Free radical polymerisation

R.S. Davidson, Polymeric and Polymerisable Free Radical Photoinitiators, J. Photo-chem. Photobiol. A Chem. 69 (1993)... 

Emulsion polymerisation is characterised by the fact that a free radical initiator (usually water-soluble) is used to polymerise free radically polymerisable monomers to give a water-insoluble polymer. The main ingredients of an emulsion polymerisation system include monomer, dispersant, emulsifier and initiator. Water is often used as the dispersant. A water-insoluble monomer can be dispersed in water by means of an oil-in-water emulsifier and polymerised with a water-soluble initiator. Styrene, MMA and VA are systems for which emulsion polymerisation are commonly used. 

Secondary amines give only a monosubstituted product. Both of these reactions are thermally reversible. The product with ammonia (3,3, 3 -nitrilottispropionamide [2664-61-1C H gN O ) (5) is frequently found in crystalline acrylamide as a minor impurity and affects the free-radical polymerisation. An extensive study (8) has determined the stmctural requirements of the amines to form thermally reversible products. Unsymmetrical dialkyl hydrasines add through the unsubstituted nitrogen in basic medium and through the substituted nitrogen in acidic medium (9)). 

The synthesis of the high molecular weight polymer from chlorotrifluoroethylene [79-38-9] has been carried out in bulk (2 >—21 solution (28—30), suspension (31—36), and emulsion (37—41) polymerisation systems using free-radical initiators, uv, and gamma radiation. Emulsion and suspension polymers are more thermally stable than bulk-produced polymers. Polymerisations can be carried out in glass or stainless steel agitated reactors under conditions (pressure 0.34—1.03 MPa (50—150 psi) and temperature 21—53°C) that require no unique equipment. 

Free Radical Initiators for the Suspension Polymerisation of Vinyl Chloride, Technical Publication 30.90, Lucidol Division, Pennwalt Corp., Buffalo, N.Y., 1976. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

K. K. Diediker and P. Oldring, "Chemistry and Technology of UV EB Formulations for Coatings, Inks Paiats," Vol. 3, Photoinitiators for Free Radical and Cationic Polymerisation, ShoHum International, 1991. 

Free-Radical Addition. In free-radical addition polymerisation, the propagating species is a free radical. The free radicals, R-, ate most commonly generated by the thermal decomposition of a peroxide or aso initiator, 1 (see Initiators, free-RADICAl) ... 

ESBR and SSBR are made from two different addition polymerisation techniques one radical and one ionic. ESBR polymerisation is based on free radicals that attack the unsaturation of the monomers, causing addition of monomer units to the end of the polymer chain, whereas the basis for SSBR is by use of ionic initiators (qv). 

Eree-radical initiation of emulsion copolymers produces a random polymerisation in which the trans/cis ratio caimot be controlled. The nature of ESBR free-radical polymerisation results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstmcture is not amenable to manipulation, although the temperature of the polymerisation affects the ratio of trans to cis somewhat. 

Oiganometallic usage is shown in the piepaiation of titanium- oi vanadium-containing catalysts foi the polymerisation of styrene or butadiene by the reaction of dimethyl sulfate with the metal chloride (145). Free-radical activity is proposed for the quaternary product from dimethylaruline and dimethyl sulfate and for the product from l,l,4,4-tetramethyl-2-tetra2ene and dimethyl sulfate (146,147). 

Continuous Polymerization. A typical continuous flow diagram for the vinyl acetate polymerisation is shown in Figure 12. The vinyl acetate is fed to the first reactor vessel, in which the mixture is purged with an inert gas such as nitrogen. Alternatively, the feed may be purged before being introduced to the reactor (209). A methanol solution containing the free-radical initiator is combined with the above stream and passed directiy and continuously into the first reactor from which a stream of the polymerisation mixture is continuously withdrawn and passed to subsequent reactors. More initiator can be added to these reactors to further increase the conversion. 

The mutual polymerisation of two or more monomers is called copolymerisation. This topic has been comprehensively reviewed (4,5). Monomers frequentiy show a different reactivity toward copolymerisation than toward homopolymerisation. In fact, some monomers that can be bomopolymerised only with great difficulty, can be readily copolymerised. One such monomer is maleic anhydride. It is rather inert to free-radical homopolymerisation yet can be copolymerised convenientiy with styrene and many other monomers under free-radical conditions. 

