Polymerization by free-radicals

Olig omerization and Polymerization. Siace an aHyl radical is stable, linear a-olefins are not readily polymerized by free-radical processes such as those employed ia the polymerization of styrene. However, ia the presence of Ziegler-Natta catalysts, these a-olefins can be smoothly converted to copolymers of various descriptions. Addition of higher olefins during polymerization of ethylene is commonly practiced to yield finished polymers with improved physical characteristics. 

Pyrazolecarbinols can be dehydrated to vinylpyrazoles, (438) — (446) (72JHC1373), or transformed into chloromethyl derivatives (81T987). Compound (440 R = CH2C1) thus prepared is the starting material for the synthesis of the macrocycles (226)-(228) (Section 4.04.2.1.2(vi)). Vinyl- and ethynyl-pyrazoles have been extensively studied (B-76MI40402) and many vinylpyrazoles are polymerized by free radical initiators. 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

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. 

Example 13.7 A 50/50 (molar) mixture of st5Tene and acrylonitrile is batch polymerized by free-radical kinetics until 80% molar conversion of the monomers is achieved. Determine the copolymer composition distribution. 

This polymeric lipid can first be polymerized by free radical initiator in organic solutions before making the vesicles. The proton NMR spectrum of the polymerized lipid shows that vinyl protons of the cyclic acrylate between 85.00 ppm and 86.00 ppm disappeared from the spectrum, compared with that of monomeric lipid. Also in the IR spectrum (Figure 6) the absorption peak at 1670 cm"1 for the cyclic acrylate carbon carbon double bond disappeared as the result of polymerization. The carbonyl absorptions of the esters at 1740 cm 1 and the lactone at 1805 cm"1 still remain in the spectrum. 

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. 

Up to the present time, use has been made almost entirely of dimethacrylates which are polymerized by free radical mechanisms to yield crosslinked products (81. Polymerization is initiated by redox systems, such as benzoyl peroxlde/aromatic amine, and by... 

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

The vinyl monomer and benzene were mixed and polymerized by free radicals. The isolated polymer had a number average molecular weight M = 10 and an activity of aP = 18.5 d/sec/g. 

Curing or hardening of organic protective coatings (paints, inks) by exposure to infrared, UV, or electron-beam radiation. Required are a monomer or oligomer and a photoninitiator. which induces polymerization by free radical formation. 

Step 3—In a separate step, styrene-acrylonitrile (SAN) resin is prepared by emulsion, suspension, or mass polymerization by free-radical techniques. The operation is carried out in stainless-steel reactors operated at about 75°C (167°F) and moderate pressure for about 7 hours. Tlie final chemical operation is the blending of the ABS graft phase with the SAN resin, plus adding various antioxidants, lubricants, stabilizers, and pigments. Final operations involve preparation of a slurry of fine resin particles (via chemical flocculation), filtering, and drying in a standard fluid-bed dryer at 121-132°C (250-270°F) inlet air temperature. 

Meth)acrylates are polymerized by free-radical polymerization. Because of their high reactivity, acrylate double bonds are preferred their polymerization rate is 10 times faster than that of methacrylate monomers. 

Vinyl fluoride is polymerized by free radical processes as most common commercial fluororopolymers, but it is more difficult to polymerize than TFE or VDF and requires higher pressures.62 The temperature range for the polymerization in aqueous media is... 

Generally, macromonomers are (co)polymerized by free-radical processes owing to convenient experimental conditions, availability of a large number of comonomers, and insensitivity of most chemical functions to the polymerization conditions. Nevertheless, some macromonomers with a suitable end group have been (co)polymerized by cationic polymerization. Provided that living cationic polymerization conditions are applied, well-defined graft homopolymers or copolymers can be prepared with a predetermined and uniform number of branches. 

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. 

When the less hindered 2,4-tolylene diisocyanate is reacted with a phospholene oxide catalyst linear oligomeric carbodiimides are obtained which have been reacted with a variety of nucleophiles to give poly(ureas), poly(acyl ureas), poly(formamidines) and poly-(guanidines) by addition across the N=C=N group. Also, reaction of the oligomeric carbodiimides with acrylic or methacrylic acid affords linear polymers, which can be further polymerized by free-radical type processes. Also, reaction of the carbodiimide oligomers obtained from 2,4-TDI with adipic acid in DMF produces a polyureid. ... 

When the styrene is polymerized by free-radical initiation, it reacts by adding across the double bonds of other styrene and rubber units, and the resulting product contains polystyrene grafts on the rubber as well as ungrafted rubber and polystyrene molecules. This mixture has better impact resistance than unmodified polystyrene. 

A linear polymer is one in which each repealing unit is linked only to two others. Polystyrene (1-1), poly(methyl methacrylate) (1-34), and poly(4-methyl pentene-1) (1-35) are called linear polymers although they contain short branches which arc part of the monomer structure. By conirast, when vinyl acetate is polymerized by free-radical initiation, the polymer produced contains branches which were not present in the monomers. Some repeating units in these species are linked to three or four other monomer residues, and such polymers would therefore be classified as branched. 

The ether oxygen is a Lewis base (electron donor), and polymerization of vinyl and cyclic ethers can be initiated by reaction with an ion pair comprising an acidic cation and a weakly nucleophilic base. These monomers do not polymerize by free-radical or anionic processes. Thio ethers behave similarly. 

Isobutene is polymerized commercially by a cationic mechanism initiated by strong acids like AICI3. It is not polymerized by free-radicals or anionic initiators. Acrylonitrile is polymerized commercially by free-radical means. It can also be polymerized by anionic initiators like potassium amide but does not respond to cationic initiators. Account for the difference in behavior of isobutene and acrylonitrile in terms of monomer structure. 

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. 

Styrene is one of the few monomers that may be polymerized by free-radical, anionic, cationic, or coordination (Ziegler-Natta) methods. This property, common to styrene and most of its derivatives, is the consequence of the availability of a benzylic position in these monomers, which is capable of stabilizing a radical, carbanionic, or carbocationic center, as well as possessing a polarizability amenable to the charge distributions required by coordination methods of polymerization. 

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. 

Emulsion polymerization is a heterogenous reaction process in which unsaturated monomers are dispersed in continuous phase with the aid of emulsifiers and polymerized by free-radical initiators. The resulting product is a dispersion of polymer particles, typically smaller than 1 pm in size, in water and is referred to as polymer latex. 

The previous section demonstrated the utility of polymerization reactions in aqueous systems. Carrying out transition metal-catalyzed polymerizations in aqueous systems is of great interest as they can afford a variety of new materials not accessible by established free radical techniques. Thus, various monomers subject to catalytic polymerization can not be polymerized by free radical polymerization, and catalytic polymerization also offers access to other polymer microstructures. 

Manufacture A conventional methacrylate monomer mixture is polymerized by free-radical initiation in an oil solution of about 10-15% of an OCP VI improver at a temperature of 120-140°C. This part of the reaction is carried out at a fiilly converted polymer solids content of 40-50%. When methacrylate conversion is complete, additional solid OCP VI improver is added to bring the total OCP content up to the desired level. The final product is then diluted with ester solvent to invert the phases, and sufficient additional mineral oil is added to reach the final product solids. 

---