Liquid vinyl monomers

Fadner (42) reported preliminary results of electrically initiated polymerization experiments where liquid vinyl monomers are isolated from the electrodes. Liquid ethyl acrylate, for example, absorbed on a filter paper was polymerized in an alternating electric field. The filter paper was sandwiched between two layers of 1.5-mil Mylar film and placed between flat, parallel aluminum electrodes. Conversions of monomer to polymer ranged from 10 to 85% in the range between 20 and 240 sec at up to 25 kcps power. Acrylic acid and its ester polymerized most readily, others, such as styrene and vinyl chloride, resulted only in low yields in the same condition. 

In many processes (e.g. polymerizations, catalytic reactions) even trace amounts (< 50 ppm) of water can cause problems and the only practical solution for dehydration of these liquids is the use of an appropriate zeolite. For example, Stannet et al. (4 ) report on the use of molecular sieves to dehydrate liquid vinyl monomers prior to radiation-induced ionic polymerization. ... 

Mixtures of liquid vinyl monomers and polymer powders serve as a basis for a special group of engineering materials. Vinyl acetate and acrylonitrile monomers are injected into polymer powders to accelerate swelling and gel formation. Styrene is added to improved the molding characteristics of reactive mixtures. Polystyrene, poly(vinyl acetate), poly(vinyl chloride), etc. are the most commonly used polymer powders. 

Maleic anhydride [R1 equals -CH=CH- in Eq. (2.7) cis isomer] is reacted with aliphatic diols to form low molar mass unsaturated polyesters, UP. For molar masses higher than 1000 g/mol, products are diluted with a liquid vinyl monomer, most often styrene. This reactive mixture, generally called unsaturated polyester, UP resin, can be transformed into crosslinked polymers through a free-radical chain polymerization (see Sec. 2.3). 

Many different vinyl monomers (9) have been used to make wood-polymers during the past ten years, but methyl methacrylate (MMA) appears to be the preferred monomer for both the catalyst-heat and radiation processes. In fact, MMA is the only monomer that can be economically polymerized with gamma radiation. On the other hand, all types of liquid vinyl monomers can be polymerized with Vazo or peroxide catalysts. In many countries styrene and styrene-MMA mixtures are used with the Vazo or peroxide catalysts. 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

The development of the study of radiation-induced ionic polymerization of liquid vinyl monomers has been a story of the struggle against adventitious impurities. The contribution of several workers has led to the establishment of preparative techniques which yield reproducible data. 

Plasma-Initiated Polymerization and Copolymerization of Liquid Vinyl Monomers... 

Ellis and Rust solved this problem by dissolving the solid unsaturated polyesters in a liquid vinyl monomer, such as vinyl acetate.-2>12 However, the less volatile and more widely available styrene monomer was used instead of vinyl acetate for most of the fiber glass-reinforced "low pressure" polyester plastics that were produced during World War II and since that time. 

In 1978 Shen and Bell [36] developed a technique called plasma-initiated polymerization in which the plasma was used to initiate conventional polymerization of liquid vinyl monomers, resulting in soluble linear polymers with high or even ultrahigh molecular weights [37]. Moreover, we found that if the polymerization conditions were set up appropriately, the polymer molecules would be aligned and packed more regularly, even leading to the formation of crystallized products [38]. 

It is possible to cross-link unsaturated linear polyester chains directly one to another however, reaction is slow and a low degree of cross-linking is achieved. These limitations are overcome by the introduction of a material which forms bridges between the chains. The materials most commonly used to cross-link unsaturated linear polyesters in this way are vinyl monomers. The addition of a liquid vinyl monomer to the polymer also leads to a reduction in viscosity and this facilitates the impregnation of glass-fibre in the preparation of laminates. 

Commercially, the degree of polymerization x is maintained low (say 8-10 repeat units) so that the product is a viscous liquid. The linear, unsaturated polyester is then diluted with a liquid vinyl monomer, most often styrene. Before use, an initiator chemical that promotes addition polymerization (as discussed in Chapter 10) is added, causing the vinyl monomer to undergo addition copolymerization with the double bonds in the polyester (which has a fimctionahty of 16-20, twice the number of double bonds). This forms a highly cross-linked rigid network. 

In the case of liquid vinyl monomers and related ones, the value of the enthalpy of polymerization is generally in the range -30 to -155kJ-mol it is definitely lower (in absolute value) for heterocycles. 

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