Starve feeding

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

The process is favored by low monomer concentrations as occurs at high conversions and in starved feed polymerizations.307 Theoretical calculations suggest that the incidence of backbiting should be strongly dependent on the tacticity of the penultimate dyad. 08 Double backbiting in VC or VAc polymerization will lead to 2-chloroethyl or 2-acetoxy ethyl branches respectively (as for E in Scheme 4.41 ).302... 

Table 9.9 Block Copolymers Prepared by Macromonomer RAFT Polymerization under Starved-Feed Conditions.380"595...

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

In the case of network formation controlled by (irreversible) kinetics programmed polymerization regime (starved feed conditions, etc.). 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

A method for calculating apparent reactivity ratios based on run number theory has been applied to "starved-feed" styrene/ ethyl acrylate systems. The reactivity ratios found are in agreement with those determined from solution polymerization data. The further confirmation of the observed agreement between reactivity ratios determined at low conversions and those determined by run number theory in "starved-feed" high conversion copolymerization requires the analysis of other comonomer pairs. 

Monteiro et al. have used a RATF Transurf in the ab initio emulsion polymerization of methyl methacrylate at 70° C. The Transurf was synthesized by esterifying a methyl methacrylate dimer with 1,10 decandiol followed by sulfonation. The authors found that only a small amount of Transurf was incorporated and suggested that, in order to increase the Transurf incorporation, the ratio of monomer to Transurf should be kept as low as possible, as achieved, e.g. in starved-feed conditions [12]. 

For conditions where ECT starved feed conditions, and coverage of the substrate is non-uniform, i.e., in spotty or wavy patterns. 

The variation of copolymer composition during the course of a batch polymerization can be reduced by conducting the reaction as a so-called semibatch process. This is a starved feed operation in which part of the charge is fed to the reaction vessel and polymerization is started. The remainder of the monomer feed is pumped in continuously or intermittently at a rate sufficient to keep the... 

Starved-feed emulsion polymerization can be conducted without emulsifiers if suitable comonomers and procedures are utilized.341 Polymerization of a water-soluble methacrylate like HEMA in the presence of a CCT agent is carried out initially. The resulting HEMA oligomer is further copolymerized with hydrophobic monomers so that the resulting diblock copolymer serves as a surfactant (see, for instance, sections 5.3 and 5.4). During the cross-linking process, all of this surfactant is incorporated into the polymer backbone and is thus immobilized, overcoming the problem of residual surfactant in the final product. 

To prepare water-soluble polymers employing CCT, it is necessary to modify the polymerization conditions.312 439 Use of a standard batch reaction leads to hydrolysis of catalyst, changing the catalyst level over the course of the polymerization, yielding a mixture of products and poor control of the reaction. A feed or starved-feed process that adds catalyst over the course of the reaction maintains a constant catalyst level and high conversion. The approach can be applied to a range of monomers such as methacrylic acid, 2-aminoethyl methacrylate hydrochloride, 2-hy-droxyethyl methacrylate, 2-methacryloxyethyl phos-phoryl choline, glycerol monomethyl methacrylate, and 3- O-methacryloyl-1,2 5,6-di- O-isopropylidene-D-glucofuranose. 

Figure 288(a) depicts on the left side the conditions in a machine with a narrow roll face and on the right side the wide-faced machine, both with gravity feed. Since there is little cross-flow in the nip the starved feed conditions in the roll border zones result in less compaction and, in extreme cases, excessive leakage at the cheek plates. It is obvious that this effect is more noticeable if the roll face is narrow because, although the absolute amount of less densified border zone material is constant for a given material, roll diameter, and feeder geometry, the relative proportion is smaller if wide-faced rolls are used. 

Solution Copolymerizations. Our primary objective in this preliminary study was to gain a qualitative understanding of the copolymerization behavior of VEC with various types of unsaturated monomers. Particularly, we wanted to determine if VEC could be incorporated into a variety of polymer types of interest to the coatings industry. Since VEC is used to provide cyclic carbonate functionality for subsequent reaction or crosslinking, limited amounts of VEC are used in the copolymerizations. A semi-batch process was used in the copolymerization experiments to approach starved-feed conditions. Starved-feed conditions can result in copolymers with more uniform composition since the conversion is kept high in the reactor. While there are a large number of variables to consider, we elected to focus on monomer composition, polymerization temperature, and initiator level. 

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