Rate-time profiles

The crosslinking of such types of silicones can be described by means of a polymerization reaction. The reaction rate (rP) of this process is a function of the light intensity, the exposure time, the acrylate content, the molecular weight of the uncrosslinked silicone, the photoinitiator and also of the oxygen content of the system. A typical reaction rate/time profile is shown in the Fig. 1. 

The kinetics of Ziegler-Natta polymerization, like the mechanism of the reaction, are complex. The relatively few polymerizations that are homogeneous behave in a manner generally similar to noncoordination ionic polymerizations (Chapter 8). However, as has been pointed out earlier, the heterogeneity of the reaction system is more of a rule in Ziegler-Natta polymerization than an exception. The heterogeneous systems usually exhibit complicated behavior [2,5,7], as can be seen from some typical kinetic rate-time profiles, types (a)-(f) in Fig. 9.6. Such profiles are significant in... 

Figure 9.6 Some typical kinetic rate-time profiles, (a) Rate increases in an initial acceleration or settling period to reach a more or less steady value, (b) Rate increases in an initial settling period to reach a maximum and then decreases, (c) No initial settling period and rate decreases rapidly from beginning, (d) Rate rises very rapidly to a maximum value and then decreases rapidly, (e) No settling period and rate decreases slowly, (f) No settling period and rate remains constant. (After Ref. 7.)...

A number of simple kinetic models [12,13] have been developed for catalysts that have relatively low activities and are characterized by Unetic rate-time profiles of the type shown in Fig. 9.6 (a to f). These models are based on the assumption that the total concentration of active centers, C, remains constant throughout the polymerization consisting of three steps chain initiation, chain propagation, and chain transfer. 

Equation (9.39) has been found applicable to the polymerization systems characterized by kinetic rate-time profile of the type (a) in Fig. 9.6. Comparative values for some of the principal kinetic parameters derived in this way are listed in Table 9.6. 

Complete addition of the less reactive monomer to the reaction vessel at the beginning of the growth stage followed by feeding of the more reactive monomer according to a calculated rate-time profile. 

Independent addition of both monomers using calculated rate-time profiles. 

Fig. 27 Video microscopy kinetics polymerization rate-time profiles for several particles...

Fig. 2 Rate-time profiles of propene polymerization with 1 activated by dMMAO and dMMAO/SiOj squares, dMMAO (Ti = 20 pmol) open circles, dMMA0/Si02 (Ti = 20 filled circles, dMMAO/Si02 (Ti = 10 omol) triangles, MMAO (Ti = 20 omol) [12]...

The rate-time profile of propene polymerization with the dMMAO system is illustrated in Fig. 2, indicating a steady rate after a certain induction period. The... 

A typical vitro release rate-time profile of scopolamine from such a therapeutic system into an infinite sink at isotonic and isothemal conditions is shown in Figure 6. 

Variation of the trlalkylaluminium concentration can bring about considerable changes in the rate-time profiles for hgCl - supported catalysts. The changes observed are quite complex and are believed to arise from simultaneous adsorption and reduction reactions (11). 

The rate of polymerization Increases rapidly with polymerization time for Al Tl > 58 1, reaching a maximum value In 2-6 min, depending on the Al Tl molar ratio. Thereafter the rate of polymerization decreases with the polymerization time. At lower A1 T1 molar ratios, l.e., Al Ti = 38 1, much lower activity Is obtained, although the rate-time profile shows good stability. 

The EP copolymerization rate with time showed decay type characteristics. Fig.4 shows the rate-time profile of the EP copolymerization and those of the corresponding homopolymerizations. In the decay period, the decrease of the EP copolymerization rate with time is even rapider than that of propylene homopolymerization rate. The decay kinetics can be described by the following equation ... 

Fig.4 Rate-time profiles of ethylene (e), propylene (A) homopolymerization and EP copolymerization (O).

Figure 83 Effect of monomer concentration on polymerization rate-time profiles and initial polymerization rates (Rpo) with silica-supported Cp Ti(OCH3)3/MAO catalyst.

Figure 4. In vitro release rate/time profiles (at 37 C) for the 20 yg/hr and 40 yg/hr ocular therapeutic systems. (Copyright 1974 ALZA Corp.)...

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