pressure cell were closed and the cell was used as a batch reactor. Polymerization was followed by NIR as described in subchapter 3.3. The upper temperature limit for measurements with this mode of operation was set by initiator decay. At temperatures above 140 °C, VA-086 decayed so fast that significant X had already been reached at the starting point.
With this setup, it was not possible to draw samples during polymerization.
The procedure is described in more detail here.[148] .
3.13.2 Polymerization in a Tubular Reactor
Polymerization was carried out inside the tubular reactor. Residence time, , was set by flow rate and was calculated from mass flow, density of the reaction mixture and geometric volume of the reactor. This theoretical value had to be corrected (v.i.).
The lower and upper temperature limits for measurements with this mode of operation were set by initiator decay. At temperatures above 170 °C, VA-086 decayed so fast that high final monomer conversion was reached even at the highest flow rate. Measuring a conversion vs. time profile is not possible under these conditions.
At temperatures below 130 °C, VA-086 decayed so slowly even for the lowest flow rate, which still allowed for turbulent flow, high conversion could not be reached.
Samples were taken at each flow rate. They were dried in vacuo at temperatures up to 90 °C and measured by NMR (subchapter 3.5.1 and 3.5.2).
Correction of residence time in the tubular reactor
0 10 20 30 40 50
0.00 0.03 0.06 0.09 0.12
intens ity / a .u.
time / s
Figure 3-9 Residence time distribution of the whole setup (red) and without the tubular reactor (blue) measured as time-dependent signal intensity (pulse-response) for residence time experiments applying marker (AA) as approximate delta function.
Residence time of the tubular reactor was measured by flowing water through the apparatus with and without the reactor being included. AA was injected as marking substance as an approximate delta function. Increase of the center peak of the interferogram (to increase time resolution) was used to measure the arrival of marking substance. The result is shown in Figure 3-9.
The curves from experiments with the tubular reactor have, within experimental accuracy, the same shape as the curves from experiments without the tubular reactor—there is only a time offset. Thus, no significant broadening by the high-pressure cell can be observed.
The curves were integrated to obtain , see Figure 3-10. The true residence time of the tubular reactor was calculated as the difference between residence time including the tubular reactor and residence time not including the reactor.
0 10 20 30 40 50 0.0
0.2 0.4 0.6 0.8 1.0
F (t)
time / s
Figure 3-10 Sum functions of the curves shown in Figure 3-9. This is used to compute the true residence time of the tubular reactor as the difference between residence time including the tubular reactor (red) and residence time not including the reactor (blue).
This was done for different flow rates and the residence time calculated from mass flow, density of the reaction mixture and geometric volume of the reactor. This theoretical value,
calc, was plotted against the true residence time, . This is shown in Figure 3-11. A linear relationship was found and an empirical correction function was determined, eq. (3.2), with which all experimental results given later have been corrected.The procedure is described in more detail elsewhere.[148]
/ s calc/ s 2.38 4.58
(3.2)3.0 4.5 6.0 7.5 9.0 0
4 8 12 16
/ s
calc/ s
Figure 3-11 Calculated and directly measured residence times are compared (green squares).
These values are fitted to a straight line (red, eq. (3.2)).
As it was only possible to measure for high flow rates, for lower flow rates, and thus higher conversion, residence time was corrected via linear extrapolation, eq. (3.2). The time has to be more precise for the beginning of the polymerization, as the rate of polymerization is highest here. The comparison of the conversion-time profiles of polymerization with the stopped-flow operation to conversion-time profiles from polymerization in the tubular reactor before and after correction suggest that the correction and even the extrapolation works well. An example is given in Figure 3-12. Small symbols give the result from batch polymerization (stopped-floe operation) used as reference here. Big open symbols belong to a polymerization of the same reaction mixture in the tubular reactor. The rate of polymerization seems to be smaller for the tubular reactor. Correction of residence time of the tubular reactor via eq. (3.2) yields the big, solid symbols. The corrected conversion-time profile of the polymerization in the tubular reactor agrees with the one of the batch reactor, thus the rates of polymerization are the same.
0 50 100 150 200 0.0
0.2 0.4 0.6 0.8 1.0
X
time / s
Figure 3-12 Conversion-time profiles of polymerization with the stopped flow operation (small symbols) is compared to conversion-time profiles from polymerization in the tubular reactor before correction (big open symbols) and after correction (big filled symbols) are compared. 0.1 g g1 AA was polymerized with 0.002 g g1 VA-086 in H2O at 140 °C.