Radicals polymerization

Radical polymerization was used on acrylamide adsorbed onto a PDMS surface. The hydrophilic surface thus generated had a twenty-fold improvement in resisting irreversible adsorption of lysozyme and was stable against reorganization to a hydrophobic surface for at least a month. 

1 Free-Radical Polymerization Using Flow Microreactor Systems 

The effects of mixing in radical polymerization of MMA are interesting [168]. The use of a 5 mm static mixer leads to fouling in the reactor. In contrast, the use of an interdigital multilamination micromixer with 36 lamellae of 25 pm thickness results in a reduction in fouling. This numbering-up approach enables production of 2,000 tons per year without the fouling problem [169]. 

Serra and coworkers studied the outstanding effect of mixing on conversion, molecular weight and polydispersity in free-radical polymerizations of styrene by a numerical simulation using different micromixer geometries [170, 171]. 

Latex production by miniemulsion polymerizations [172-174] in continuous tubular reactors has also been reported by McKenna and coworkers [175]. 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

Alternatively, liquid phase polymerization (in bulk monomer at a temperature of 20° C) furnishes an isomer (II) characterized by a cis-transoid (or trans-cisoid) configuration of the main chain, with carboxyl groups located on both sides of it. These isomers will be shown later to differ in chemical and physicochemical properties. 

In order to model radical polymerization kinetically, the various reactions— initiation, propagation, and termination— must be understood. 

By convention, the initiation step consists of two elementary reactions  

Combination of these primary radicals with a single monomer molecule, as in the formation of 

This section focuses on heat-sensitive initiators, primarily because of their overwhelming usage in industry. The homolytic decomposition of initiator molecules can be represented schematically as follows  

Equations (5.2.2) and (5.2.4) imply that aU the radicals generated by the homolytic decomposition of initiator molecules, I2, are used in generating the polymer chain radicals Pi, and no primary radicals are wasted by any other reaction. This is not true in practice, however, and an initiator efficiency is defined to take care of the wastage of the primary radicals. 

To determine the several rate constants in the photopolymerization of vinyl acetate. 

The reaction was followed dilatometrically under steady illumination at 15.9 C. The fractional volume contraction —AVjV at times t is given in table 1. 

The density of vinyl acetate is 0.86 g cm and, since its molecular weight is 86, its molar volume is 100 cm.  

In the presence of para-benzoquinone the reaction does not begin until a certain time h after which the rate has its normal value. Table 2 gives values of the time ti for several values of w the mass of quinone added to 13 cm of vinyl acetate. 

These mejisurements were made at 25 °C with illumination of the same intensity as in the dilatometric measurements. 

In this reaction scheme, the initiating species might be a free radical, anion, cation, or some more complex species. Broadly speaking, free radical initiators are produced by homolysis, i.e. the breaking of a covalent bond in such a way that its electrons are shared equally between the two fragments of the original molecule. Ionic initiators may function by the heterolytic scission of a covalent 

From a chain structure standpoint, the stereochemical nature of the propagating species and the role that the propagating centre plays in deter- 

The activation energies for syndiotactic and isotactic propagation only differ by approximately 1 kJ/mol for propagating radicals, i.e. there is little preference for either stereochemical isomer, and the polymers produced in radical polymerizations are atactic. 

Mechanistically, radical polymerizah ons are addition polymerizations, and they follow the exact same radical chain process discussed in Chapters 10 and 11. The key steps of initiation, propagation, and termination are again involved, as recapitulated in Figme 13.13 B. Based upon studies of substituents on the carbon beta to the site of radical addition, it has been concluded that the adding radical has almost completely formed a bond at the transition state. Secondary isotope effects support this notion of a late transition state, with values between 1.05 to 1.17 for deuterium at the site beta to addition. 

As noted above, the key factor influencing polymer length in these chain reactions is the competition between propagation and termination. One can expect that it will be crucial to exclude any impurities that can react with the growing radical chain and terminate the polymerization, if you want high MW polymer. On the other hand, if smaller molecular weights are desired, judicious addition of inhibitors and chain transfer agents could be useful. 

A wide range of initiators are employed in radical polymerization chemistry. These include organic peroxides such as benzoyl peroxide, azo compounds such as azoisobutyroni-trile [AIBN, Me2C(CN)N=NC(CN)Me2], redox agents such as persulfates or soluble metal ions, heat, radiation, and electrolysis. 

