Solvents polymerization conditions influence

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

The presence of initiator in the monomer/template/solvent mixture and the influence of polymerization conditions can result in a wide variety of imprinted polymer morphologies and imprinting efficiencies as demonstrated by Bruggemann [28]. 

We believe that in the near future, the use of computational methods in conjunction with spectroscopic techniques and thermodynamic considerations should allow the in silico simulation to be used broadly for the analysis of the influence of polymerization conditions (solvent, cross-linker, temperature) on the performance of imprinted polymers, for optimization of the monomer composition and for tailoring polymer performance for specific applications. The computational approach described here represents a first step towards the truly rational design (tailoring) of MlPs and prediction of polymer properties. Nonetheless, improvements in our capacity to predict polymer performance should be benefited through the combination of molecular modeling, further physical characterization of the imprinting process, and combinatorial strategies (Chapter 8). 

The influence of the nature of the donor monomer (allyl or vinyl ethers) and of the polymerization conditions (in diluted media or in solvent-free conditions) on the microstructural characteristics of polymers were studied. The molecular weights of copolymers synthesized in both conditions obtained by Size Exclusion Chromatography are presented in Table 2. 

Still another new approach was the combined use of fractionation, osmometry and viscometry to study the influence of the polymerization conditions Ctemperature, extent of polymerization, solvent) on the infrastructure of addition polymers it was originally exemplified with three polystyrene samples prepared at different temperatures but has since been used in refined form and with such improved fractionation techniques (GPC) to study such problems as branching, stereoregulation and grafting. 

There are many faaors that influence a catalyst s polymerization behavior and the properties of the obtained polymer, including the identity of the transition metal, the cocatalyst catalyst ratio, the role of the solvent, the influence of the ligand stmaure, the effect of the bridge (for anM-metallocenes), the polymerization temperature, the polymerization pressure, and so on. Since reports on catalyst performance employ a very large number of different polymerization conditions, it is quite difficult to reach unanimous conclusions about catalyst behavior. 

Proposals for the mechanism of PPS formation include nucleophilic aromatic substitution (Sj Ar) (2radical-cation (27), and radical-anion processes (28,29). Some of the interesting features of the polymerization are that the initial reaction of the sodium sulfide-hydrate with NMP affords a soluble NaSH-sodium 4-(N-methylamino)butanoate mixture, and that polymers of higher molecular weight than pi edicted by the Caruthers equation are produced at low conversions. Mechanistic elucidation has been hampered by the harsh polymerization conditions and poor solubility of PPS in common organic solvents. A detailed mechanistic study of model compounds by Fahey provided strong evidence that the ionic S]s Ar mechanism predominates (30). Some of the evidence supporting the S s(Ar mechanism was the selective formation of phenylthiobenzenes, absence of disulfide production, kinetics behavior, the lack of influence of radical initiators and inhibitors, relative rate Hammet values, and activation parameters consistent with nucleophilic aromatic substitution. The radical-anion process was not completely discounted and may be a minor competing mechanism. 

Logically, this section should discuss the other rate constant characterizing the ATRP process, namely the deactivation rate constant, kdeact- However, the values of kdeact are typically rather large and difficult to determine experimentally. They are often calculated as the ratio kdeact = kact/KAXRP of the much easier to determine rate constant of activation and equilibrium constant of ATRP. This is why this section is dedicated to the experimental determination of Katrp as well as to the factors (initiator and catalyst structure, solvent, etc.) that influence its values. As seen from eqn (2), the rate of polymerization under classical ATRP conditions depends on the value of the equilibrium constant. 

The rate of ion propagation, is independent of the counterion and has been found to be about 46 X 10 in all cases for CF SO", AsF, SbF, SbCFg, PF g, and BF/ counterions. Conditions were the same for all counterions, ie, 8.0 M of monomer in CCI4 solvent and 25°C polymerization temperature. With less stable counterions such as SbCF and BF at most temperatures, the influence of transfer and termination reactions must be taken into account (71). 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Substituted styrenes are often used for investigating influences of structure, solvent and initiators on the cationic polymerization 1,2). Under constant outer conditions,... 

Hexaepoxy squalene, HES (Scheme 70) was used as a multifunctional initiator in the presence of TiCU as a coinitiator, di-f-butylpyridine as a proton trap, and N,N-dimethylacetamide as an electron pair donor in methylcy-clohexane/methyl chloride solvent mixtures at - 80 °C for the synthesis of (PIB-fc-PS)n star-block copolymers [145]. IB was polymerized first followed by the addition of styrene. The efficiency and the functionality of the initiator were greatly influenced by both the HES/IB ratio and the concentration ofTiCL, thus indicating that all epoxy initiation sites were not equivalent for polymerization. Depending on the reaction conditions stars with 3 to 10 arms were synthesized. The molecular weight distribution of the initial PIB stars was fairly narrow (Mw/Mn < 1.2), but it was sufficiently increased after the polymerization of styrene (1.32 < Mw/Mn < 1.88). 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

To find a suitable membrane for a certain application, an important parameter is the molecular weight cut-off (MWCO). The MWCO is defined as the molecular weight at which 90% of the solutes are retained by the membrane. It should be taken into account that the pore size of many ultra- and nano filtration membranes is greatly influenced by the solvent and by the temperature used under experimental conditions. This particularly concerns polymeric membranes as will be discussed in the next paragraph. 

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