When heated to 120°C, AIBN decomposes to form nitrogen and two 2-cyanoisopropyl radicals. The ease with which AIBN forms radicals, and the fact that the rate of information does not vary much in various solvents has resulted in wide use of AIBN as a free-radical initiator. AIBN is used commercially as a catalyst for vinyl polymerisation (see Initiators). 

Acrylate esters can be polymerised in a variety of ways. Among these is ionic polymerisation, which although possible (6—9), has not found industrial apphcation, and practically all commercial acryUc elastomers are produced by free-radical polymerisation. Of the four methods available, ie, bulk, solution, suspension, and emulsion polymerisation, only aqueous suspension and emulsion polymerisation are used to produce the ACMs present in the market. Bulk polymerisation of acrylate monomers is hasardous because it does not allow efficient heat exchange, requited by the extremely exothermic reaction. 

Type AD-G is used in an entirely different sort of formulation. The polymer is designed for graft polymerisation with methyl methacrylate. Typically, equal amounts of AD-G and methyl methacrylate are dissolved together in toluene, and the reaction driven to completion with a free-radical catalyst, such as bensoyl peroxide. The graft polymer is usually mixed with an isocyanate just prior to use. It is not normally compounded with resin. The resulting adhesive has very good adhesion to plasticised vinyl, EVA sponge, thermoplastic mbber, and other difficult to bond substrates, and is of particular importance to the shoe industry (42,43). 

Free-Radical Polymerization. The best method for polymerising isoprene by a free-radical process is emulsion polymerisation. Using potassium persulfate [7727-21-1] as initiator at 50°C, a 75% conversion to polyisoprene in 15 h was obtained (76). A typical emulsion polymerisation recipe is given as follows (77). 

Trimetbyl vinyl silane [754-05-2] M 100.2, b 54.4 /744mm, 55.5 /767mm, d S(25,4) 0.6865, n D 1.3880. If the H NMR spectrum shows impurities then dissolve in Et20, wash with aq NH4CI soln, dry over CaCl2, filter, evaporate and distil at atmospheric pressure in an inert atmosphere. It is used as a copolymer and may polymerise in the presence of free radicals. It is soluble in CH2CI2. [J Org Chem 17 1379 7952.]... 

In the case of mechanism (6) there are materials available which completely prevent chain growth by reacting preferentially with free radicals formed to produce a stable product. These materials are known as inhibitors and include quinone, hydroquinone and tertiary butylcatechol. These materials are of particular value in preventing the premature polymerisation of monomer whilst in storage, or even during manufacture. 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

Elementary Kinetics of Free-radical Addition Polymerisation... 

In a simple free-radical-initiated addition polymerisation the principal reactions involved are (assuming termination by combination for simplicity)... 

An increase in temperature will increase the values of and k. In practice it is observed that in free-radical-initiated polymerisations the overall rate of conversion is approximately doubled per 10°C rise in temperature (see Figure 2.20). Since the molecular weight is inversely related to k and k it is observed in practice that this decreases with increase in temperature. 

A further feature of anionic polymerisation is that, under very carefully controlled eonditions, it may be possible to produee a polymer sample which is virtually monodisperse, i.e. the molecules are all of the same size. This is in contrast to free-radical polymerisations which, because of the randomness of both chain initiation and termination, yield polymers with a wide molecular size distribution, i.e. they are said to be polydisperse. In order to produce monodisperse polymers it is necessary that the following requirements be met ... 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Although in principle the high-pressure polymerisation of ethylene follows the free-radical-type mechanism discussed in Chapter 2 the reaction has two particular characteristics, the high exothermic reaction and a critical dependence on the monomer concentration. 

Most vinyl monomers will polymerise by free-radical initiation over a wide range of monomer concentration. Methyl methacrylate can even be polymerised... 

Polybutadiene was first prepared in the early years of the 20th century by such methods as sodium-catalysed polymerisation of butadiene. However, the polymers produced by these methods and also by the later free-radical emulsion polymerisation techniques did not possess the properties which made them desirable rubbers. With the development of the Ziegler-Natta catalyst systems in the 1950s, it was possible to produce polymers with a controlled stereo regularity, some of which had useful properties as elastomers. 

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