Methyl methacrylate Poly(methyl methacrylate) (PMMA)—Lucite, Plexiglas, and Perspex 

While living polymerizations can be exploited to produce block copolymers, a copolymerization should give polymer chains that contain both monomers distributed throughout. You might expect that a radical chain polymerization would give a truly random copolymer. Radicals are quite reactive and not known for their selectivity. In fact, though, radical copolymerizations are not totally random, and some quite distinctive polymer compositions can be achieved. Consider a radical polymerization progressing in the presence of two different monomers. 

The general reaction scheme for fi ee-radical polymerization can be expressed as follows  

Physical Chemistry of Macromolecules Basic Principles and Issues, Second Edition. By S. F. Sun ISBN 0-471-28138-7 Copyright 2004 John Wiley Sons, Inc. 

Most of the initiators are peroxides and aliphatic azo compounds, such as the following  

Styrene has been described as a model monomer for radical homopolymerization of hydrophobic monomers in many reports. The polymerization of acrylates and methacrylates is also well known. It could be also shown that the miniemulsion process also easily allows polymerization of the ulhahydrophobic monomer lauryl methacrylate without any carrier materials as are necessary in emulsion polymer- 

It has been shown that the principle of aqueous miniemulsions could be transferred to nonaqueous media [19]. Here, polar solvents, such as formamide or glycol replace water as the continuous phase, and hydrophobic monomers are miniemulsified with a hydrophobic agent, which stabihzes the droplets against molecular diffusion processes. 

In the case of inverse systems, hydrophilic monomers such as hydroxyethyl acrylate, acrylamide, and acrylic acid were miniemulsified in nonpolar media such as cyclohexane or hexadecane [10,19]. 

The miniemulsion is also well suited to the preparation of copolymers. Here, a mixture of hydrophobic monomers can be used, for example a mixture of styrene and MMA [20] or styrene and butyl acrylate [21], MMA and p-methylstyrene, vinyl hexanoate, or vinyl 2-ethyUiexanoate [22]. Fluorinated monomers can also be copolymerized with monomers such as MMA and styrene [14]. Miniemulsification of such mixed monomer species allows efficient copolymerization reactions to be performed with standard hydrophobic and hydrophilic monomers in a common heterophase situation, resulting either in core-shell latexes or in statistical copoly- 

The polymerization process of two monomers with different polarities in similar ratios is difficult due to solubility problems. Using the miniemulsion process, it was possible to start from very different spatial monomer distributions, and this resulted in very different amphiphilic copolymers in dispersion [23]. The monomer, which is insoluble in the continuous phase, is miniemulsified in order to form stable and small droplets with a small amount of surfactant The monomer with the opposite hydrophilicity dissolves in the continuous phase (but not in the droplets). The formation of acrylamide/MMA (AAm/MMA) and acrylamide/ styrene (AAm/Sty) copolymers was chosen as examples of the miniemulsion process. In all cases, the syntheses were carried out in water as well as in cyclohexane as the continuous phase. If the synthesis is performed in water, the hydrophobic monomer with a low water solubiHty (styrene or methyl methacrylate) 

Propose a mechanism for the formation of a segment of poly(vinyl chloride) containing three units of vinyl chloride and initiated by hydroxyl radical. 

The hydroxyl radical will add to the alkene to give the more stable, more substituted radical  

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec- 

In a more conventional approach, poly(S-b-CL) and poly(MMA-b-CL) block copolymers have been prepared from the same components as described previously, but in a two-step process via macromolecular initiators [47]. In a first step. 

Concerning the synthesis of graft copolymers, Jedlinski et al. have prepared poly(MMA-g-(3BL) copolymers via anionic grafting of 3BL from a modified PMMA backbone [85]. PMMA chains were partially saponified by potassium hydroxide and complexed by 18C6 crown ether so as to act as multifunctional mac- 

Although are no examples of stereocontrol aided by bulky or well-designed aluminum reagents in radical polymerization, the remarkable effects of aluminum species are still worthy of comment 

The search for an efficient catalyst for cationic polymerization has attracted much 

Further studies revealed [Cp2Sm(/z-Me)2AlMe2]2 with co-catalyst Al(i-Bu)3/ 

Among the aforementioned abstractors MAO- or MM AO-promoted reactions are complicated, and intractable species are produced. Despite the elusiveness of MAO and MMAO, the reaction of metallocene dialkyls with electrophiles which either generate or contain very weakly coordinating anions has proved a particularly successful strategy for the generation of highly active, MAO-free polymerization catalysts. Marks reported isolable and X-ray crystallographically characterizable catalysts for study of the molecular basis of this type of polymerization catalysis [30]. 

