Vinyl conversion

Skrtic et al. [104] studied methacrylate conversion and volumetric contraction in photo-polymerized composites with amorphous calcium phosphate (ACP). ACP was used because it is highly soluble in aqueous media and can be quickly transformed to HAp. Ideally, on polymerization, the resin systems used in composites should achieve the contradictory goals of both high vinyl conversion and minimal volumetric contraction. A high degree of vinyl conversion, with... 

This review summarizes the recent progress on the synthesis and application of covalently crosslinked hydrogels prepared using ATRP techniques. Section 2 briefly covers the fundamentals of the ATRP mechanism with particular emphasis on the recent progress to understand the structure/reactivity relationship and the development of new ATRP conditions that decrease the amount Cu catalyst. In Sect. 3, theoretical gel points and experimental gel points based on vinyl conversions are described. Emphasis focuses on understanding of the gelation process in... 

The rather unreactive chlorine of vinyl chloride can be displaced with nucleophiles by the catalytic action of PdCb. The conversion of vinyl chloride to vinyl acetate (797) has been studied extensively from an industrial standpoint[665 671]. DMF is a good solvent. 1,2-Diacetoxyethylene (798) is obtained from dichloroethylene[672]. The exchange reaction suffers steric hindrance. The alkenyl chloride 799 is displaced with an acetoxy group whereas 800 and 801 cannot be displaccd[673,674]. Similarly, exchange reactions of vinyl chloride with alcohols and amines have been carried out[668]. 

The term allylic refers to a C=C—C unit The singly bonded carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic carbon Conversely doubly bonded carbons are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents... 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

The properties of these new materials are being studied. Hydroboration is also appHed for the conversion of double bonds in polymers into hydroxyl groups (450—454). Well-defined copolymers of ethylene—vinyl alcohol can be prepared (455). 

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

The three steps in equation 3 are carried out in one vessel. This affords a wide variety of disilylaminoorganophosphines (8), including those with vinyl substituents (65), in yields of 40—85%. The oxidation of (8) to (9) and the reaction of (9) with alcohol (eq. 4) are carried out in a second reactor to provide the "monomer" phosphoranimines (10) in overall 30—65% yield based on starting PCl or CgH PCl2. The use of in place of Br2 in the conversion of (8) to (9) makes it possible to carry out all the reactions leading to (10) in one vessel, and this has significantly increased yields of the monomer, with overall yields up to 80% (66). 

Films from prepolymer solutions can be cured by heating at 150°C. Heating the prepolymer in molds gives clear, insoluble moldings (38). The bulk polymerisation of DAP at 80°C has been studied (35). In conversions to ca 25% soluble prepolymer, rates were nearly linear with time and concentrations of bensoyl peroxide. A higher initiator concentration is required than in typical vinyl-type polymerisations. 

Constmction of multilayers requires that the monolayer surface be modified to a hydroxylated one. Such surfaces can be prepared by a chemical reaction and the conversion of a nonpolar terminal group to a hydroxyl group. Examples of such reactions are the LiAlH reduction of a surface ester group (165), the hydroboration—oxidation of a terminal vinyl group (127,163), and the conversion of a surface bromide using silver chemistry (200). Once a subsequent monolayer is adsorbed on the "activated" monolayer, multilayer films may be built by repetition of this process (Fig. 8). 

Olefin isomerization is often catalyzed by titanium. An example is the conversion of vinyl norhornene to the comonomer ethylidenenorhornene (141). The catalyst is a mixture of a sodium suspension, AlCl, and (RO)4Ti or Cp2TiCl2. Although isomerization is slow, the yield is high. The active reagent is doubdess a Ti(III) compound. 

Pyrolysis. Vinyl chloride is more stable than saturated chloroalkanes to thermal pyrolysis, which is why nearly all vinyl chloride made commercially comes from thermal dehydrochlorination of EDC. When vinyl chloride is heated to 450°C, only small amounts of acetylene form. Litde conversion of vinyl chloride occurs, even at 525—575°C, and the main products are chloroprene [126-99-8] and acetylene. The presence of HCl lowers the amount of chloroprene formed. 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

Continuous emulsion copolymerization processes for vinyl acetate and vinyl acetate—ethylene copolymer have been reported (59—64). CycHc variations in the number of particles, conversion, and particle-size distribution have been studied. Control of these variations based on on-line measurements and the use of preformed latex seed particles has been discussed (61,62). 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

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