The results are reported for a series of measurements of the propagation rate coefficient (k p) of butyl acrylate obtained from pulsed-laser polymerization (PLP). Previous attempts reported in the literature to use PLP for this monomer have failed because the data did not satisfy the internal consistency tests afforded by PLP. The problem was obviated by carrying out measurements at very low temperatures and with very short times between laser pulses. Data for k p were obtained over the range −65 to −7 °C which satisfy PLP consistency tests (invariance of the apparent k p value to laser pulse frequency, etc.). The results fit k p (dm3 mol-1 s-1) = 107.2 exp(−17.3 kJ mol-1)/RT); the confidence ellipse for these parameters is provided. These data extrapolate to a value of k p = 2.7 × 104 dm3 mol-1 s-1 at 50 °C. The higher value of the frequency factor of butyl acrylate compared to that of butyl methacrylate can be rationalized in terms of hindered rotations in the transition states.

Transfer constants to chain-transfer agent (CTA) and to monomer can be obtained by consideration of the complete molecular weight distribution, using the slope of a plot of the natural logarithm of number-molecular weight distribution against molecular weight, in the limit of high molecular weight and low radical flux (P. A. Clay, R. G. Gilbert, MacromolecuIes28,552 (1995)). This method is applied to the bulk polymerisations of methyl methacrylate (MMA) with added triethylamine (TEA) and with added tert-dodecylmercaptan (mixture of 2,4,4,6,6-pentamethyIheptane-2-thiol and 2,2,4,6,6-pentamethylheptane-4-thiol, TDM). The transfer rate coefficients are found to be 74 (+20) dm3 * mol-' . s -l at 60°C for TEA, and 56 (+13) dm3.mol-' . s -l at 25°C for TDM.

The IUPAC Working Party on modeling free-radical kinetics' noted that pulsed-laser polymerization (PLP) (e.g., ref 2) provides ameans of measuring the propagation rate coefficient (k,) that is virtually assumption-free. PLP involves pulsed-laser illumination of monomer and photoinitiator, under conditions such that a significant amount of macroradicals is terminated by a very short radical species formed in the pulse immediately subsequent to that in which the longer macroradical was formed. If one can identify the degree of polymerization up of polymer terminated as above, then k, is given by up/ [MI tp, tf being the time between pulses and [MI the monomer concentration. This paper shows that matrix-assisted laser desorptionlionization (MALDI) mass spectrometry can be used to measured up and provides an alternative to the only present means of carrying out this measurement, gel permeation chromatography (GPC).Modeling polymerization under PLP conditions-shows that (i) up can be identified as the point of inflection occurring at the lowest molecular weight in the number molecular weight distribution (MWD) of the formed polymer (this point of inflection is within a few percent of vp for all models for polymerization kinetics tested thus far) and (ii) up also corresponds closely to the lowmolecular-weight point of inflection on the weight MWD, and hence on a GPC trace.Despite its advantages, PLP suffers from some major limitations,interaZia from the need to use GPC to identify v , . These include (i) the need for molecular-weight calibration, through either a reliable monodisperse standard or "universal" calibration and (ii) the necessity of accumulating sufficient polymer. Optimization with regard to these constraints is often opposed by having to ensure that (i) laser illumination of the sample is uniform (including negligible attenuation of the laser beam over the length of the sample cell); (ii) [MI changes negligibly over the course of the experiment5i6 (both of which restrict the total amount of polymer that can be formed); (iii) a large proportion of polymer is formed by termination involving one chain that started in a given pulse and a second chain of a very low degree of polymerization that formed in the next pulse (as distinct from chain stoppage by termination between two chains that both have a large degree of polymerization, etc.); and (iv) the GPC is calibrated to provide accurate MWDs.MALDI offers the potential for obtaining the number MWD directly and is applicable to polymers of moderate molecular weight (up to lo5 under favorable condition^).'^ derivative c 10 10 molecular weight weight distribution calculated from MALO1 (b) MALDI I I : , , : ' I t MALDI signal = number distribution) derivative I 10 I O molecular weight Figure 1. (a) GPC trace (full line), which is the weight MWD, number MWD (broken line) and derivative of weight MWD (points), for product from PLP of MMA at -8 "C, as described in the text. (b) Corresponding quantities for the same sample from MALDI, deduced from the data of Figur...

Spontaneous initiation in emulsion polymerization may be more important than in the corresponding bulk free-radical polymerization. A methodology is derived for finding the locus of spontaneous polymerization in emulsion polymerization, through use of spin traps and catalysts which can accelerate decomposition of peroxides. Applying this to both styrene and chloroprene (chlorobutadiene), it is found that this generation occurs to some degree within all phases present. The rate of spontaneous initiation is relatively small in styrene emulsion polymerization but large in chloroprene. A means of including this effect in modeling rates and molecular weight distributions is derived, which also shows how rate parameters for the process may be obtained from experimental molecular weight distributions. This methodology is applied to these two monomers, with a series of seeded emulsion polymerizations using polystyrene host seeds for both. For styrene polymerization, the spontaneous initiation rate is low and varies with latex preparation; consistent values for this rate coefficient for a given latex are obtained by independent measurements involving two different techniques, thereby verifying the methodology. Applying this methodology to chloroprene, it is found that the effect of spontaneous initiation is much larger and probably arises from peroxides formed by exposure to oxygen. For chloroprene, spontaneous radical generation occurs both within the particles and in any monomer droplets present, with different chain-stopping mechanisms occurring in these two phases. It is a major influence on rates and molecular weight distributions, even in the presence of large amounts of added initiator; chain stoppage in droplets is largely by transfer to monomer, whereas chain stoppage within particles is by termination with short radicals formed by spontaneous initiation. Arrhenius parameters for the rate coefficient for transfer to monomer are obtained from the molecular weight distributions for the chloroprene system:  k tr/dm3 mol-1 s-1 = 104.3 exp(−30.9 kJ mol-1/RT).