Polymerization of VAc

We have found that in the system of presulfate initiator, the PVAc latexes are not dissolved transparently in the methanol-water mixture [8], and in the system of HPO initiator, the extraction of the polymer from the PVAc latex films with acetone greatly depends on the polymerization condition [9]. These results suggest that if a polymerization method can be found in which the grafting polymerization of VAc onto PVA is controlled to the minimum, a large portion of PVAc in the latex film will have a chance of extraction with solvents. In this Chapter, the preparations of the unique porous films from the PVAc latexes containing PVA as a protective colloid by an extraction of the PVAc particles with acetone and the characteristic properties of the porous films are summarized. 

Therefore, the porous film obtained by using the polymerization of VAc in the presence of PVA as a protective colloid is certainly the porous PVA-PVAc composite film. 

The extent of branching, of whatever type, is dependent on the polymerization conditions and, in particular, on the solvent and temperature employed and the degree of conversion. Nozakura et at.1 1 found that, during bulk polymerization of VAc, the extent of transfer to polymer increased and the selectivity (for abstraction of a backbone vs an acetoxy hydrogen) decreases with increasing temperature. 

The processes described in this section should be contrasted with RAFT polymerization (Section 9.5.3), which can involve the use of similar thioearbonylthio compounds. A. A -dialkyl dithiocarbamates have very low transfer constants in polymerizations of S and (mctb)acrylatcs and arc not effective in RAFT polymerization of these monomers. However, /V,A -dialkyl dithiocarbamates have been successfully used in RAFT polymerization of VAc. Certain O-alkyl xanthates have been successfully used to control RAFT polymerizations of VAc, acrylates and S. The failure of the earlier experiments using these reagents and monomers to provide narrow molecular weight distributions by a RAFT mechanism can he attributed to the use of non-ideal reaction conditions and reagent choice. A two part photo-initiator system comprising a mixture of a benzyl dithiocarhamate and a dithiuram disulfide has also been described and provides better control (narrower molecular weight distributions).43... 

The polymerization of MA with 7 was carried out in the presence of 13, i.e., 7 and 13 were used as two-component iniferters [175]. When an identical amount of 13 to 7 was added to the system, the polymerization proceeded according to a mechanism close to the ideal living radical polymerization mechanism. Similar results were also obtained for the polymerization of VAc. These results indicate that the chain end of the polymer was formed by the competition of primary radical termination and/or chain transfer to bimolecular termination, and that it could be controlled by the addition of 13. 

Oscillations in the number of polymer particles, the monomer conversion, and the molecular weight of the polymers produced, which are mainly observed in a CSTR, have attracted considerable interest. Therefore, many experimental and theoretical studies dealing with these oscillations have been published [328]. Recently,Nomura et al. [340] conducted an extensive experimental study on the oscillatory behavior of the continuous emulsion polymerization of VAc in a single CSTR. Several researchers have proposed mathematical models that quantitatively describe complete kinetics, including oscillatory behavior [341-343]. Tauer and Muller [344] proposed a simple mathematical model for the continuous emulsion polymerization of VCl to explain the sustained oscillations observed. Their numerical analysis showed that the oscillations depend on the rates of particle growth and coalescence. However, it still seems to be difficult to quantitatively describe the kinetic behavior (including oscillations) of the continuous emulsion polymerization of monomers, especially those with relatively high solubility in water. This is mainly because the kinetics and mech-... 

This can be explained by the fact that the flow in the CCTVFR became closer to plug flow as the Taylor number was dropped closer to. Therefore, the steady-state particle number and the steady-state monomer conversion could be arbitrarily varied by simply varying the rotational speed of the inner cylinder. Moreover, no oscillations were observed, and the rotational speed of the inner cylinder could be kept low, so that the possibility of shear-induced coagulation could be decreased. Therefore, a CCTVFR with these characteristics is considered to be highly suitable as a pre-reactor for a continuous emulsion polymerization process. In the case of the continuous emulsion polymerization of VAc carried out with the same CCTVFR, however, the situation was quite different [365]. Oscillations in monomer conversion were observed, and almost no appreciable increase in steady-state monomer conversion occurred even when the rotational speed of the inner cylinder was decreased to a value close to. Why the kinetic behavior with VAc is so different to that with St cannot be explained at present. 

Wang and Schork [73] used PS, PMMA and PVAc as the costabilizers in miniemulsion polymerizations of VAc with PVOH as the surfactant. They found that, while PMMA and PS were effective kinetic costabihzers (at 2-4%wt on total monomer) for this system, PVAc was not. While the polymeric costabilizers did not give true miniemulsions, Ostwald ripening was retarded long enough for predominant droplet nucleation to take place. 

The effect of mass transfer of vinyl versatate on the mini/macroemulsion polymerization of VAc/VEOVA in batch and semibatch systems was explored. For the batch experiments, the addition of neat VEOVA formed poor dispersions of VEOVA, which resulted in smaller particles, lower polymerization rates and different polymer composition tracks compared to normal mini/macroemulsion polymerization of VAc/VEOVA. The well-dispersed VEOVA seemed to help the monomer-swollen particle to gain more radicals in the nucleation period. 

