Polymerization of acetals

A common feature of all lipids is that, biologically, their hydrocarbon content is derived from the polymerization of acetate followed by reduction of the chain so formed. (However, this process also occurs for the synthesis of some compounds that are not lipids and therefore cannot be used as a definition of lipids.) For example, polymerization of acetate can give rise to the following ... 

Thus, halonium ions (e.g. Et2Cr, A ) can initiate polymerization of acetals, ethers, sulfides, or amines. Oxonium ions also initiate more or less successfully the polymerization of these monomers while ammonium ions can only be applied in the polymerization of some cyclic amines. 

In the polymerization of several cyclic acetals and ethers autoacceleration was observed. There might be a number of reasons for this behaviour, and some of these have already been discussed, namely the preinitiation equilibria and the inequality ki enhanced reactivity of the hydroxy end group in polyacetals toward active species (in comparison with acetal groups) is another, not yet considered, reason for induction periods. When the pol5mierization d ee increases with conversion the proportion of the active tertiary oxonium ions also increases, at the cost of the less reactive secondary oxonium ions (cf. Ref. 164), because the b k-biting or intermolec-ular transfer become more important than the end-biting. Thus, there b no need to make speculative assumptions about the nature of the active species and to propose the two stage polymerization of acetals in order to explain the induction... 

A number of studies on the polymerization of acetal derivatives of sugars by means of Lewis catalysts,166,167 or 6-O-alkene derivatives,168 have been published. 

Ethylene-vinyl acrylate resins n. Co-polymers of the polyolefins family derived from random co-polymerization of acetate and ethylene. 

Commercial polymerization of acetal homopolymer starts with anhydrous formaldehyde monomer from formaldehyde solution. Water is evaporated from the aqueous solution, forming paraformaldehyde, poly-oxymethylene, and hemiformal, which are purified and thermally decomposed to form anhydrous formaldehyde [6]. Methanol and formic acid are removed, either by freeze-trapping at a temperature just above the boiling temperature of formaldehyde or by washing with a nonvolatile polyol [6]. The anhydrous formaldehyde is fed into a reactor containing the inert hydrocarbon solvent, initiator, and dispersant, where the... 

Figure 13.7 ADMET polymerization of acetal and alcohol dienes.

Acrolein produced in the United States is stabilized against free-radical polymerization by 1000—2500 ppm of hydroquinone and is protected somewhat against base-catalyzed polymerization by about 100 ppm of acetic acid. To ensure stabiUty, the pH of a 10% v/v solution of acrolein in water should be below 6. 

Vinyl alcohol does not exist as a monomer, but Herrmann and Haehnel (1) were able to obtain the desired product poly(vinyl alcohol) [9002-89-5] (PVA), by polymerizing vinyl acetate and then hydrolyzing the resultant poly(vinyl acetate). This process is employed for the commercial production of PVA even now. The principal concern of the discoverers was development of a suture for surgical operations the fiber then obtained was not suited for clothing use (2). 

PVA used for the manufacture of fiber generally has a degree of polymerization of about 1700 and, for general-purpose fiber, a high degree of hydrolysis of vinyl acetate units of at least 99 mol %. 

Cyclic ether and acetal polymerizations are also important commercially. Polymerization of tetrahydrofuran is used to produce polyether diol, and polyoxymethylene, an excellent engineering plastic, is obtained by the ring-opening polymerization of trioxane with a small amount of cycHc ether or acetal comonomer to prevent depolymerization (see Acetal resins Polyethers, tetrahydrofuran). 

Methyl Vinyl Ketone. Methyl vinyl ketone [78-94-4] (3-buten-2-one) is a colorless Hquid with a pungent odor. It is stable only below 0°C, and readily polymerizes on standing at room temperature. It can be inhibited for storage and transportation by a mixture of acetic or formic acid and hydroquinone or catechol (266). This ketone is completely soluble in water, and forms a binary azeotrope with water (85 MVK 15 H2O vol %) at 75.8°C. 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

In studies of the polymerization kinetics of triaUyl citrate [6299-73-6] the cyclization constant was found to be intermediate between that of diaUyl succinate and DAP (86). Copolymerization reactivity ratios with vinyl monomers have been reported (87). At 60°C with benzoyl peroxide as initiator, triaUyl citrate retards polymerization of styrene, acrylonitrile, vinyl choloride, and vinyl acetate. Properties of polyfunctional aUyl esters are given in Table 7 some of these esters have sharp odors and cause skin irritation. 

Catalysts. Silver and silver compounds are widely used in research and industry as catalysts for oxidation, reduction, and polymerization reactions. Silver nitrate has been reported as a catalyst for the preparation of propylene oxide (qv) from propylene (qv) (58), and silver acetate has been reported as being a suitable catalyst for the production of ethylene oxide (qv) from ethylene (qv) (59). The solubiUty of silver perchlorate in organic solvents makes it a possible catalyst for polymerization reactions, such as the production of butyl acrylate polymers in dimethylformamide (60) or the polymerization of methacrylamide (61). Similarly, the solubiUty of silver tetrafiuoroborate in organic solvents has enhanced its use in the synthesis of 3-pyrrolines by the cyclization of aHenic amines (62). 

Besides direct hydrolysis, heterometaHic oxoalkoxides may be produced by ester elimination from a mixture of a metal alkoxide and the acetate of another metal. In addition to their use in the preparation of ceramic materials, bimetallic oxoalkoxides having the general formula (RO) MOM OM(OR) where M is Ti or Al, is a bivalent metal (such as Mn, Co, Ni, and Zn), is 3 or 4, and R is Pr or Bu, are being evaluated as catalysts for polymerization of heterocychc monomers, such as lactones, oxiranes, and epoxides. An excellent review of metal oxoalkoxides has been pubUshed (571). 

Emulsion Polymerization. Poly(vinyl acetate)-based emulsion polymers are produced by the polymerization of an emulsified monomer through free-radicals generated by an initiator system. Descriptions of the technology may be found in several references (35—39). 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Bulk Polymerizations. In the bulk polymerization of vinyl acetate the viscosity increases significantly as the polymer forms making it difficult to remove heat from the process. Low molecular weight polymers have been made in this fashion. Continuous processes are known to be used for bulk polymerizations (68). 

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. 

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). 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

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