Conventional precipitation polymerization

Conductivities as high as 0.05 S/cm were obtained - which are indicated to be higher than obtainable for polypyrrole prepared in conventional solvents. In contrast to globular morphology obtained in conventional precipitation polymerizations, the polypyrrole samples prepared in the supercritical carbon dioxide medium lead to fibrillar morphology. 

The free-radical retrograde-precipitation polymerization (FRRPP) process distinguishes itself from the conventional free-radical precipitation polymerization in both mechanisms and final product properties. In short, the FRRPP process proceeds above the LCST of the system, whereas the conventional-precipitation polymerization generally happens within the UCST region. For the different mechanisms of the UCST and LCST, one can expect that the polymerization process must be controlled by different schemes above the LCST and below the UCST. Furthermore, the product properties may possess some unique characteristics for the aforementioned reasons. 

As a new kind of precipitation polymerization, the FRRPP process is claimed to differ from the conventional-precipitation polymerization mentioned above and has its own unique features. These process and product features will be discussed in subsequent sections. 

The above data provide the kinetics, solubility, and emulsification results for three solvents ether, pyridine, and cyclohexane. Note that S polymerization in ether occurs via FRRPP, but AA polymerization in ether occurs via conventional precipitation polymerization (CPP). Pyridine is a solvent for both PS and PAA thus, copolymerization of AA and S in pyridine is via solution polymerization. Finally, AA polymerization in cyclohexane is via CPP and PS formation in cyclohexane is via solution polymerization. 

Cortes et al. [18] have quantitatively determined polymer additives in a polycarbonate homopolymer and an ABS terpolymer. In that case, a multidimensional system consisting of a microcolumn SEC was coupled on-line to either capillary GC or a conventional LC system. The results show losses of certain additives when using the conventional precipitation approach. An at-column GC procedure has been developed for rapid determination (27 min) of high-MW additives (ca. 1200Da) at low concentrations (lOOppm) in 500- xL SEC fractions in DCM for on-line quality control (RSD of 2-7%) [36], Also, SEC-NPLC has been used for the analysis of additives in dissolution of polymeric... 

Poly(4-vinylpyridine) was used also as a template for polymerization of maleic anhydride. Maleic anhydride is very difficult to polymerize by conventional radical polymerization, but in the presence of poly(4-vinylpyridine) in chloroform or in nitromethane, polymerization proceeds at room temperature just after mixing 0.5% solution of poly(4-vinylpyridine) with 1% solution of maleic anhydride. A yellow precipitate is obtained. The precipitate is a mixture of poly(maleic anhydride), poly(4-vinylpyridine), and unreacted maleic anhydride. In the absence of oxygen, polymerization is much slower. The reaction stops on the stage of donor-acceptor complex formation. 

It is unavoidable to generate a stoichiometric amount of triorganotin-based waste, that often disturb isolation of the desired product because of low polarity and good solubihty in many organic solvents, in most of cases. Development of process to separate such a waste from the product is therefore one of the central issues in this protocol. Treatment of the reaction mixture with aqueous KF solution to precipitate polymeric trialkyltin fluoride is the most widely used procedure [22, 23, 80]. Partition between acetonitrile and pentane can effectively remove the tin waste and unreacted nonpolar organotin reagent if the desired product is polar enough [53]. Use of combinatorial technique may be one of recent solutions (see Sects. 4.7.2 and 4.7.3). Conventional and widely used procedures are nicely summarized in the review by Farina et al. [25,78,81 - 83]. 

Recently, Charpentier et al. [36] demonstrated the synthesis of PVDF by a continuous precipitation polymerization in SCCO2, using diethyl per-oxydicarbonate (DEPDC) as an initiator. Low molecular weight polymers (Mjj<20,000 g/mol) were reported in this first work. However, subsequent research has yielded PVDF with number-average molecular weights upwards of 79,000 g/mol [51,52]. PVDF made in the continuous CO2-based system had molecular weights comparable to commercial polymers, but also exhibited unique properties not observed in PVDF made by conventional processes. 

Although quite complex hybrid block copolymer architectures can now be synthesized, obtaining these materials in a state of high purity typically requires additional measures. As discussed above, many of the hybrid copolymers contain homopolymer impurities, which must be removed by selective solvent extractions or fractional precipitation when possible. Since conventional NCA polymerizations also usually give polypeptide segments with large chain length distributions, these samples are ideally also fractionated... 

The radical ring-opening elimination polymerization of 4-methylene-l,3-dioxolane stimulated us to construct a novel template polymerization (3). The concept is that polymers bear polymers. Polymer-supported monomer, which had a structure of 2,2-dipheny 1-4-methylene-1,3-dioxolane, reacted with radical species to afford polyketone and copolymer of styrene with vinylbenzophenone as newborn polymer and template, respectively (Figure 12). These polymers were easily separated by fractional precipitation without any particular chemical treatment after polymerization. On the other hand, common template polymerization requires annoying procedures for the separation of obtained polymers form template. On this point, our novel template polymerization system differs from conventional template polymerization. 

