Neutral comonomers

Neutral comonomers Anionic comonomers Cationic comonomers... 

An example of a continuous aqueous dispersion process is shown in Figure 12.5 [92]. A monomer mixture composed of acrylonitrile and up to 10% of a neutral comonomer, such as methyl acrylate or vinyl acetate, is fed continuously. Polymerization is initiated by feeding aqueous solutions of potassium persulfate (oxidizer), sulfur dioxide (reducing agent), ferrous iron (promoter), and sodium bicarbonate (buffering agent). The aqueous and monomer feed... 

The choice of comonomers (e.g. acrylic or itaconic acids) can influence the relative ease of processing (Chapter 4). Preferred neutral comonomers for AN would be MA and MMA. A comonomer breaks up the structure and could be termed a plasticiser rendering the pol5mer more readily soluble in the spinning solvent and improving the quality of spinning. 

Virtually all acrylic fibers are made from acrylonitrile combined with at least one other monomer. The comonomers most commonly used are neutral comonomers. 

Both acrylic and modacrylic fibres are based on atactic polyacrylonitrile. The generic name acrylic fibre refers to fibres made from linear copolymers that consist of not less than 85 wt % acrylonitrile units. The majority of commercial acrylic fibres contain between 5 and 8% of neutral comonomers, namely vinyl acetate, methyl acrylate or methyl methacrylate. In addition, smaller quantities of various ionic comonomers e,g, sodium styrenesulfonate) are used to provide, together with the ionic end-groups formed from sulfonate and sulfate initiators, the dye sites in the fibres. 

Most commercial processes involve copolymerization of ethylene with the acid comonomer followed by partial neutralization, using appropriate metal compounds. The copolymerization step is best carried out in a weU-stirred autoclave with continuous feeds of all ingredients and the free-radical initiator, under substantially constant environment conditions (22—24). Owing to the relatively high reactivity of the acid comonomer, it is desirable to provide rapid end-over-end mixing, and the comonomer content of the feed is much lower than that of the copolymer product. Temperatures of 150—280°C and pressures well in excess of 100 MPa (1000 atm) are maintained. Modifications on the basic process described above have been described (25,26). When specific properties such as increased stiffness are required, nonrandom copolymers may be preferred. An additional comonomer, however, may be introduced to decrease crystallinity (10,27). 

Buffers are frequently added to emulsion recipes and serve two main purposes. The rate of hydrolysis of vinyl acetate and some comonomers is pH-sensitive. Hydrolysis of monomer produces acetic acid, which can affect the initiator, and acetaldehyde which as a chain-transfer agent may lower the molecular weight of the polymer undesirably. The rates of decomposition of some initiators are affected by pH and the buffer is added to stabilize those rates, since decomposition of the initiator frequently changes the pH in an unbuffered system. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4—5, eg, with phosphate or acetate, but buffering at neutral pH with bicarbonate also gives excellent results. The pH of most commercially available emulsions is 4—6. 

Ionomers are made in a two-stage process. In the first step, we copolymerize ethylene with small amounts of an organic acid containing a vinyl group, such as acrylic or methacrylic acid, in a high pressure reactor. In the second step, we neutralize the acid comonomers to form metal salts. We can create ionomers with a variety of metal salts, including sodium, calcium, and zinc. 

Unsaturated polyesters that are terminated by carboxylic acid groups at both ends of the chain after neutralization are efficient emulsifiers for lipophilic monomers [110] and thus act as self-emulsifying crosslinking agents in the ECP of these systems. Normal emulsions of EUP and comonomers have a white, milky appearance. With an appropriate structure and molar mass of the EUP and within a certain range of EUP/comonomer ratios, however, microemulsions are... 

By first dispersing the EUP in water containing the base for neutralization of the carboxyl acid groups of the EUP and then adding the comonomer with intensive stirring, normal emulsions are obtained. They are favorable because, with multiple emulsions, insoluble polymers are formed, which decrease the yield of microgels. 

Apart from the kind of components used in preparing microgels from EUP and comonomers, the yield essentially depends on the composition of the reactive components, on the water/monomer ratio, the W/M (serum ratio), the degree of neutralization of the EUP [91] and on the concentration of electrolytes. 

Base copolymers with dicarboxylic acid comonomers, even those in which one acid radical has been esterified, when neutralized with metal ions, which have two or more ionized valences, result in intractable ionic copolymers at the level of neutralization essential to obtain significant improvement in solid state properties. 

Similarly, base copolymers with mono-carboxylic acid comonomers result in intractable ionic copolymers when neutralized to the indicated degree with metal ions, which have four or more ionized valences. It is believed that the nature of the ionic bond in these instances is too strong to be suitable for the formation of ionic copolymers, which exhibit solid state properties of crosslinked resins and melt properties of uncrosslinked resins. 

Figure la shows results for MMA/DMA gels with different comonomer ratios. Gels were swollen in buffered media with I = 0.1 M. As expected, swelling decreases as the content of MMA increases and of DMA decreases. This is explained by noticing that (1) the gel becomes more hydrophobic, and (2) the density of ionizable groups decreases. More interesting is the pH-dependent behavior. At neutral and alkaline pH only minimal ( < 10% w/w) water uptake... 

Manufacture and Processing. Most commercial processes involve copolymerizaiion of ethylene with the acid comonomer followed by partial neutralization, using appropriate metal compounds. 

For different applications, water-soluble neutral and ionic comonomers can be incorporated into or attached to the PNIPAM chain backbone to form amphiphilic PNIPAM copolymers via free-radical copolymerization. In this section, we will use the folding of neutral PNIPAM amphiphilic copolymer chains in extremely dilute solutions ( pg/mL) to illustrate a general feature of the folding of hydrophilically modified copolymer chains. 

Using this approach, hydrophilic (neutral or ionic) comonomers, such as end-captured short polyethylene oxide (PEO) chains (macromonomer), l-vinyl-2-pyrrolidone (VP), acrylic acid (AA) and N,N-dimethylacrylamide (DMA), can be grafted and inserted on the thermally sensitive chain backbone by free radical copolymerization in aqueous solutions at different reaction temperatures higher or lower than its lower critical solution temperature (LCST). When the reaction temperature is higher than the LOST, the chain backbone becomes hydrophobic and collapses into a globular form during the polymerization, which acts as a template so that most of the hydrophilic comonomers are attached on its surface to form a core-shell structure. The dissolution of such a core-shell nanostructure leads to a protein-like heterogeneous distribution of hydrophilic comonomers on the chain backbone. 

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