Vinyl alcohol main effects

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

The main disadvantage of the EVOHs is their water sensitivity, the more so as the vinyl alcohol content increases. Simultaneously, the barrier effect decreases. 

These calculations show that, if vinyl alcohol and vinyl thiol indeed mainly exist in planar syn conformations, as previously stated, the situation can be very different when halogen substituents are present. In some cases, the anti conformation is more or less out-of-plane staggered and can become more stable than the syn one. The general trends of substituent effects can be discussed by molecular orbitals analysis and electrostatic interactions. The results are subject to experimental confirmation, at least in the case of enols since their conformers can be prepared by the action of singlet oxygen atom on substituted ethylen, and trapped in low temperature matrices. 

The temperature dependence of hole formation and hole profile is affected by four factors decrease in the Debye-Waller factor, broadening of the hole width, spectral diffusion, and laser-induced hole filling. The first two effects are reversible phenomena and recover at low temperatures. The latter two are irreversible and their influence cannot be eliminated by cooling the sample again. The temperature dependence of the Debye-Waller factor (DiV(T) — S0(T)/S 4)) for TPP/PMMA and TPP/phenoxy resin systems, shown in Table 2.13 by a dotted line, agrees well with the slope of 0 at 4-20 K. The temperature dependence of the Debye-Waller factor is smaller in poly(vinyl alcohol), which shows a higher Es value (23 cm4). Thus, hole formation efficiency is controlled by the temperature dependence of Debye-Waller factor for temperatures below T and, for temperatures above T it is affected mainly by the simultaneous occurrence of spectral diffusion and laser-induced hole filling due to structural relaxation. 

Similar trends have been recently (P) obtained by Miiller et al. for the interaction of poly(vinyl alcohol) (PVA) with the BZ reaction mixture. They compared the effect of PVA widi that of isopropmiol (IPA), a small molecular weight secondary alcohol whose structure is rather close to that of the polymer repeating unit in PVA. They have found that the secondary alcoholic group of the IPA was a stronger perturbant compared to the same group in PVA even though quantitative modelling of the perturbation effects could not be done mainly for two reasons the reaction of the BZ subsystem PVA-acidic bromate was found to be too complicated for a systematic kinetic study, and the PVA-IPA comparison was difficult because acetone (which can be brominated) is the product of the reaction between IPA and bromate. 

Another approach to biodegradability is to blend a biodegradable natural polymer with a thermoplastic synthetic pol5nner. Blends of starch with PE or with poly(vinyl alcohol) have been commercialized as packaging materials (176), although they have mainly been used in niche markets. Early materials were rather poor because the encapsulating effect of the PE on the starch protected it from water access, and, even if the starch was biodegraded, the PE was left... 

Metallation has also been tried as a method for imparting weather- and heat-resistance to polymers. The heat stability of poly(vinyl alcohol) improves on treatment with aluminium isopropoxide which reacts at hydroxy groups with the elimination of propan-2-ol. About 12% of the hydroxy groups can be substituted in this way and these are believed to comprise mainly vicinal hydroxy groups and possibly end and other labile groups. A similar stabilizing effect was noted when free hydroxy groups in poly(vinyl butyral) were removed by this means. 

The second mechanism by which flocculation may be prevented is that of steric stabilisation. This is produced using nonionic surfactants or polymers that adsorb at the liquid/liquid interface with their hydrophobic portion, leaving a thick layer of hydrophilic chains in bulk solution, e.g. poly(ethylene oxide) (PEO) or poly( vinyl alcohol). These thick hydrophilic chains produce repulsion as a result of two main effects. The first, usually referred to as mixing interaction (osmotic repulsion), re-... 

Lamination of Paper and Board. Adhesives based on starch, dextrin, glutin and poly(vinyl alcohol), and also emulsion adhesives, mainly polyfvinyl acetate), are used for the lamination of paper and board. Only when high initial tack and a lay-flat effect are required are hot glues based on glutin still in use. Starch-based adhesives modified with silicate, casein-stabilized copolymer dispersions, and rubber latices are used in the lamination of paper to aluminum foils. Modified polyacrylate solutions and reactive polyurethane adhesives are used mainly for paper-to-plastic lamination. 

The two main approaches which can be used in the preparation of functional polymers consist of the polymerization or copolymerization of suitably functionalized monomers or the chemical modification of pre-formed polymers The first approach is often considered to be the most attractive due to its apparent simplicity, although it is often ill suited for the preparation of polymers with fairly complex functionalities In some cases, even simple polymers such as poly(vinyl alcohol) are only accessible via a chemical modification route In other cases it may be desirable to effect a simple chemical modification reaction to prepare a less common or more reactive polymer such as poly(iodomethyl styrene) from a more readily accessible but less reactive precursor such as polyCchloromethyl styrene). 

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