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

In 1994, Matyjaszewski group first reported the controlled/ living radical polymerization of VAc initiated by the complex of Al(/Bu)3/Bpy/TEMPO (Bpy = 2,2 -bipyridyl, TEMPO = 2,2,6,6-tetramethyl-l-piperidinyloxy)." In benzene at 60 °C, the semilogarithmic plots of ln[M] t were linear at different initiator concentrations (Figure 1), indicating the polymerization was first order with respect to monomer concentration and the concentration of growing radicals remained constant during the polymerization. 

Figure 3 shows the molecular weight versus conversion plot for the polymerization of VAc mediated by Co(acac)2. The plot was hnear and passed the origin, and the polydispersity of the prepared PVAc was lower than 1.3, indicative of living features of the polymerization. The hving features were further confirmed by a chain extension experiment. " ... 

The kinetic semilogarithmic plot for this polymerization was linear but didn t pass the origin (Figure 4). A significant induction time was observed in the polymerization of VAc by Co(acac)2. This induction time was attributed to the slow conversion of Co(ll) to Co(lII) complex by in situ generated radicals. " This group also conducted a successful suspension polymerization of VAc in water using Co(acac)2 and prepared PVAc-PS and PVA-PS block copolymers by combination of the cobalt-mediated polymerization and ATRP. ... 

Besides organocobalt complexes, organostibine has also been found to be able to mediate controlled/ diving polymerization of many vinyl monomers. " For example, Yamago et al reported that at 60 °C the polymerization of VAc mediated by a-dimethylstibanyl ester reached 92% conversion in 5 h and produced narrowly dispersed PVAc (PDl = 1.26), but no detailed reaction kinetics was provided in their study.Kamigaito et al. very recently found that a manganese carbonyl complex [Mn2(CO)io] coupled with an alkyl iodide ( -I)... 

The polymerization of VAc by NMP was reported by Matyjaszewski group in 1996. To reduce the bimolecular radical termination dnring the polymerization, the authors attached the stable nitroxyl radical (TEMPO 2,2,6,6-tetramethylpiperidine-l-oxyl radical) to the interior of a dendrimer and used this modified TEMPO ([G-3]-TEMPO) as a scavenger combined with 2,2 -azobis(2-methyl-propionitrile) (AIBN) to polymerize VAc. Fignre 5 shows the kinetic plot and the dependence of molecular weight on monomer conversion for the bnlk polymerization of VAc at 80 °C in the presence of [G-3]-TEMPO/AIBN. 

Figure 5. Bulk polymerization of VAc at 80 °C. [[G-3J-TEMPO/AIBN] o= 0.03 M. (A) Kinetics. (B) Evolution ofPVAc molecular weight andpolydispersity with monomer conversion (Reproduced with permission from American Chemical Society).

Figure 6. Kinetic plots for bulk polymerization of VAc with EtlAc as transfer agent and CPD as initiator (50 °C). ( ) [VA]q [EtlAc]y[CPD]o = 500 1.0 0.15 ...

Figure 8. The dependence of molecular weight on monomer conversion in the bulk polymerization of VAc at 60 °C using AIBN as initiator and MESA as RAFT agent. RAFT agent concentration Cmadix = T1 xlO M (o) 2.2 xKT M (k) ...

Sawamoto et al. reported in 2002 the polymerization of VAc mediated by dicaibonylcyclopentadienyliron dimer [Fe(Cp)(CO)2)]2 using iodide compounds as initiators and Al(0-i-Pr)3 or Ti(0-/-Pr)4 as an additive." However, this catalyst system was found complicated in mechanism. The metal alkoxide additives and the iodide compounds played important roles in the polymerization of VAc. Without the additive or iodide compounds, the polymerization became extremely slow or even no polymerization occurred. Additionally, the iodine-degenerative transfer process could not be excluded in this polymerization because alkyl-iodides alone could mediate degenerative transfer polymerization of VAc, as discussed in the above section.Thus, the mechanism of this polymerization system was proposed as shown in Scheme 6, but it was not verified and unclear. 

Otha Victors affect initiatrx decompositian rate. Lepizzera and Hamielec [8] studied the rate of decompositioi of potassium persul te in the polymerization of VAc in the presence of various grades of poly(vinyl alcohol) (P A1). While all grades of PVAI increased the rate of decompositioi of initiator, the higher molar mass PVAI materials increased the rate of decomposition more effectively than the lower molar mass materials at equal loading. 

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. 

While the various intervals in emulsion polymerization of VAc have a profound affect on the development of molar mass, other components added during the polymerization can also affect molar mass. Lee and Mallinson [21] determined that simple components such as surfactants can profoundly influence molar mass. They studied Aerosol OT [sodium bis-(2-ethylhexyl) sulfosuccinate] (AOT) and determined it can change the molar mass of a product by broadening the molar mass distribution. They reported an increase in polydispersity from 4.2-14 when AOT was substituted for sodium dodecyl sulfate in a VAc system. They attributed these results to significant chain transfer effects of AOT. 

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