At certain temperature, the polymerization reaches equilibrium rate of polymerization rate of depolymerization this temp is called the ceiling temperature tc. During radical polymerization, initially start a radical species with a single electron, which arises from, for example, the decay of peroxides or azo compounds fig. It was also possible to determine the rate constants of catalytic chain transfer by the cobalt catalysts. Freeradical polymerization is a type of chaingrowth polymerization, along with anionic, cationic and coordination polymerization. The new species may initiate polymerization or may truncate it. Most chain growth polymers are formed from unsaturated hydrocarbons, with the unsaturation present as a double bond between carbon atoms table 1. Understanding free radical reactions universalclass. Propagation or growth of the active chain by sequential addition of monomers. Freeradical polymerization frp is a method of polymerization by which a polymer forms by. The most common example of chain growth polymerization is the conversion of ethylene.
Cationic polymerization the initiator is an acidcation. The active center is transferred from the growing chain to a different species chain transfer agent, initiator, monomer, or polymer. Chain growth polymerization an overview sciencedirect. All molecules present monomer, oligomer, polymer can react with any other molecule. Each time a free radical hits an ethene molecule a new longer free radical is formed. Free radical is most versatile type of chain growth. Difference between chain growth and step growth polymerization.
Here the addition polymer forms by addition of atoms with a free electron in its valence shells. Atom transfer radical polymerization and the synthesis of. Logiccontrolled radical polymerization with heat and. Kinetics of the freeradical polymerization of isobornyl. Fundamental steps initiation, propagation, termination. There are many different types of emulsion polymerizations depending on. Free radical polymerization is one of the most common processes used to produce. The rate and molecular weight in both reversible and irreversible polymerizations are discussed together with the process conditions needed for producing different polymers. Effects of temperature and pressure on chain polymerization. Sarac department of chemistry, faculty of science, istanbul technical university, maslak 80626, istanbul, turkey received 15 july 1996. Free radical polymerization plastic bags and film wrap high density polyethylene hdpe low degrees of branching essentially linear density 0. To develop expressions for the average degree of polymerization and molecular weight distribution for anionic polymerization. Chain grow and free radical polymerization introduction chain growth polymerization monomers for chain. Controlled radical polymerization guide sigmaaldrich.
Describes the radical mechanism for making polymers from alkenes. Free radical polymerization stable free radical polymerization. This chapter describes the characteristics of radical chain polymerization, with emphasis on the parameters that control polymer molecular weight. Free radical mechanisms of ethene polymerization for cbse chemistry board exam 2018 duration. Increased pressure alters polymerization rate and polymer molecular weight. The plastics and rubber are examples of the most common polymers which are commonly used in both everyday life and in medical applications. During propagation, only monomers react to the active site at the end of the growing chain. The chain is initiated by free radicals, ra, produced by reaction between some of the ethene and the oxygen initiator. We discuss this distribution further after we discuss chain reactions. Anionic polymerization the initiator is a nucleophile. Introduction to the basics of uveb chemistry and formulations. A polymer from polymeaning many and mer meaning part is a chain of repeating chemical structures. Kinetic chain length will decrease as concentration and efficiency of the initiator increase this is reasonable because increasing the number of. Free radical polymerization frp is a type of chain growth polymerization, where the polymer is formed by successive addition of building blocks vinyl monomers to a propagating radical chain 1,2.
Free radical curing uv photoinitiator absorbs uv light and generates free radicals free radicals react with double bonds causing chain reaction and polymerization cationic curing uv photoinitiator absorbs uv light and generates a lewis acid acid reacts with epoxy groups resulting in polymerization. Chain transfer agent an overview sciencedirect topics. Radical polimerization an overview sciencedirect topics. Following its generation, the initiating free radical adds nonradical monomer units, thereby growing the polymer chain. Chain transfer and chain termination do not always happen in a chain growth polymerization.
Controlled radical polymerization has branched into three fundamental techniques which are listed below. The exact properties of polymers depends on a variety of chemical. Another important reaction that involves free radicals is polymerization, or the formation of polymers. Stable free radical polymerization mit opencourseware.
At the same time, a new primary transfer radical is formed which can start chain polymerization. A polymerization where a cation is formed causing a chain reaction. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. The chemistry of radical polymerization 2nd edition. According to the initiator used in the chain growth polymerization process, there are three types of chain growth polymerization. Mechanism and kinetics of free radical chain polymeriza. The most common technique is the free radical polymerization of mma. Publishers pdf, also known as version of record includes final page, issue and. The propagation site of radical polymerization is a carbon radical whereas propagation site. Chemical engineering 160260 polymer science and engineering.
The ionic chain polymerization will be discussed in chap. Explaining free radical addition in the polymerisation of. Unit2 free radical polymerization mechanism chemistry. Glossary of basic terms in polymer science iupac recommendations 1996 pdf. 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. There is a selectivity involved that depends upon chemical structure i. 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 technique is the most studied and utilized of the controlledliving radical polymerization techniques since a large number of monomers. Chainlength dependent bimolecular termination in freeradical. The overall process is known as free radical addition. The model was previously used to analyze the free radical polymerization of pure isobornyl methacrylate iboma, 14 and is now extended to describe the reaction in the presence of polyisobutylene pib. Summary this chapter describes the characteristics of radical chain polymerization, with emphasis on the parameters that. Comparison of common polymerization methods stepgrowth chain growth anionic growth method cons. They join in a successive chain during free radical polymerization. While switching of controlled radical polymerization has been achieve using light, applied voltage, allosteric effects, chemical reagents, ph, and mechanical force, it is still challenging to introduce multiple external switches using the same catalyst to. Stiile colorado state university, fort collins, co 80523 as illustrated in the articles on chain polymerization, a chain growth 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. The free radical polymerization of acrylates and methacrylates is a chain polymerization across the double bond of the monomer figure 2. This process is similar to radical polymerization, as demonstrated by the following equations.
Some basics on free radical polymerization european commission. 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. Polymerization of isobutylene 2methylpropene by traces of strong acids is an example of cationic polymerization. Virtually all of the monomers described above are subject to radical polymerization. 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. Monomers are added one by one to the active site on the growing polymer. Pdf thiols as chain transfer agents in free radical. 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. Therefore, controlled radical polymerization approaches, such as atom transfer radical polymerization atrp and reversible additionfragmentation chain transfer raft were typically used to. Atom transfer radical polymerization and the synthesis of polymeric materials by timothy e. The chain growth polymers count more than 80 % total usage of polymers in our daily life. If the radical that was extended by a monomer unit is able to add an additional monomer and to form a macro. Atom transfer radical polymerization atrp has proven to be a powerful technique to obtain polymers with narrow polydispersities and controlled molecular weight. Chain growth polymerization proceeds by the formation of an active center of growth 8,9.
The mole fraction of polymer with a chain length j is just recalling m m o 1 p, we obtain r. Thiols as chain transfer agents in free radical polymerization in aqueous solution. Chain transfer chain transfer is a chain breaking reaction leading to a decrease in the size of the propagating polymer chain. Free radical polymerization frp is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Adronov chem 4ob3 notes free radical polymerization kinetic chain length is directly related to monomer concentration however, these polymerizations are often done in bulk monomer, which acts as solvent, and m is therefore constant.