Heat-initiated free-radical polymerisation

Decomposes upon irradiation with UV, initiate free radical polymerisations and other radical reactions. A classic example of a radical reaction that can be initiated by AIBN is the an/i-Markovnikov hydrohalogenation of alkenes. 

Only free radical polymerisation, which requires the formation of reactive free radical species to initiate polymerisation, appears to have been used to form MIPs. Free radicals are produced by the decomposition of an initiator species by the action of heat or light. Commonly used initiators are benzoyl peroxide and azobis compounds such as azobisisobutyronitrile (AIBN) or 2,2 -azobis(2,4-dimethylvaleronitrile) (ABDV) Figure 6.20). 

With the exception of nylon 6, all the polymers mentioned in the above paragraph are made from a monomer containing a double bond. The principle of polymerisation is quite simple, all we need to do is open up the double bond and then join the pieces together, but the actual mechanism of polymerisation is more complex than this. Polymerisation only takes place when an initiator or catalyst is added to the monomer. The initiator provides free radicals, anions or cations, which are the active centres for polymerisation. Benzoyl peroxide is a commonly used free radical initiator that decomposes upon heating into free radicals which then react with monomer (here vinyl chloride is our example). 

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). 

The monomers used in chain polymerisations are unsaturated, sometimes referred to as vinyl monomers. In order to carry out such polymerisations a small trace of an initiator material is required. These substances readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation from around or just beyond the blue end of the spectrum. The two most commonly used free radical initiators for these reactions are benzoyl peroxide and azobisisobutyronitrile (usually abbreviated to AIBN). They react as indicated in Reactions 2.1 and 2.2. 

In solution polymerisation, the reaction is carried out in presence of a solvent. The monomer is dissolved in a suitable inert solvent along with the chain transfer agent. A large number of initiators can be used in this process. The free radical initiator is also dissolved in the solvent. The ionic and coordination catalysts can either be dissolved or suspended in the medium. The solvent facilitates the contact of monomer and initiator and helps the process of dissipation of exothermic heat of reaction. It also helps to control viscosity increase. 

The fluorinated ethenes CF2=Cp2, CF2=CFH, CF2=CH2, CF2=CFC1 and CF2=CFBr each form homopolymers in conventional free-radical initiation procedures [220] and it is notable that the heat of polymerisation for tetrafluoroethene is much greater than for ethene [2]. Indeed, tetrafluoroethene and trifluoropropene are relatively dangerous monomers to handle because of the risk of explosive polymerisation. In marked contrast, quite drastic conditions are required in order to form a homopolymer from hexafluoropropene (HFP) [221], although commercially successful copolymers of CF2=CFCF3 with CF2=CF2 (i.e. FEP) and with CF2=CH2 (Viton rubber) have been developed. 

Tetrafluoroethylene is obtained from chloroform and hydrogen fluoride in a multistage reaction via a series of intermediates. Since the monomer is a gas at room temperature, and has a large heat of polymerization, it is polymerized free radically with, for example, K2S2O8 as initiator. The powdery polymerisate is insoluble in all solvents, so the molar mass is determined by assay of the carbonyl end groups produced by saponification of the primary initiator fragments ... 

Spontaneous polymerisation of some monomers such as styrene can occur in the presence of sunlight and heat, conditions in which free radicals may be generated. Such an uncontrolled initiation is a dangerous hazard. As polymerisation is an exothermic process, temperature increases in the bulk of the monomer and the process can lead to a blast. 

The system involved in the bulk polymerisation process is the simplest from the point of view of composition and is used for large-scale radical polymerisation. In this process, the initiator is mixed with the monomer, usually under pressure of an inert gas, and the mixture is heated to induce generation of free radicals by thermal decomposition of the initiator. Depending on whether the monomer is a gas or a liquid, the system can be homogenous or heterogeneous... 

When styrene is polymerised, polyst5rene is formed. The polymerisation of styrene is a chain growth reaction and it is induced by any known initiation techniques such as heat, free radical, anionic or cationic addition. The product polystyrene is a white polymer with high clarity and good physical and electrical properties. 

A free radical catalyst or initiator is dissolved in the monomer which is then heated and stirred in a suitable vessel. The polymerisation is exothermic and dissipation of heat through cooling may be required. As the reaction progresses, tlie system becomes viscous making stirring difficult. The method is economical and the product is of high purity. The technique is used for preparing polyvinyl chloride. (PVC), polystyrene (PS) and polymethymethacrylate (PMMA). 

Polymerisation is then brought about by free radical initiation using the water phase as a heat sink to help dissipate the heat of reaction. The polymer formed is stabilised within the emulsion by absorption onto surfactants and protective colloids as the polymerisation proceeds. The resultant product, unlike that obtained with suspension polymerisation is an homogenous, relatively stable emulsion of high molecular weight polymer in water. 

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