Pulsed-Laser Experiments

The experiments for the testing of the MWD method were performed in the laboratory of Polymer Chemistry at the Eindhoven University of Technology. Also this experimental set-up has been described in detail elsewhere [4]. The pulse frequency was controlled by an external pulse generator [5] and varied between 0.01 to 100 Hz. Experiments were performed at a temperature of 25 °C, monitored by a thermocouple (Pt-100, Netsushin, MG-1505). Generally, per sample approximately 20 pulses were applied at 351 nm (XeF line), though pulse sequences with as low as 20 pulses and as high as 130 pulses were also applied. 

Directly after irradiation, the polymerization cell was opened and the contents were mixed with a tiny amount of inhibitor ( 500 ppm, hydroquinone, Merck, fotopur) to prevent further polymerization. Samples were dried in the dark at reduced pressure ( 10 Pa) for at least 72 hours. The conversion was determined gravimetrically and was typically 1 to 2 %. 

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The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

The shortest time delay in the pulsed laser experiments is limited either by the pulse width (in the picosecond time range) or by the shortest time the probe pulse can be delayed from the photolytic pulse (in the picosecond time range). The convenient long time delay could be accomplished by sending the probe pulse around the laboratory a few times (producing a delay time in the nanosecond range). 

In many cases, it is possible to prove that energy transfer is the predominant mechanism of quenching if the excited state of the acceptor fluoresces or phosphoresces at a characteristic wavelength, In a pulsed laser experiment, the rise in fluorescence intensity from Q with a characteristic time which is the same as that for the decay of the fluorescence of S is often taken as indication of energy transfer from S to Q. 

My thanks go to Dr. Hari Virdee for help with the literature survey which preceded this review, to the SERC Rutherford Laboratory for help with pulsed laser experiments, and to the SERC for financial support. 

The propagation rate coefficient (kp) of methyl methacrylate is measnred in miniemulsions. The molecular weight distributions show many higher order peaks, which seems to be specific for pulsed initiation polymerisation in compartmentalised liquids. The kp values are very similar to bulk values although a frequency dependence, overlooked in earlier research, is also present in pulsed laser experiments in bulk. 18 refs. 

Upon irradiation by laser light, the initiator molecule changes its stereochemical configuration from the trans to the cis isomer. This cis/trans isomerism becomes of importance if azoinitiators are employed in pulsed laser experiments The time at which the laser pulse hits the reaction mixture is no longer identical with the generation of the primary radicals. The time delay is usually in the order of microseconds and may be observed in time-resolved pulsed laser experiments. 

Pig. 11. Simulated and normalized number distribution for a pulsed laser experiment, with termination exclusively via disproportionation (a) without and (b) with Poisson broadening. 

The polydispersity of the chain length distribution of a poljmier produced via a pulsed laser experiment for disproportionation reads... 

Pulsed Laser Polymerization-Size Exclusion Chromatography. The careful determination of the chain length distribution of the polymer produced via a pseudostationary pulsed laser experiment allows to obtain accurate values for the propagation rate coefficient p. The polymerizable system—containing monomer and photoinitiator, occasionally solvent and transfer agents— is irradiated by a pulsed laser beam and the chain length distribution formed is subsequently analyzed by SEC. The determination of the additional peaks and its points of inflection on the low molecnlar side, respectively, gives a value for Lo,n which can easily be inserted into eqnation 123. 

In the sections below it will be discussed how the kinetic information regarding the termination reactions can be extracted from the MWD. After a brief discussion of some of the work reported in literature, it will be shown that also here the single-pulse laser experiments have some very attractive features. Similarly to the case for studying the time-resolved data from these kind of experiments, also the MWD can reveal model-independent information about the chain-length dependence of kt. 

Buback M, Kowollik C. Termination kinetics of methyl methacrylate free-radical polymerization studied by time-resolved pulsed laser experiments. Macromolecules 1998 31 3211-3215. 

The quantitative analysis of an experiment of this type involves solving a set of rate equations in the dressed frame — extra time dependence may be present in pulsed laser experiments (the commonest method for producing intense fields) and can also be taken into account. 

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