Starved conditions

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

Polymerizations of methacrylic monomers in the presence of methacrylic macromonomers under monomer-starved conditions display many of the characteristics of living polymerization (Scheme 9.36). These systems involve RAFT (Section 9.5.2). However, RAFT with appropriate thiocarbonylthio compounds is the most well known process of this class (Section 9.5.3). It is also the most versatile having been shown to be compatible with most monomer types and a very wide range of reaction conditions.382... 

Kinetic studies show that insertion (the enantioselection step) is very rapid, and that the rate-determining step is the hydrogenolysis of the M-C bond. Nonetheless, under H2-starving conditions, there is evidence that fi- I elimination can be competitive with hydrogenolysis. />-H elimination of the alkyl intermediate gives back the starting alkene and, through an equilibration process, it... 

There is competition between conventional and AM ROPs. Initiation in conventional ROP is first-order each in protonated monomer and unprotonated monomer. AM ROP is first-order each in protonated monomer and alcohol. The ratio of the rates of AM-to-conventional ROP depends on [ROH]/[M] and the ratio of the rate constants for the two reactions. Assuming that the two rate constants are comparable, AM ROP becomes the dominant process at high [ROH] and low [M], Thus, AM ROP is carried out under monomer-starved conditions. The instantaneous monomer concentration is very low, but monomer is continuously added to the reactor at a rate equal to its rate of consumption. 

The same occurs with the stirring speed. A gas-starved condition can be achieved by a very low stirring speed. This may be desirable in some hydrogenations where it is necessary not to allow too much hydrogen to be transferred into the liquid (for selectivity reasons). Then, if stirring is increased, the reaction rate increases up to a point where it does not increase further. Since the gas/liquid resistance has already been lowered, the reaction rate becomes the maximum possible. 

Both HsSisO and the related polymer (resin) are made by hydrolysis of HSiCE under water-starved conditions. One method that gives high yields of the polyhedral... 

PaMeSt chain end] >10) before the addition of IB. This is based on a recent finding that the living cationic polymerization of pClaMeSt can be accomplished under conditions identical to those used for the synthesis of poly (aMeSt-fc-IB) copolymer [22, 23]. Importantly, the living PpClaMeSt chain end is very stable and there is no loss of livingness even after 5 h under monomer starved conditions. This is attributed to the reduced tendency of intramolecular alkylation due to the particularly large deactivating effect of the p-chloro substituent on the 2,5-positions of the aromatic ring. 

The power exponent, x determines what the monomer feed profile will look like as a function of of. Some examples of more common feed profiles are shown in Figure 1. If the polymerization is carried out under monomer-starved conditions, the composition of the polymer being formed at any instant is the same as the feed composition, C-. Therefore, the cumulative polymer composition at any timeef may be obtained from ... 

The process usually starts with the polymerization of a small proportion of the reagents at a very low monomer to water ratio (the seed stage), followed by the feeding of the remaining monomer (which may take several hours) and of other materials (if needed) once the conversion in the reactor has reached 70% or more. The in-reactor conversion will then depend upon the rate of polymerization compared to the rate of feed. If the reaction is continued under the so-called monomer-starved conditions, the in-reactor conversion is kept at a high 80-90%, which reduces the polymerization rate. To compensate, temperature is raised however, then the initiator depletes faster and more has to be added during the reaction. 

Ikeda, T. (1977). The effect of the laboratory conditions on the extrapolation of experimental measurements to the ecology of marine zooplankton. IV. Changes in respiration and excretion rates of boreal zooplankton species maintained under fed and starved conditions. Mar. Biol. 41, 241-252. 

Pietsch noted that the peripheral roll speed and particulate powder speed are not equivalent in the entire compaction zone. Throughput does not increase proportionally with roll speed. There are two effects that hinder throughput starved conditions in the feed zone, and secondly, too much squeezed air from the particle mass flows upward and against the powder flow, reducing fhe supply of maferial fo fhe nip... 

Sajjadi, S. Particle formation under monomer-starved conditions in the semibatch emulsion polymerization of styrene. I. Experimental. J. Polym. Sci. Pt. A Polym. Chem. 2001, 39, 3940-3952. 

Roll speed For most considerations and approximations it is assumed that the peripheral speed of the rollers and the speed of the particulate matter are identical in the entire compaction zone. In reality this is not true throughput does not increase proportionately with roll speed. The maximum speed is determined by two effects starved conditions in the compaction zone develop if ... 

Due to wall and interparticle friction, the flow of particulate matter is slower near the feeder wall and produces starved conditions at the edges of the rollers, resulting in more or less pronounced zones of less compacted material to overcome this problem, at least partially, the feeder should be wider than the roller, or /[Pg.278]

According to the core-shell model, the growing particle is actually heterogeneous rather than homogeneous, and it consists of an expanding polymer-rich (monomer-starved) core surrounded by a monomer-rich (polymer-starved) outer spherical shell. It is the outer shell that serves as the major locus of polymerization and Smith-Ewart (on-off) mechanism prevails while virtually no polymerization occurs in the core because of its monomer-starved condition. Reaction within an outer shell or at the particle surface would be most likely to be operative for those polymerizations in which the polymer is insoluble in its own monomer or under conditions where the polymerization is diffusion-controlled such that a propagating radical cannot diffuse into the center of the particle. 

---