Bulk polymerization of VAc

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. 

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) ...

Moreover, the reactivation of a cobalt-terminated polymer in the presence of second monomer leads to block copolymerization. In this respect, CMRP has aheady contributed to the preparation of the valuable copolymers listed in Table 4.1. For example, well-defined poly(acrylate) block copolymers were prepared via a sequential polymerization of acrylic monomers with cobalt porphyrin la or cobaloximes 2 [14, 20]. The synthesis of well-defined poly (acrylate)-b-poly(VAc) block copolymers was also achieved with complex la [26]. Co(acac)2 (3a see Figure 4.1) is the most prolific complex for the preparation of block copolymers, until now. Indeed, the sequential CMRP of VAc with NVP [33], AN [48], or vinyl pivalate (VPi) [49] leads to the corresponding block copolymers, in controlled fashion. Throughout the polymerization, the experimental conditions were necessarily adjusted, taking into consideration the reactivity of the second monomer. As an illustration of this, well-defined PVAc-b-poly(acrylonitrile) (PAN) copolymers could only be prepared via a bulk polymerization of VAc at 30 °C, followed by the AN polymerization at 0°C in solution in DMF [48]. In this case, the DMF not only serves as the solvent but also binds the metal and adjusts its reactivity. As a rule, the PVAc sequences of these copolymers were hydrolyzed in order to provide poly(vinyl alcohol) (PVA)-containing derivatives, such as hydrosoluble PVA-b-poly... 

Hie system of Koumura et al. based on the photo-induced polymerization in the presence of Mn2(CO)io and ethyl-2-iodoisobutyrate at 40 °C was tested for the bulk polymerization of VAc (Scheme 18d). The addition of n-Bu3N accelerated the polymerization while maintaining a rather good control of the pol5mierization. For instance, poly(VAc) of = 18 000 and... 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

In 2005, Jerome et al reported a very efficient and well-controlled radical polymerization of VAc in the presence of Co(acac)2 (acac = acetylacetonate), VI (Figure 9), withV-70 as radical initiator in bulk, using a V-70/Co ratio of 6.5. Polymers with up to 10 were obtained with... 

Figure 22 (a) Kinetic scheme and rate iaw for the OMRP of VAc initiated by XViii. (b) First-order piot, Mn, and MJM for the bulk polymerization in the presence of a variabie amount of water [VAc]o/[XXViii]o = 358, [H20]o/[XXViii]o = 30 (fiiied triangie), 60 (fiiied square), 120 (filled diamond). 

Figure L Solution of kinetic equations for molecjdar weight and LCB development in bulk VAc polymerization. Ms, Mw, and Bs are plotted as a function of conversion. Model predictions Cm = 2.0 X Cp = 3.0 X i0 K == 1.0.

RAFT, allowing for predictable molecular weight with low polydispersi-ties, is applicable to a wide range of vinyl monomers [173-176], some of them not always being polymerizable by NMP or ATRP (i.e., VAc [167] or monomers bearing protonated acid groups). Hence, RAFT is employed in many polymerization processes, such as bulk, solution, suspension, emulsion, and miniemulsion [177-180]. 

Unlike St and MMA, the propagating radical of PVAc shows intractably high reactivity and less stability because of the lack of a conjugated substituent in the VAc monomer. Thus, achievement of a narrow molecular weight distribution of PVAc has still been limited, except for a few controlled living radical polymerization systems [83-88]. However, PVAc obtained from the nanochannels of PCPs showed small MJM values (e.g., MJM of PVAc prepared in [Cu2(bdc)2ted] was 1.7) by GPC measurements, while PVAc prepared under similar bulk conditions... 

The structure of Bj corresponds to a monomer unit of vinyl alcohol, while the structure B2 is two monomer units. By comparison of observed chemical shifts with those of the model compounds, the signals at 61.30 and 61.24ppm are assigned to the ultimate carbon atom in the end-group in model Bj and two peaks also coming from essentially dyad tacticity. The content of the PVA was about 0.027 B2 end-groups per 100 repeat units. When VAc was polymerized in bulk or methanol solution at the range of conversion 40-90%, the concentrations of short branches in the PVA s were almost same. The correlation between the concentration and the conversion was not found. 

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