Influence of the Polymerization Time

The polymerization time as a polymerization parameter for adjustment of the porous properties of thermally initiated copolymers has recently been characterized [111]. A polymerization mixture comprising methylstyrene and l,2-bis(p-vinylbenzyl)ethane as monomers was subjected to thermally initiated copolymerization for different times (0.75, 1.0, 1.5, 2, 6, 12, and 24h) at 65°C. The mixtures were polymerized in silanized 200pm I.D. capillary columns as well as in glass vials for ISEC and MIP/BET measurements, respectively. 

FIGURE 1.7 Influence of the polymerization time on the porosity of monolithic MS/BVPE polymer networks, determined by MIP. Reduction of the polymerization time converts a narrow monomodal pore distribution into a broad bimodal distribution, comprising mesopores. 

BET measurements (Table 1.2) prove the increase in mesopores, as decreasing the total polymerization time from 24 h to 45 min causes to raise by a factor of 3, resulting in 5p 80mVg, which is comparable to silica particles with a mean pore diameter of 300 A [112,113]. 

Even if MIP and BET are widely accepted regarding the characterization of HPLC stationary phases, they are only applicable to the samples in the dry state. In order to investigate the impact of polymerization time on the porous properties of wet monolithic columns, ISEC measurements of 200 jm I.D. poly(p-methylstyrene-co-l,2-bis(vinylphenyl)ethane) (MS/BVPE) capillary columns (prepared using a total polymerization time ranging from 45 min to 24 h) have been additionally evaluated (see Table 1.2 for a summary of determined e values). On a stepwise decrease in the time down to 45 min, the total porosity (St) is systematically increasing to about 30% in total (62.8% for 24 h and 97.2% for 45 min). This is caused by a simultaneous increase in the fraction of interparticulate porosity (e. ) as well as the fraction of pores (Cp). The ISEC measurements are in agreement with those of the MIP as well as BET analyses, as an increase in should be reflected in an increase in 8p and as the relative increase in the total porosity (caused by decreasing the polymerization time 

Influence of the Polymerization Time on the Porous Properties of Monolithic MS/BVPE Networks, Considering Capillary Columns (80x0.2 mm I.D.) for ISEC and Glass Vial Bulk Polymers for MIP and BET Measurements 

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Figure 1.8 shows the influence of the polymerization time on the separation efficiency and resolution of MS/BVPE columns toward biomolecules (e.g., oligonucleotides) and small molecules... 

The phase separation in the presence of an inert medium occurring during the crosslinked formation was studied using Flory s thermodynamic theory (Dusek and Duskova-Smrckova 2000). Because the monomers themselves act as diluents, phase separation will be strongly influenced by the polymerization time, the phase equilibrium being described by the conversion function. 

Our results show that the network density (vg - 1/Q Q = swelling degree) of the crosslinked polymers is a function of the light intensity, the exposure time, the acrylate content, the molecular weight of the uncrosslinked silicone, and also of the length of the spacer group between the acrylate or methacrylate unit and the silicone backbone. Oxygen influences only the polymerization kinetics, but it does not influence the network density. 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

The effect of temperature on the RP-HPLC behaviour of /(-carotene isomers has been extensively investigated and the results were employed for the separation of carotenoids of tomato juice extract. Carotenoids were extracted from food samples of 2g by adding magnesium carbonate to the sample and then extracted with methanol-THF (1 1, v/v) in a homogenizer for 5min. The extraction step was repeated twice. The collected supernatants were evaporated to dryness (30°C) and redissolved in methanol-THF (1 1, v/v). Separations were performed on a polymeric ODS column (250 X 4.6 mm i.d. particle size 5/.an). The isocratic mobile phase consisted of methanol-ACN-isopropanol (54 44 2, m/m). The flow-rate was 0.8 or 2.0 ml/min. The effect of temperature on the retention times of lycopene and four /(-carotene isomers is shown in Table 2.11. The data indicated that the temperature exerts a considerable influence on the retention time and separation of /(-carotene isomers. Low temperature enhances the efficacy of separation. 