Whereas Cp2ZrMe [F-Al(Ci2F9)4] and 176 have negligible activity in the poly- 

Many vinyl monomers are polymerized by light irradiation. Photoinitiators are effective with polymerization. Irradiation to benzoin produces radicals  

Those radicals initiate polymerization of vinyl monomers. For example, methylmethacrylate is polymerized by the benzoin radical as follows  

free radicals are emitted from benzoin propagate polymerization of an acryl monomer. 

The polymerization is interrupted by radical scavengers such as oxygen or impurities existing in the coated monomer layer. Oxygen is a strong scavenger for radical polymerization. It invades the layer from the surface. Therefore, polymerization is disturbed, especially on the surface. 

Acrylic acids and methacrylic acids are mostly applied to polymerize monomers. However, esters of their acids are generally used. Monofunctional monomers are basic monomers for photopolymerization and are used for dilution because of low viscosity. Bifunctional monomers induce crosslinking and hard coating. Polymerization of 1,4-butanediol diacrylate (BDDA) is shown below  

It was shown that E-isomers of 2-, 3-, and 4-fluorostilbene, 4,4 -difluorostilbene, and Z-isomers of 4-fluorostilbene were involved as initiators in radical copolymerization 

Upon pulse radiolysis of traws-stilbene (t-St) solutions in THE, the radical anion of trans-stilbene was demonstrated to be formed by the reaction of electrons with St (reaction with the rate constant ks = (1.16 0.03) x 10 dm /(mol s)) [86]. The transient absorption spectrum observed with Xmax 500 and 720 nm was attributed to the unassociated radical anion St . This species reacted with the countercation of THE formed upon radiolysis and with radiolytically generated radicals. Addition of sodium tetrahydridoaluminate (NAH) resulted in the radical anion being associated with Na as a contact ion pair. In the presence of the lithium salt, formation of solvent-separated ion pairs has been detected. 

What is the expected regioselectivity (Markovnikov or ant/-Markovnikov) The presence of peroxides indicates that the reaction proceeds via an ant/-Markovnikov addition. That is, the Br is expected to be placed at the less substituted carbon  

What is the expected stereospecificity In this case, one new chirality center is created, which results in a racemic mixture of the two possible enantiomers  

PRACTICE the skill 11.20 Predict the products for each reaction. In each case, be sure to consider 

21 The initiation step for radical addition of HBr is highly endothermic  

This polymerization can occur via a radical mechanism (Mechanism 11.4). 

VFc rmdergoes oxidation with peroxide initiators for which azoinitiators have been used almost exclusively. Unlike most vinyl monomers the molecular weight of poly(vinylferrocene) (PVFc) does not increase with the decrease of the initiator concentration. This is the consequence of an anomalously high chain-transfer constant [11]. 

The experimental results can be described under the assumption that termination is an intramolecular reaction, 

In this case the degree of polymerization DPn) is given by the equation fcpc(VFc) 

Since the transfer to the solvents applied in these investigations is known to be small, one can simplify the last equation and write 

Transfer to polymer was also considered unimportant because monomer conversion was kept below 10% in all experiments. 

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Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

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. 

These materials are obtained through free-radical polymerization of acrylic or methacrylic monomers, or of fumarates. 

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

My faculty colleagues of the Institute also bring great expertise in the areas of anionic, cationic, and radical polymerization to the transformation of low-molecular-weight hydrocarbons into macromole-... 

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... 

In the free radical polymerization of ethylene ethylene is heated at high pressure in the presence of oxygen or a peroxide... 

FIGURE 6 17 Mechanism of peroxide initiated free radical polymerization of ethylene... 

The mechanism of free radical polymerization of ethylene is outlined m Figure 6 17 Dissociation of a peroxide initiates the process m step 1 The resulting per oxy radical adds to the carbon-carbon double bond m step 2 giving a new radical which then adds to a second molecule of ethylene m step 3 The carbon-carbon bond forming process m step 3 can be repeated thousands of times to give long carbon chains... 

Teflon IS made in a similar way by free radical polymerization of tetrafluoroethene... 

In their polymerization, many individual alkene molecules combine to give a high molecular weight product Among the methods for alkene polymerization cationic polymerization coordination polymerization and free radical polymerization are the most important An example of cationic polymerization is... 

