Polymerization control

Further improvements in reaction rates and polymerization control have led to the commercial availabihty of poly(ethylene oxide) of varying molecular weights. 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in these polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine-end with an ester side chain, attack of DMF by the amine-end, or chain transfer to monomer) [11, 12], this conclusion does not seem to be well justified. It is likely that the role of impurities (e.g., water) in these polymerizations is very complex. A possible explanation for the polymerization control observed under high vacuum is that the impurities act to catalyze side reactions with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain-end and thus influence their reactivity. 

Generally, these equilibrium constants are very solvent dependent. In particular, the values of KEA are expected to be relatively high in protic solvents because halide anions can be stabilized through solvation in such media [143], Also, on the other hand, Kx values will be likewise affected with changes in solvent polarity [50], Low values of Kx in polar and protic media have direct implications on the degree of polymerization control because of the decreased amounts of deactivator (Mt"+1X/L), as a result of halide anion dissociation from the metal center. 

Radical grafting, 10 206 Radical-induced decompositions, 14 280 of dialkyl peroxydicarbonates, 14 289 Radical ozone reactions, 17 774 Radical polymerization, 22 40. See also Free-radical polymerization controlling, 14 297 of methacrylic ester polymers, 16 279-290... 

Savage, K.E., C.L. McCormick, and B.H. Hutchinson, "Biological Evaluation of Polymeric Controlled Activity Herbicide Systems Containing Pendant Metribuzin,"... 

The polymerization control strategy which is based on the fact that each of the monomers in a co- or ter- polymerization is lost in a first order manner has been shown to be satisfactory for the synthesis of homogeneous multi-component polymers. The MMA/TBTM/2EHA is not a demanding system in that only a small amount of the relatively unreactive 2EHA have been used. 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

Another way to deal with the problem of the toxicity of the metal is the enantiopure synthesis of p-BL from racemic PO. The extremely low activity and enantiomeric excess of this process prohibits its application on industrial scale. In addition, there is still a lack of catalysts that convert enantiopure p-lactones to isotactic polymers with good polymerization control and whilst retaining high activities. 

ATRP and NMP control chain growth by reversible termination. RAFT living polymerizations control chain growth through reversible chain transfer [Bamer-Kowollik et al., 2001, 2003 Chiefari and Rizzardo, 2002 Cunningham, 2002 D Agosto et al., 2003 Goto et al., 2001 Kwak et al., 2002 Moad et al., 2002 Monteiro and de Brouwer, 2001 Stenzel et al.,... 

Keywords Crystal engineering Solid-state photoreaction Topochemical polymerization Controlled radical polymerization Dimerization Isomerization Topotactic reaction... 

The first reports of LA ROP using yttrium complexes focused on homoleptic alkoxide complexes, such as cluster complexes of the form Ln5(p-0)(0R)i3 [27]. A patent, and preprint, published by DuPont described the application of a homoleptic yttrium alkoxide, Y(OCH2CH2NMe2)3, formed in situ by reaction of yttrium fm-Ao-propoxide with ALV-dimethy I am i noethanol. The complex showed a very high rate (kob = 0.5 s 1, [Y]0 = 3 mM) and reasonable polymerization control [28]. 

Stevels et al. studied homoleptic yttrium aryloxide complexes (Fig. 9) [29]. The complexes were rather slow initiators however, in the presence of isopropanol, the rate was accelerated and the polymerization control was excellent. 

Our research groups have investigated various b/s(thio/oxo phosphinic)diamido yttrium initiators (e.g. complex 5) [36, 54—56]. These complexes show excellent rates for LA ROP and reasonable control polymerization control is improved in the presence of exogenous alcohol. We have shown that the nuclearity of the initiator has an important influence over polymerization stereochemistry, with a mononuclear complex enabling high degrees of heteroselectivity (Fig. 12). 

Particle size distribution of a latex has a pronounced influence on its rheological properties, storage stability, and film forming capabilities Q ). In addition, latex particle size distributions provide information on polymerization mechanisms and kinetics (2), and provide a means of polymerization control (3,4). 

The driving force for drag release from a pump is a pressure difference that causes the bulk flow of a drag, or drug solution, from the device at a controlled rate. This is in contrast to the polymeric controlled release systems described above, where the driving force is due to the concentration difference of the drag between the formulation and the surrounding environment. Pressure differences in an implantable pump can be created by osmotic or mechanical action, as described below. 

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