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Handbook of Size Exclusion Chromatography and Related ...

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2.1.1 Free Radical Polymerization in aBatch Reactor<br />

Idealfreeradicalpolymerizationinvolvesinitiation,propagation,<strong>and</strong>termination.<br />

The production <strong>of</strong> the free radicals with rate constant kd is expressed by<br />

I2 ! kd<br />

whereI2 istheinitiator<strong>and</strong>I theprimaryradicalformedbyinitiatordecomposition.<br />

The second initiation step is the production <strong>of</strong> the first monomer radical R1<br />

by combination <strong>of</strong> I with monomer m, with rate constant ki<br />

2I<br />

I þm ! ki<br />

Propagation ensues with rate constant kp (assumed equal for all chain lengths).<br />

R1 þm ! kp<br />

Termination can occur by disproportionation<br />

<strong>and</strong>/or by recombination<br />

Rm þRn ! kt<br />

Rm þRn ! kt<br />

R1<br />

R2<br />

Pm þPn<br />

Pmþn<br />

An <strong>of</strong>ten used approximation is the so-called quasi-steady-state approximation<br />

(32), in which the concentration <strong>of</strong> radical [R], varies slowly with respect to the<br />

time scale for propagation <strong>and</strong> termination reactions. This yields<br />

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

2Fkd[I2]<br />

[R] ¼<br />

(9)<br />

If [I2] decreases negligibly during conversion, the monomer disappears in afirstorder<br />

process<br />

[m] ¼[m] 0e kt<br />

(10)<br />

where the rate constant kis given by k¼kp[R].<br />

Figure2showstypicalrawdataforfreeradicalpolymerization<strong>of</strong>acrylamide<br />

(AAm) initiated by sodium persulfate at t ¼ 608C (33). Pure water is pumped through<br />

the detector train during the first 500s, in order to obtain baselines for each instrument.<br />

After this the reactor withdrawal pump begins to pull the initial aqueous monomer<br />

solution (0.034g/mL <strong>of</strong> AAm) from the reactor to achieve 4% <strong>of</strong> the flow to the<br />

detectors. The other 96% <strong>of</strong> the flow comes from the pure water reservoir, so that the<br />

total amount <strong>of</strong> monomer initially in the detector train was 0.00136g/mL. The flow<br />

rate was 2mL/min, so that 4.8mL per hour <strong>of</strong> reactor fluid was withdrawn during the<br />

reaction. The increase in the UV absorbance monitored at 225nm during the<br />

monomer pumping period is due to the double bonds in the AAm, which are lost,<br />

© 2004 by Marcel Dekker, Inc.<br />

kt

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