Polymers free radical polymerization

A number of different materials were used as chain transfer agents to control molecular weight. These results are shown in Table 6.1. The effect of varying concentration of t-butyl alcohol and reaction temperature is shown in Figure 6.1. The results are consistent with normal free radical polymerizations. Polymer output was characterized by inherent viscosity and ZST tests. 

Most free radical polymerized polymers exhibit a preponderance of syndiotactic diads. The syndiotacticity normally increases with decreasing temperature. This phenomenon has lead some authors to conclude that the stereocontrol is governed by the bulkiness of the substituent. Implicit in this argument are the assumptions that only repulsive forces... 

Nitroxide-Mediated Controlled Radical Polymerization (NMCRP) was first discovered by Solomon et al., who patented their discovery in 1985 [205]. This opened up new pathways in the field of free-radical polymerization. Polymer architectures, which were the domain of the anionic polymer chemist, became accessible to the free-radical polymer chemist. However, it was not until the work of Georges et al. [206] was published in 1993, that the world of polymer chemistry became aware of the possibihties of this new class of free-radical polymerization. This was the beginning of what is today one of the leading topics in free-radical polymer chemistry Controlled or Living Free Radical Polymerization. This initiated the search for new Controlled or Living Free Radical Polymerization techniques, and soon afterwards other methods (which will be discussed later) were developed. 

Tip 2 Chain stereoregularity and active sites. In free radical polymerization, polymer chain configuration and MWD are often independent of initiator type and initiation mechanism, depending strongly on reaction temperature, initiation rate, and monomer concentration. One can, therefore, often predict chain stereoregularity and MWD without a detailed knowledge of the initiation mechanism. 

S. Blomberg, S. Ostberg, E. Harth, A.W. Bosman, B. van Horn, C.J. Hawker, Production of crosslinked, hollow nanoparticles by surface-initiated living free-radical polymerization,/. Polym. Sci., Part A Polym. Chem. 2002, 40,1309-1320. 

A. Kasseh, A. Ait-Kadi, B. Riedl, J.F. Pierson, Organic/inorganic hybrid composites prepared by polymerization compounding and controlled free radical polymerization. Polymer 2003, 44, 1367-1375. 

Berger, K.C. and Meyerhoff, G., 1989. Propagation and Termination Constants in Free-radical polymerization. Polymer Handbook. Immergut. New York, Wiley-Intercsience II/67-II/79. 

Bignozzi, M. C., et al. (1999). Lithographic results of electron beam photoresists prepared by living free radical polymerization. Polym. Bull., 45(1) 93-100. 

Gabaston, L.I., Furlong, S.A., Jackson, R.A. and Armes, S.P. (1999) Direct synthesis of novel acidic and zwitterionic block copolymers via TEMPO-mediated living free-radical polymerization. Polymer, 40,4505-4514. 

With respect to the above it is noteworthy that Kent et al. [107] performed their study using narrow polydispersity probe and matrix polymers. The insensitivity of Rg versus polymer concentration below C only occurs if the molar mass of the probe and background polymer are similar. If the matrix polymer is of much lower molar mass, it can freely penetrate the probe polystyrene molecules and act as a poor viscous solvent inside the probe coils. In that case a decrease in Rg can also be observed at polymer concentration below C [107], In real free-radical polymerizations, polymer molecules with a wide variety of molar masses will be present simultaneously and it can thus be expected that all macroradicals will experience coil contraction to some extent in dilute solutions (except for the very smallest macroradicals). The magnitude of this effect will thus be dependent upon the molecular weight distribution (MWD) of the polymer and thus also upon the systems polymerization history. 

Tonge MP, Kajiwara A, Kamachi M, Gilbert RG. ESR measurements of the propagation rate coefficient for styrene free radical polymerization. Polymer 1998 39 2305-2313. 

Pasquale, A. J., Long, T. E., Synthesis of star-shaped polystyrenes via nitrogen-mediated stable free-radical polymerization /. Polym. Sci. A, Polym Chem. (2000) 39, pp. 216-223... 

Kartavykh, V R, Y. N. Barantsevice, V A. Lavrov, and S. S. Ivanchev, The Effect of the Change in Volume of the Reaction System on the Kinetics of Free-Radical Polymerization, Polym. Sci. USSR, 22, 1319-1325, 1980. 

Polymeric vinylidene chloride generally produced by free radical polymerization of CH2 = CCl2. Homopolymers and copolymers are used. A thermoplastic used in moulding, coatings and fibres. The polymers have high thermal stability and low permeability to gases, and are self extinguishing. 

Barton J 1996 Free-radical polymerization in inverse microemulsions Prog. Polym. Sc/. 21 399-438... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

The elastomer produced in greatest amount is styrene-butadiene rubber (SBR) Annually just under 10 lb of SBR IS produced in the United States and al most all of it IS used in automobile tires As its name suggests SBR is prepared from styrene and 1 3 buta diene It is an example of a copolymer a polymer as sembled from two or more different monomers Free radical polymerization of a mixture of styrene and 1 3 butadiene gives SBR... 

Section 1117 Polystyrene is a widely used vinyl polymer prepared by the free radical polymerization of styrene... 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

The molecular weight distribution for a polymer like that described above is remarkably narrow compared to free-radical polymerization or even to ionic polymerization in which transfer or termination occurs. The sharpness arises from the nearly simultaneous initiation of all chains and the fact that all active centers grow as long as monomer is present. The following steps outline a quantitative treatment of this effect ... 

That the Poisson distribution results in a narrower distribution of molecular weights than is obtained with termination is shown by Fig. 6.11. Here N /N is plotted as a function of n for F= 50, for living polymers as given by Eq. (6.109). and for conventional free-radical polymerization as given by Eq. (6.77). This same point is made by considering the ratio M /M for the case of living polymers. This ratio may be shown to equal... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Acrylamide—acrylic polymers are made by free-radical polymerization of monomers containing the acryHc stmcture, where R is —H or —CH and is —NH2 or a substituted amide or the alkoxy group of an ester. 

Allyl polymers are made by free-radical polymerization of diaHyl compounds, most frequently diallyl dimethyl ammonium chloride (DADMAC) [7398-69-8] forming a chain containing a five-membered ring (28) poly(DADMAC) [26062-79-3]. 

Photoinitiation. Since photolysis of polysdanes generates sdyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated organic monomers (135,136). Though about one-tenth as efficient as other organic photoinitiators, polysdanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

Free-radical polymerization processes are used to produce virtually all commercial methacrylic polymers. Usually free-radical initiators (qv) such as azo compounds or peroxides are used to initiate the polymerizations. Photochemical and radiation-initiated polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of methacrylic monomers is first-order with respect to monomer concentration, and one-half order with respect to the initiator concentration. Rate data for polymerization of several common methacrylic monomers initiated with 2,2 -azobisisobutyronitrile [78-67-1] (AIBN) have been deterrnined and are shown in Table 8. 

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