Molecular weight distribution bulk polymerization

The predictive capabilities of the new kinetic model were demonstrated by a direct comparison of model predictions with experimental measurements on monomer conversion, number and weight average molecular weights and molecular weight distribution. The polymerization was carried at different temperatures in a batch, bulk polymerization system. In the temperature range of 100 - 150 °C, a chemical initiator (e.g., Dicumyl Peroxide, DCP) was employed in combination with the thermal initiation of styrene. On the other hand, at higher temperatures (150 - 180 °C), the polymerization was initiated exclusively by the thermal initiation mechanism. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

Recently, gel permeation chromatography (GPC) and other dilute solution techniques have been directly applied to the characterization of II (17-20). In our laboratory we have examined II prepared by the uncatalyzed bulk and solution polymerization processes. Polymers obtained from the former process have high molecular weights (MWs) and broad molecular weight distributions (MWDs, Mw/Mn=5). The dilute solution... 

A combination of anionic and ATRP was employed for the synthesis of (PEO-b-PS) , n = 3, 4 star-block copolymers [148]. 2-Hydroxymethyl-l,3-propanediol was used as the initiator for the synthesis of the 3-arm PEO star. The hydroxyl functions were activated by diphenylmethyl potassium, DPMK in DMSO as the solvent. Only 20% of the stoichiometric quantity of DPMK was used to prevent a very fast polymerization of EO. Employing pentaerythritol as the multifunctional initiator a 4-arm PEO star was obtained. Well-defined products were provided in both cases. The hydroxyl end groups of the star polymers were activated with D PM K and reacted with an excess of 2-bromopropionylbro-mide at room temperature. Using these 2-bromopropionate-ended PEO stars in the presence of CuBr/bpy the ATRP of styrene was conducted in bulk at 100 °C, leading to the synthesis of the star-block copolymers with relatively narrow molecular weight distributions (Scheme 72). 

The bulk polymerization of styrene to give a narrow molecular weight distribution has appeared in a U.S. patent [45]. The polydispersity reported was... 

In a bulk polymerization study initiated with BF3-ethylene oxide catalyst at 0° C, Ofstead (34) obtained quite narrow molecular weight distributions over the range from five to forty percent conversion. The comparison of his data for a PTHF of DP 96 with the theoretical curve for a Poisson distribution is shown in Fig. 23. 

The polymerizations are generally carried out in bulk or in solution (THF, di-oxane, toluene, etc.). The dispersion polymerization of e-CL using a mixture of 1,4-dioxane and heptane and surface-active agents yields a polymer in the form of microspheres with a narrow molecular weight distribution [63]. 

Initial results on the two zone wall temperature optimization of bulk polymerizers in tubular reactors shows that the molecular weight distribution and product quality can be controlled for conversion levels aroung 20%. Further investigations into the use of optimal wall temperatures in tubular polymerizers are underway. 

Fig. 15. Molecular weight distribution of polymer in concentrated emulsion polymerization and in bulk polymerization (SDS 0.3 g, water 3 ml, styrene 40 ml, 40 °C, 30 h)...

Figure 18, which plots conversion vs monomer volume fraction, exhibits a maximum at about during polymerization, some cells of the gel coalesce and form a bulk phase in which the conversion is smaller. Visual observations indeed indicated a separated thin layer at the upper part of the tube after polymerization. Since no appreciable separated liquid phase was observed before polymerization, it is likely that during polymerization some cells did coalesce. Molecular weight distribution curves have been determined for various values of (j>. The GPC curves (see Fig. 19) have a tail which is consistent with the molecular weight distribution of the polymer prepared by bulk polymerization. Therefore it is likely that this tail is due to the polymerization in bulk. The greater amount of bulk phase formed for values of < ) greater than 0.9 is probably due to the decreased stability of the concentrated emulsion in such cases. 

Figure 22 presents the GPC curves of polystyrenes obtained in concentrated emulsions at various temperatures. The molecular weight distribution broadens because of a greater amount of low molecular weight polymers generated in the bulk as the polymerization temperature increases. The greater the temperature, the greater is the coalescence and hence the amount of bulk phase formed. 

Figure 26 compares the conversion as a function of time in concentrated emulsion and bulk polymerization and shows that polymerization proceeds much faster in a concentrated emulsion. The concentrated emulsion has an internal phase ratio of 0.93 and a molar ratio of MAA/styrene of 0.036. The molecular weight distributions of the polymers generated by both processes are presented in Fig. 27, which shows that concentrated emulsion polymerization leads to molecular weights an order of magnitude higher. Since the copolymer composition changes with conversion, the GPC curves were recorded at the same conversion. 

Fig. 27. Molecular weight distribution of the polymers produced in (a) concentrated emulsion (polymerization time = 9 h) (b) bulk polymerization (polymerization time = 41.5 h) (styrene 37 ml, methacrylic acid 1 ml, AIBN 0.3 g, SDS 0.4 g, water 6 ml)...

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