Kinetics of free radical addition polymerization youtube. Understanding free radical reactions universalclass. Chain initiation edit chain initiation is the process of initially generating a chain carrier chain carriers are some intermediates such as radical and ions in chain propagation process in a chain polymerization. Effects of temperature and pressure on chain polymerization. Controlled radical polymerization guide sigmaaldrich. Whereas thermodynamics will assist us in predicting whether a chain polymerization will proceed, we. At the same time, a new primary transfer radical is formed which can start chain polymerization. Addition polymerization an overview sciencedirect topics. In this unit we will look at those formed by the process of free radical polymerization. Chain transfer is a chain breaking reaction leading to a decrease in the size of the propagating polymer chain.
All molecules present monomer, oligomer, polymer can react with any other molecule. The new species may initiate polymerization or may truncate it. During propagation, only monomers react to the active site at the end of the growing chain. Chain transfer to small molecules such as monomer, solvent or chain transfer agent usually involves the abstraction of hydrogen from the small molecule by the chain radical and leads to the termination of the live chain. The reaction schemes for radical polymerization initiation, propagation, termination, and transfer, sect. Chain transfer to solvent has been investigated in the conventional radical polymerization and nitroxide. The three steps in chainreaction polymerization are initiation, propagation and termination. Suppose we have a container full of ethene ethylene monomers and we unpair the pi electrons by a suitable method, either by appropriate radiation, or a free radical initiator. Freeradical polymerization frp is a method of polymerization by which a polymer forms by the successive addition of freeradical building blocks. A polymerization where a cation is formed causing a chain reaction. Explaining free radical addition in the polymerisation of ethene.
Today free radical polymerization accounts for a large portion of massproduced polymers. Here a live polymer chain of j monomer units tranfers its free radical. Student authored video explaining the initiation, propagation and termination steps in radical polymerization. Polymerization occurs by the propagation of the reactive center through successive additions of large numbers of monomer molecules in a chain reaction. Chainreaction polymerization, sometimes called addition polymerization, requires an initiator to start the growth of the reaction. Chain transfer and chain termination do not always happen in a chaingrowth polymerization.
Polymers include polyethylene, polypropylene, polyvinyl chloride pvc and polystyrene. This study deals with modeling the propagation and the chain transfer reactions in the free radical polymerization of ethylene, methyl methacrylate mma, and acrylamide am. Radical chain polymerization is a chain reaction consisting of a sequence of three stepsinitiation, propagation, and, termination. Polymerization reactions are divided into two groups known as step reactions also called condensation reactions and chain reactions also known as addition reactions. The chain is initiated by free radicals, ra, produced by reaction between some of the ethene and the oxygen initiator. Once a chain reaction gets started, it is able to keep itself going. First, the free radical polymerization of methyl methacrylate mma monomer. New chains are constantly started and growing ones terminated. The kinetics and mechanism of a radical chain polymerization reaction initiated from a selfassembled monolayer of an azo initiator attached to the surfaces of silica particles are investigated. The free radical polymerization of acrylates and methacrylates is a chain polymerization across the double bond of the monomer figure 2.
All the monomers from which addition polymers are made are alkenes or functionally substituted alkenes. Inhibitors stop all radical polymerization until consumed. Polymer chemistry university of south carolina upstate. Since this can be initiated by traces of oxygen or other minor impurities, pure samples of these compounds are often stabilized by small amounts of radical inhibitors to avoid unwanted reaction. Polymers products that are commonly referred to as plastics by the general public are known to chemists as polymers. Dec 29, 2017 for the love of physics walter lewin may 16, 2011 duration. Addition polymerization is the successive addition of alkene monomers to one another. Polymerization kinetics step growth slow can use statistical methods as well as kinetics to describe mol. Step reactions require bifunctional or polyfunctional monomers, while chain reactions require the presence of an initiator. Following its generation, the initiating free radical adds nonradical monomer units, thereby growing the polymer chain. Pdf origins and development of initiation of free radical.
The first is the production of free radicals by homolytic dissociation of an initiator species i to yield a pair of radicals r. To emphasize the importance of stabilization of the charged reactive center on the growing chain. Like freeradical polymerizations, these chain reactions take place via chaininitiation, chainpropagation, and chaintermination steps. To provide enough thermal stability, some polymerization inhibitor is added to the commercial product, which is a radical scavenger. Chain transfer reactions are especially prevalent in the high pressure radical polymerization of ethylene, which is the method used to make ldpe low density polyethylene. In chain growth or chain polymerization, the only chain extension reaction step is the addition of a monomer to a growing chain with an active center such as a free radical or ion. The process of chain initiation involves two steps. They join in a successive chain during free radical polymerization. The chemistry of radical polymerization request pdf. Suppose we have a container full of ethene ethylene monomers and we unpair the pi electrons by a suitable method, either by appropriate radiation, or a.
