Monomer-starved conditions

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... 

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. 

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. 

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. 

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. 

Effects of monomer-starved conditions upon molar... 

Rjjf < R. This situation is of great significance and defines what are known as monomer-starved conditions. Under these conditions, the concentration of monomer in the particles falls below the saturation value and is controlled directly by R ,. 

In order to take full advantage of semi-batch emulsion polymerization, it is necessary to use monomer-starved conditions. The following section is devoted to the reasons why these conditions are so important. Except where indicated otherwise, statements such as addition of monomer and monomer added simply mean... 

El-Aasser et al. [20] studied the difference between batch and semi-continuous polymerization of VAc and determined that batch polymerization produced a narrower molar mass distribution than the semi-continuous process. The semi-batch polymerization produced a high molar mass fraction which was attributed to CTP due to monomer-starved conditions. 

Figure 7.1 Dynamic mechanical spectra showing the variation of storage modulus ( ) and loss modulus ( ") with temperature for copolymers prepared by emulsion copolymerization of vinyl acetate (71 mol%) with n-butyl acrylate (29 mol%) using (i) a batch process (T, E V, E"), and (ii) a semi-batch process operated under monomer-starved conditions ( , " o, "). (Adapted from ref. 1.)...

The simplest way to establish the value of Rm below which monomer-starved conditions can be achieved, is to perform an experiment in which the chosen seed stage is carried out as planned for the semi-batch process, and to then add the remaining monomer over a short period of time to ensure that monomer-flooded conditions are attained. Measurement of the rate of polymerization (which should be i p ) after the initial period required for swelling of the seed particles, gives the limiting value of R ) below which monomer-starved conditions can be expected. 

In the following sections, the importance of monomer-starved conditions in controlling the kinetics of the polymerization and the properties of the polymer produced are described. 

J2 Control of copolymer composition using monomer-starved conditions... 

Other strategies for controlling copolymer composition Although the use of monomer-starved conditions for control of copolymer composition is widespread, the low monomer feed rates which need to be used give rise to low rates of copolymerization and have significant effects upon the molar mass and molar mass distribution of the copolymers formed (see Section 7.4.4.4). Hence, alternative procedures have been developed which facilitate higher feed rates, but nevertheless allow for control of copolymer composition. These procedures are briefly described in this section. 

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