The fact that these polymers do not dissolve in flieir own monomers leads to complex phase separation at veiy low conversions during the polymerization process. The phase separation is even more complex than in a conventional emulsion polymerization since, not only is there phase separation between the newly formed polymer and the continuous wat phase, but also betweoi the polymer and monomer present. Due to this phase s aradon the locus of polymerization is not the interior of the polymer particles, simply because there is no monomer in the interiors. Free radicals formed in the water phase toid to precipitate or adsorb at the surface of existing particles r idly after their fonnation. Thus the major part of the propagation process will take place at the surface of the polymer latex particles [5,6]. Evidence for this is that the rate of polymerization is proportional to the total surface area of the latex particles this bend has been determined by varying both the latex particle concentration and size and observing the polymerization rate [5]. 

In emulsifier-free (hereafter referred to as soap-free ) polymerizations, the polymerization is carried out in the same way as described above, except that no surfactant is used. Nucleation occurs by oligoradical precipitation into unstable nuclei which collide to form larger particles. Polymerization takes place mainly within these monomer-swoUen particles, and the particles grow in similar manner to conventional emulsion polymerization. 

The monomers presented here have been prepared via multistep syntheses, each having been derivatized with desirable functional groups as per device/appli-cation requirements. Cited papers for the polymerizations presented here will refer the reader to the appropriate journals for the corresponding monomer synthesis. For our own work, glassware consists mainly of three-necked round-bottomed flasks, Schlenk tubes, microwave vials, that have been baked, dried effectively and kept under vacuum to remove any residuals of water or air. All polymerization reactions are performed under Schlenk conditions with care taken to eliminate the presence of air and water. The crude reaction mixture is conventionally precipitated into excess methanol (MeOH) followed by filtration of the solid, which is then subjected to successive Soxhlet extraction... 

Several general disadvantages of bulk polymerization (removal of the reaction heat, shrinkage, nonsolubility of the resulting polymer in the monomer, side reactions in highly viscous systems such as the Trommsdorff effect or chain transfer with polymer) are responsible for the fact that many polymerization processes are carried out in the presence of a solvent. A homogeneous polymerization occurs when both monomer and polymer are soluble in the solvent. When the polymer is insoluble in the solvent, the process is defined as solution precipitation polymerization. Other heterogeneous polymerization reactions in liquid-solid or liquid-liquid systems such as suspension or emulsion polymerizations are described later. Conventional solution polymerization is compared with solution precipitation polymerization for the synthesis of acrylic resins in Ref. [34]. 

Talamini and Peggion [145] visualize the process as a modified heterogeneous solution polymerization. The monomer has an appreciable solubility in the aqueous phase. These authors estimate the solubility to be on the order of 0.5 moles per liter. (Presumably this is under the pressure conditions of a typical reactor. Our Table I gives the solubility as 0.1 or approximately 0.02 moles per liter at standard temperature and pressure.) Polymerization starts in the aqueous solution. The polymer that forms separates. The emulsifier in the solution protects the particle from coagulation. By imbibing monomer on the surface of the polymer particle, growth takes place until latex-sized particles form. When the surfactant is consumed by adsorption on these particles, radicals precipitate from solution onto existing particles. Then the number of particles remains constant, very much as in a conventional emulsion polymerization. The total surface area of the polymer particle appears to be involved in the polymer process. 

In precipitation polymerization (c) the polymer is separated in the reactor from the solvent and the unconverted monomer. A filtration or centrifugation is required to recover the polymer. In processes of the types c, d, e and / the polymer is present as a dispersed phase, so that high concentrations can be used while the viscosity of the dispersion is relatively low. Conventional stirred tank reactors are widely used. In all four processes the polymer has to be separated (f after coagulation) and dried. 

The foregoing results indicate that the emulsion polymerization of VC is influenced in a complex way by the high water solubility of the monomer and the limited miscibility of polymer and monomer and deviates strongly from the conventional emulsion polymerization. This is discussed in terms of the water-phase polymerization of VC, precipitation polymerization of VC, polymerization in the emulsifier layer zone (a water/particie interphase), desorption of radicals from particles, chain transfer to monomer and polymer, formation of unsaturated and branching structures of PVC, formation of occluded radicals and diffusion of more volatile radical species to the vapor phase. 

Adamsky and Beckman polymerized acrylamide in inverse emulsions in supercritical CO2 (345 bar, 60°C) (55). The surfactant used in this study contained a polar head group composed of an amide group, to accomplish micelle formation, and a C02-philic tail composed of hexafluoropropylene. The absence of surfactant resulted in precipitation of a single solid mass of polymer in the reaction vessel. When surfactant was present, the solution had a milky-white appearance of an emulsion polymerization with yields higher than when no surfactant was used. The polymers produced exhibited a higher degree of linearity when compared with conventional emulsion polymerization of acrylamide. 

Free-radical initiators, like azobisisobutyronitrile (AIBN), and peroxides readily polymerize NVK. Thermal bulk polymerization of NVK is possible [47] but gives rather irreproducible results and coloured products even with purified monomer. An almost colourless polymer of high molecular weight can be achieved by bulk polymerization of NVK initiated with AIBN [88]. Besides bulk polymerization, which is carried out as a technical process, PVK can be obtained by precipitation polymerization [89], suspension polymerization [90,91] and photopolymerization [92]. Biswas and Roy [91] performed the suspension polymerization of NVK in a biphasic system (water and toluene) and obtained a polymer with appreciably higher conductivity (ca 1 x 10 S cm ) than that of conventionally prepared PVK. 

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