Polymerization, any process in which relatively small molecules, called monomers, combine chemically to produce a very large chainlike or network molecule, called a polymer. The extent of chain transfer to dmf can significantly impact the maximum attainable molecular weight in both systems. An appropriate mechanism was found to be radical polymerization of dimethacrylated pegbased hydrogels 222, whereby the chain growth polymerization is a fast and random crosslinking reaction. The main difference between chain growth polymerization and step growth polymerization is that the chain growth polymerization involves the growth a polymer chain at its ends whereas. Does solvent plays an important role in free radical. Free radical polymerization frp is a type of chain growth polymerization, along with anionic, cationic and coordination polymerization. Free radical polymerization is one type of chain polymerization. And in condensation reactions, functional groups of two monomers react together releasing a small molecule to form a polymer.
Virtually all of the monomers described above are subject to radical polymerization. In chain growth polymerization, the position of the equilibrium between polymer and monomers can be determined by the thermodynamics of the polymerization. The chemical reaction needs a catalyst and forms a free radical before polymerization is complete. For example, if c1, the transfer agent will be entirely consumed when not all of the monomer has been polymerized in a batch all ingredients present in the reaction vessel before the process is started. Stiile colorado state university, fort collins, co 80523 as illustrated in the articles on chain polymerization, a chaingrowth polymerization is one in which each polymer chain, after being started by a free radical initiator, a cationic catalyst or an anionic catalyst, grows rapidly, producing a high molecular weight polymer. This chapter describes the characteristics of radical chain polymerization, with emphasis on the parameters that control polymer molecular weight. The active center is transferred from the growing chain to a different species chain transfer agent, initiator, monomer, or polymer. Radical polymerization is a type of polymerization in which the reactive center of a polymer chain consists of a radical the polymerization reaction is initiated by three classes of free radical initiators. Atom transfer radical polymerization atrp is an example of a reversibledeactivation radical polymerization. Polymerization is an important process in so many industries. The degree of polymerization depends on the velocity constants of radical formation, chain growth, chain transfer and chain termination.
For the love of physics walter lewin may 16, 2011 duration. Free radical polymerization plastic bags and film wrap high density polyethylene hdpe low degrees of branching essentially linear density 0. From the point of reaction mechanism, the radical density in conventional radical polymerization is higher than that of crp for having no substance that can combine with the radical to form. Jun 27, 2017 polymerization is an important process in so many industries. Difference between chain growth and step growth polymerization. Two reactions are involved in the initiation of free radical chain polymerization. Ordinarily, such high energy radiation does produce radical type polymerizations. Addition polymerization is a chain reaction, generally associated with the polymerization of ethene or vinyl type monomers, such as, ethene, propene, styrene, and vinyl chloride.
Publishers pdf, also known as version of record includes final page. The free radicals can be produced by thermal, catalytical or photochemical decomposition of organic peroxides, hydroperoxides, azo and diazo compounds. To develop expressions for the average degree of polymerization and molecular weight distribution for anionic polymerization. Mechanism and kinetics of free radical chain polymeriza. Retarders stop only a portion of the radicals from propagating. Solomon department of chemical and biomolecular engineering. The rate of the decomposition of the surfaceattached initiator is followed by differential scanning calorimetry and volumetry. Fundamental steps initiation, propagation, termination. Each time a free radical hits an ethene molecule a new longer free radical is formed. Therefore, addition polymerization is the reaction between monomers with multiple bonds, where they join together to form saturated polymers. G p of the polymerization is commonly used to quantify the tendency of a polymeric reaction.
Determine the concentration and mole fraction of polymers of chain length j in terms of. Explaining free radical addition in the polymerisation of. In this socalled initiation reaction, the chlorine molecule is homolytically cleaved into two chlorine radicals by the application of light or radiation or heat subsequently, in the first step of the chain propagation, a chlorine radical abstracts a hydrogen atom from methane. Therefore, the chain reaction is a cyclic process that is fed with the starting product b and yields the product c. The chemistry of radical polymerization graeme moad csiro molecular and health technologies bayview ave, clayton, victoria 3168, australia and david h. Lecture notes synthesis of polymers chemical engineering. Polymers can be formed by a variety of reaction pathways. When the chain reaction is initiated and carried by negatively charged intermediates, the reaction is known as anionic polymerization.
