Polymer chain modifiers

I11 addition to the primary effect of the great length of the molecule, the details of the distribution of functional groups along tlie polymer chain modify the behavior of these materials. This leads to differences in their applications. For example, natural rubber exists in the rubbery state at... 

The chains terminated by recombination are larger than those terminated by disproportionation. Deactivation of the growing chain radical always occurs through chain transfer termination, i.e., by abstraction of an atom from another molecule such as the initiator, monomer, solvent, completed polymer chain, modifier, or impurities, and thus the propagating chain radical becomes saturated. The molecule from which the atom has been abstracted will then become a free radical and may or may not start a new chain. Accordingly, the rate of polymerization does not decrease but the molecular weight decreases. 

The third electroviscous effect is due to the change of shape of suspending particles when their electrical free energy is modified by ionization and the presence of neutral salts. If a polymer molecule can undergo ionization, e.g. by reaction with a base or by reaction with some other ion-producing substance, electrostatic repulsion between the like charges introduced on the polymer chain modifies the partial molecular free energy of the polymer in the solution. With polymeric... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

This compound is soluble in most organic solvents and may be easily copolymerized with other vinyl monomers to introduce reactive side groups on the polymer chain (18). Such reactive polymer chains may then be used to modify other polymers including other amino resins. It may be desirable to produce the cross-links first. Thus, A/-methylolacrylamide can react with more acrylamide to produce methylenebisacrylamide, a tetrafunctional vinyl monomer. 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Hydrophobic fibers are difficult to dye with ionic (hydrophilic) dyes. The dyes prefer to remain in the dyebath where they have a lower chemical potential. Therefore nonionic, hydrophobic dyes are used for these fibers. The exceptions to the rule are polyamide and modified polyacrylonitriles and modified polyester where the presence of a limited number of ionic groups in the polymer, or at the end of polymer chains, makes these fibers capable of being dyed by water-soluble dyes. 

During emulsion polymerization, a high conversion of monomer to polymer produces cross-linked rubber which is insoluble. To obtain a high conversion in the polymerization reaction and a processable polymer, suitable polymer modification should be made. The use of sulphur moieties allows this goal to be reached [2]. Sulphur-modified polychloroprenes contain di- and polysulphide sequences in the polymer chains. After the polymerization reaches the desired degree, reaction is stopped by adding thiuram disulphide ... 

Mercaptan-modified polychloroprenes only contain sulphur at the ends of the polymer chain. Polymerization is carried out in the presence of a mercaptan to act... 

The most important practical application of the organometallic complex photoinitiators is the possibility of using these types of initiators in modifying the pre-existing polymer chain, e.g., block, graft, and crosslinked copolymers preparation. 

A new process to develop interface vulcanization is grafting of selective accelerators onto a polymer chain, which in the subsequent process of vulcanization acts as an effective cure accelerator for the second polymer component in the blend. Beniska et al. [6] prepared SERFS blends where the polystyrene phase was grafted with the accelerator for curing SBR. Improved hardness, tensile strength, and abrasion resistance were obtained. Blends containing modified polystyrene and rw-1,4-polybutadiene showed similar characteristics as SBS triblock copolymers. 

Although, the heat resistance of NBR is directly related to the increase in acrylonitrile content (ACN) of the elastomer, the presence of double bond in the polymer backbone makes it susceptible to heat, ozone, and light. Therefore, several strategies have been adopted to modify the nitrile rubber by physical and chemical methods in order to improve its properties and degradation behavior. The physical modification involves the mechanical blending of NBR with other polymers or chemical ingredients to achieve the desired set of properties. The chemical modifications, on the other hand, include chemical reactions, which impart structural changes in the polymer chain. 

The modified NBR samples were characterized by differential scanning calorimetry [11,78-80,98]. The glass-transition temperature (T ) decreased with the level of hydrogenation. In the case of HFNBR, Tg increased with an increase in the addition of aldehyde groups to the polymer chain. Thermogravimetric analysis of the modified polymers have also been carried out [15]. 

The efficiency of PPV may also be raised by introducing disorder into the polymer chains. The crystallinity of PPV may be lowered by employing a modified Wessling method utilizing a xanthate leaving group 63J. PPV produced by this method is believed to contain a mixture of cis- and rram-alkenc units. The efficiency of the material is 0.22% when employed in a single layer device with... 

Insertion of flexible blocks into the stiff polymer chain of PAN ensures the possibility of raising the resistance of modified PAN fibres to multiple deformations and, especially, increases significantly the abrasive resistance of fibres made of block copolymers. 

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