The results of the MIP analyses of the bulk polymers are illustrated in Figure 1.7. It could be demonstrated that the polymerization time is capable of influencing the shape of the pore distribution itself, rather than shifting a narrow macropore distribution (and thus the pore-size maximum) along the scale of pore diameter (see effect of the porogenic solvent in Section 1.3.2.2 and Figure 1.5). On... 

A polyacetylene coating applied on sulfur does not negatively influence its activity and speed as curing agent, but it can increase the scorch time. This effect is probably due to a delayed release of the sulfur out of the polymeric shell. In the SBR/EPDM blend, on the other hand, the plasma-treated sulfur results in higher torque values, an indication that the distribution of the plasma-treated sulfur over the different rubber phases is more homogenous, which is the main effect aimed for in the context of this study. 

The polymerization of light olefins using copper pyrophosphate is licensed by The M. W. Kellogg Co. under patents of the Polymerization Process Corp. The process is essentially the same as the U.O.P. process but instead uses a copper pyrophosphate catalyst (18). The first plant was built in 1939 (22) and several more have been put into operation since that time. A correlation of operating variables for this process was published in 1949 (21) it shows how conversion is influenced by catalyst activity, temperature, ratio of propene plus n-butene to isobutene, and the space velocity of olefins and of total feed when operating at 900 pounds per square inch gage pressure. A catalyst life of 100 to 150 gallons of polymer per pound of catalyst is claimed (15). 

After an iPP particle reached the FBR, co-polymerization of ethylene-propylene starts preferrably inside the porous PP matrix. Depending on the individual residence time, the particle will be filled with a certain amount of ethylene-propylene rubber, EPR, that improves the impact properties of the HIPP. It is important to keep the sticky EPR inside the preformed iPP matrix to avoid particle agglomeration that could lead to wall sheeting and termination of the reactor operation. Ideally a "two phase" structure, see Fig.5.4-3, is produced. Finally, a "super-high impact" PP results that contains up to 70% EPR. How much EPR is formed per particle depends on three factors catalyst activity in the FBR, individual particle porosity, and individual particle residence time in the FBR. All particle properties are therefore influenced by the residence time distribution, and finally, a mix of particles with different relative amounts of EPR is produced - a so called "chemical distribution" see, for example, [6]. 

An interesting effect of the ionic factors of the polymerization was found by Kuntz (59). He has shown that the homopolymerization of styrene using butyllithium catalysts is six times as rapid as that of butadiene. However, in copolymerization, butadiene polymerized initially at its own rate with relatively small amounts of the styrene being consumed. Only after 90% of the butadiene had been consumed, the styrene began to polymerize at its own rate. THF increased the rate of the polymerization but had little effect on the rate of butadiene to styrene which is polymerized. The butadiene structure is little influenced by copolymerization. The homopolymer contained 44% cis-1.4, 7% 1.2 and 49% trans-1.4 while the butadiene units of the butadiene copolymers contained 40% cis 1.4, 7% 1.2 and 53% trans-1.4 groups. 

The alkylaluminum component combined with V(acac)3 has an influence on the kinetic behavior of the propylene polymerization at —78 °C47 82 83). In the polymerization with V(acac)3 and dialkylaluminum monohalide like Al(i-C4H9)2C1, Al(n-C3H7)2C1, A1(C2HS)2C1 or Al(C2H5)2Br, M of polypropylene increased proportionally to the polymerization time, and the polydispersity (M Mj was as narrow as 1.15 0.05 (see Fig. 9). This is the case of living polymerization. As can be seen from Fig. 9, the rate of increase of Mn, i.e. the rate of propagation of living chains as expressed by lvln/(42 t), is influenced by the kind of aluminum component and decreases in the series... 

However, Hsieh and Kitchen 151 failed to consider the influence of their measurement temperature, 78 °C, on the stability of the poly(dienyl)lithium active centers (see section on Active Center Stability). As an example of this potential problem is the observation by two separate groups 47-152> that viscometric measurements of hydrocarbon solutions of poly(butadienyl)lithium fail to yield constant flow times (at 30 °C) following the completion of the polymerization, i.e., the flow times were found to increase with increasing time. This inability of the poly(butadienyl)lithium chain to exhibit constant solution viscosities renders it unsuitable for association studies of the type done by Hsieh and Kitchen 151). 

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