As the demand for rubber increased so did the chemical industry s efforts to prepare a synthetic sub stitute One of the first elastomers (a synthetic poly mer that possesses elasticity) to find a commercial niche was neoprene discovered by chemists at Du Pont in 1931 Neoprene is produced by free radical polymerization of 2 chloro 1 3 butadiene and has the greatest variety of applications of any elastomer Some uses include electrical insulation conveyer belts hoses and weather balloons... 

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... 

United States The Ziegler route to polyethylene is even more important because it occurs at modest temperatures and pressures and gives high density polyethylene which has properties superior to the low density material formed by the free radical polymerization described m Section 6 21... 

Hydroxy-2-methylpropanenitrile is then reacted with methanol (or other alcohol) to yield methacrylate ester. Free-radical polymerization is initiated by peroxide or azo catalysts and produce poly(methyl methacrylate) resins having the following formula ... 

Poly (methyl Acrylate). The monomer used for preparing poly(methyl acrylate) is produced by the oxidation of propylene. The resin is made by free-radical polymerization initiated by peroxide or azo catalysts and has the following formula ... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

In this section we discuss the initiation step of free-radical polymerization. This discussion is centered around initiators and their decomposition behavior. The first requirement for an initiator is that it be a source of free radicals. In addition, the radicals must be produced at an acceptable rate at convenient temperatures have the required solubility behavior transfer their activity to... 

This low level of concentration is typical of free-radical polymerizations. Next we inquire how long it will take the free-radical concentration to reach 0.99[M-]5, liter" in this case. Let a = (16fk jkj [1] q) ... 

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. 

There is a great deal more that could be said about emulsion polymerization or, for that matter, about free-radical polymerization in general. We shall conclude our discussion of the free-radical aspect of chain-growth polymerization at this point, however. This is not the end of chain-growth polymerization, however. There are four additional topics to be considered ... 

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... 

Bagdasan yan, K. S., Theory of Free Radical Polymerization, Israel Program for Scientific Translations, Jerusalem, 1968. 

North, A. M., The Kinetics of Free Radical Polymerization, PeTgamon, Ne < York, 1966. 

We begin our discussion of copolymers by considering the free-radical polymerization of a mixture of two monomers. Mi and M2. This is already a narrow view of the entire field of copolymers, since more than two repeat units can be present in copolymers and, in addition, mechanisms other than free-radical chain growth can be responsible for copolymer formation. The essential features of the problem are introduced by this simpler special case, so we shall restrict our attention to this system. 

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

Fox and Schneckof carried out the free-radical polymerization of methyl methacrylate between -40 and 250 C. By analysis of the a-methyl peaks in the NMR spectra of the products, they determined the following values of a, the probability of an isotactic placement in the products prepared at the different temperatures ... 

Radical initiators Radical polymerization Radical scavengers... 

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

PHOST is often prepared by polymerization of 4-acetoxystyrene followed by base-catalyzed hydrolysis (Fig. 29). The acetoxystyrene monomer s stabihty and polymerization kinetics allow production of PHOST of higher quaUty than is easily obtained by direct radical polymerization of HOST. The PHOST homopolymer product is then partially or fully derivatized with an acid-cleavable functionaUty to produce the final resist component. 

Butenediol does not undergo free-radical polymerization. A copolymer with vinyl acetate can be prepared with a low proportion of butenediol (110). 

Acrolein produced in the United States is stabilized against free-radical polymerization by 1000—2500 ppm of hydroquinone and is protected somewhat against base-catalyzed polymerization by about 100 ppm of acetic acid. To ensure stabiUty, the pH of a 10% v/v solution of acrolein in water should be below 6. 

Since the principal hazard of contamination of acrolein is base-catalyzed polymerization, a "buffer" solution to shortstop such a polymerization is often employed for emergency addition to a reacting tank. A typical composition of this solution is 78% acetic acid, 15% water, and 7% hydroquinone. The acetic acid is the primary active ingredient. Water is added to depress the freezing point and to increase the solubiUty of hydroquinone. Hydroquinone (HQ) prevents free-radical polymerization. Such polymerization is not expected to be a safety hazard, but there is no reason to exclude HQ from the formulation. Sodium acetate may be included as well to stop polymerization by very strong acids. There is, however, a temperature rise when it is added to acrolein due to catalysis of the acetic acid-acrolein addition reaction. 

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