Anionic polymerizations are difficult to carry out and are only used on an industrial scale if the monomer in question does not polymerize by freeradical mechanism or when polymers with regular structure are. The formation of a polymer by addition polymerization is an example of a chain reaction. The polymerization reaction in many chemical compounds may occur through a variety of reaction mechanisms that differ in complexity because of the presence of the functional groups in the reactants as well as due to their inherent steric effects. Chainlength dependent bimolecular termination in freeradical. Classification of polymerization reaction addition and. Jul 15, 2011 polymerization occurs by the propagation of the reactive center through successive additions of large numbers of monomer molecules in a chain reaction. The chemistry of radical polymerization 2nd edition. A polymer from poly meaning many and m er meaning part is a chain of repeating chemical structures. Sarac department of chemistry, faculty of science, istanbul technical university, maslak 80626, istanbul, turkey received 15 july 1996. In general, the use of solvent in free radical polymerization is to increase the solubility formed polymer chain and 2 increase the heat transfer in the reaction system to make sure the. Radical chaingrowth polymerization chemistry libretexts. Here the addition polymer forms by addition of atoms with a free electron in its valence shells.
The polymerization inhibitor used for the stabilization of vinyl acetate monomers is hydroquinone h. The largest family of polymers 3, vinyl polymers, are produced by chain polymerization reactions. Another important reaction that involves free radicals is polymerization, or the formation of polymers. A good example is the freeradical polymerization of styrene, which is initiated by a free radical r that reacts with styrene. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. The addition reaction may occur by way of radical, cationic, or anionic. Chain transfer to solvent in the radical polymerization of.
Chain transfer agent an overview sciencedirect topics. Polymerizations by a freeradical mechanism are typical freeradical reactions. Stiile colorado state university, fort collins, co 80523 as illustrated in the articles on chain polymerization, a chaingrowth polymerization is one in which each polymer chain, after being started by a free radical initiator, a cationic catalyst or an anionic catalyst, grows rapidly, producing a high. The mechanism in the case of radiation can be quite complex. Usually at least 100 monomer molecules must be combined to make a product that has certain unique physical properties. The monomer molecules may be all alike, or they may represent two, three, or more different compounds. That is to say, there is an initiation, when the radicals are formed, a propagation. Chain polymerization is characterized by the presence of a few active sites which react and propagate through a sea of monomers, e. The most widely used chain transfer agents are compounds with one relatively weak bond like thiols, disulfides, ccl 4, or cbr 4, but even hydrogen and propane can be used with the very reactive polyethylene radical. The three steps of this reaction to focus on are how the reaction gets started initiation how the reaction keeps going propagation. The most common technique is the free radical polymerization of mma. Mechanism of radical chain polymerizations initiated by azo. Radical polymerization is a type of polymerization in which the reactive center of a polymer chain consists of a radical the polymerization reaction is initiated by three classes of freeradical initiators. For example, if c1, the transfer agent will be entirely consumed when not all of the monomer has been polymerized in a batch all ingredients present in the reaction vessel before the process is started reaction.
Free radical polymerization frp is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. This can induce chain branching through the terminal double bond propagation reaction. Propagation of polystyrene with a phenyl radical initiator. The end group analysis of propagating chain shows the initiator radicals are incorporated into the polymer. At certain temperature, the polymerization reaches equilibrium rate of polymerization rate of depolymerization this temp is called the ceiling temperature tc. Accordingly, the radical a is again available to convert b into b. The intermediate radical a is consumed and recovered over and over again in this radical chain reaction. Once the growth of a chain is initiated by formation of an active center, chain propagation is usually rapid by addition of a sequence of monomers.
Propagation or growth of the active chain by sequential addition of monomers. Free radical polymerization is one of the processes of polymer formation. Jul 10, 2014 describes the radical mechanism for making polymers from alkenes. A polymer is a macromolecule which consists of small molecular units that are repeated over and over again to form a long chain. Like its counterpart, atra, or atom transfer radical addition, atrp is a means of forming a carboncarbon bond with a transition metal catalyst. Chemical engineering 160260 polymer science and engineering. The chain transfer agents modeled in the free radical polymerization of ethylene are the experimentally widely used species such as ethylene, methane, ethane, propane, trimethylamine, dimethylamine, chloroform, and. Understanding vinyl acetate polymerisation accidents. The overall process is known as free radical addition.