Chain polymerization by free radical

CHAIN POLYMERIZATION BY FREE RADICAL MECHANISM 2.4.1 General Kinetics... 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

Why is ethylene more readily polymerized by free radical chain polymerization than isobutylene ... 

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. 

We often want to prevent or retard free-radical reactions. For example, oxygen in the air oxidizes and spoils foods, solvents, and other compounds mostly by free-radical chain reactions. Chemical intermediates may decompose or polymerize by free-radical chain reactions. Even the cells in living systems are damaged by radical reactions, which can lead to aging, cancerous mutations, or cell death. 

Ethylene is also polymerized by free-radical chain-growth polymerization. With ethylene, the free-radical intermediates are less stable, so stronger reaction conditions are required. Ethylene is commonly polymerized by free-radical initiators at pressures around 3000 atm and temperatures of about 200 °C. The product, called low-density polyethylene, is the material commonly used in polyethylene bags. 

Radiation-induced polymerization, which generally occurs in liquid or solid phase, is essentially conventional chain growth polymerization of a monomer, which is initiated by the initiators formed by the irradiation of the monomer i.e., ion radicals. An ion radical (cation radical or anion radical) initiates polymerization by free radical and ionic polymerization of the respective ion. In principle, therefore, radiation polymerization could proceed via free radical polymerization, anionic polymerization, and cationic polymerization of the monomer that created the initiator. However, which polymerization dominates in an actual polymerization depends on the reactivity of double bond and the concentration of impurity because ionic polymerization, particularly cationic polymerization, is extremely sensitive to the trace amount of water and other impurities. 

Commercially, PFA is polymerized by free-radical polymerization mechanism usually in an aqueous media via addition polymerization of TFE and perfluoropropyl vinyl ether. The initiator for the polymerization is usually water-soluble peroxide, such as ammonium persulfate. Chain transfer agents such methanol, acetone and others are used to control the molecular weight of the resin. Generally, the polymerization regime resembles that used to produce PTFE by emulsion polymerization. Polymerization temperature and pressure usually range from 15 to 95°C and 0.5 to 3.5 MPa. 

Addition (or free radical) polymerization Polymerization in which monomers are added to the growing chains, initiated by free radical agents. 

Oxygen in the air oxidizes and spoils foods, solvents, and other compounds by free-radical chain reactions. Chemical intermediates may decompose or polymerize by free-radical chain reactions. Even the cells in living systems are damaged by radical reactions, which can lead to aging, cancerous mutations, or cell-death. We often want to prevent or retard free-radical reactions. Radical inhibitors are often added to food and chemicals to retard spoilage by radical chain reactions. Butylated hydroxyanisole (BHA) is often added to food as an antioxidant. It stops oxidation by reacting with radical intermediates to form a relatively stable free radical intermediate (BHA radical). The BHA radical can react with a second free radical to form an even more stable quinone with all its electrons paired (Scheme 4.64). 

Table 3.1, finally, introduces a summary for many monomers which are polymerizable by chain reactions. Listed are their ability to polymerize by free radical (R), cationic (C), anionic (A), or coordination reaction (Crd). Syntheses for many common macromolecules are found in this list. Now it is time to use this list to check the 22 polymers for the proper source-based and structure-based names, as in Sect. 1.2. Also, it is a suitable time to review the common applications, and properties of these polymers as known from the introductory chapter on polymer science. 

Polymerizations by free-radical mechanism are typical free-radical reactions. That is to say, there is an initiation, when the radicals are formed, a propagation, when the products are developed, and a termination, when the free-radical chain reactions end. In the polymerizations, the propagations are usually chain reactions. A series of very rapid repetitive steps follow each single act of initiation, leading to the addition of thousands of monomers. 

Compounds possessing allylic structures polymerize by free-radical mechanism only to low molecularweight oligomers. In some cases the products consist mostly of dimers and trimers. The DP for poly(allyl acetate), for instance, is only about 14. This is due to the fact that allylic monomer radicals are resonance-stabilized to such an extent that no extensive chain propagations occur. Instead, there is a large amount of chain transferring. Such chain transferring essentially terminates the reactions [151]. The resonance stabilization can be illustrated on an allyl alcohol radical ... 

Commercially, poly(vinyl acetate) is formed in bulk, solution, emulsion, and suspension polymerizations by free-radical mechanism. In such polymerizations, chain transferring to the polymer may be as high as 30%. The transfer can be to a polymer backbone through abstraction of a tertiary hydrogen ... 

Cyanoacrylates can be polymerized by free radical and ionic initiators (see Chain polymerization). In adhesive applications, ionic polymerization is by far the most important mode of chain growth. It is their marked susceptibility to initiation by anions and nucleophiles that is responsible for their usefulness as adhesives. The cyanoacrylate r-electron system is under the influence of two strongly electron-attracting groups. This results in a reduced electron density on the )8-carbon and enhanced susceptibility to nucleophilic attack (Scheme 2). 

Vinyl monomers containing P-lactam groups were prepared by ketene-imine [2 - - 2] cycloaddition followed by modification of the side chains, and by free radical polymerization gave polyacrylate P-lactams (Scheme 4.41). Cationic polymerization of dimethylketene catalyzed by aluminum tribromide with tetra- -butylammonium bromide in dichloromethane... 

In this section we present a short overview of the polymerization of VC. The most common method is polymerization by free radicals [305]. According to the ease of homolytic splitting of the n bond in the monomer, radical polymerization takes place in the presence of suitable initiation systems. In general, there are three methods for producing radicals available for the polymerization of VC (A) thermal cleavage of azo or peroxo compounds, (B) oxidation-reduction processes, and (C) metal alkyls in connection with oxygen. After the initiation step, chain growth takes place rapidly ... 

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