1、3.2 Initiator and Initiationn3.2.1 Types of Initiatorsn3.2.2 Kinetics of Initiator Decomposition (1)Initiator Decomposition Rate (2)Initiator Efficiency(3)Choice of Initiator n3.2.3 Other Initiation(1)Thermal-initiated polymerization(2)Light initiated polymerization(3)High energy radiation initiated
2、 polymerization.3.2 Initiator and InitiationnInitiation is a process which converts olefinic monomers to radicals.nPrimary radicals can be produced through light,heat,or high energy radiation directly acting on monomers.nBut,what used more often is initiator.nInitiators have weak bonds,nThermal deco
3、mposition can produce two primary radicals.3.2.1 Types of InitiatorsnAzo initiator nOrganic peroxide initiatornInorganic peroxide initiatornRedox initiation system.（1）Azo Initiator nazobisisobutyronitrile（AIBN）azo-bis-iso-heptonitrile.（2）Organic Peroxide Initiator dibenzoyl peroxide dialkyl peroxide
4、.（3）Inorganic Peroxide Initiatornpersulphate.（4）Redox initiation systemncomponents：inorganic and organic nproperties：oleiferous solubility and aqueous solubility.for instanceThermal decomposition activation energyFrom 200 downgrade to 40KJ/mol From 125 downgrade to 50KJ/molFrom 140 downgrade to 50KJ
5、/molIn the redox system of aqueous solubility,inorganic reducers are usually used,and a small amount of organic reducers,such as alcohol,amine,and so on,are used too.Systems of Oleiferous Solubility are Often Used Initiatorsntertiary amine，naphthenate，mercaptan，metallo organic compound.Examplenbenzo
6、yl peroxide and N，Ndimethyl aniline initiating system.Some metallo organic compoundsnTake trialkylboranel for example,it can react with the oxygen in the air,and form a redox initiation system,which can initiate olefinic monomers polymerization at a low temperature.3.2.2 Decomposition Kinetics of In
7、itiatornIn each elementary reaction of radical polymerizationnInitiation rate is the lowest one.nHave a strong effect on both overall polymerization rate and molecular weight.nChain initiation ratenControlled by the initiator decomposition rate.nBy studying the initiator decomposition rate,we can fi
8、nd the quantitative relationship between the radical generation rate and the initiator concentration,temperature,and time.（1）Thermal Decomposition Rate of Initiator Initiator thermal decomposition is a first order reaction.initiatorPrimary radicalDecomposition rate of initiator mol/(Ls)Rate constant
9、 for decomposition s-1 integratedAt a given temperature，through the plot of ln(I/I0)to time t Rate expression.（1）Thermal Decomposition Rate of Initiator the plot of ln(I/I0)to t Its slope=-kd ln(I/I0)t/min0.0-0.4-0.80.080160240320=(-0.4)-(-0.8)/(160-320)60=-4.2x10-5(s-1).The halflife of the first or
10、der reaction-t1/2 nTo the first order reaction,we usually take the halflife to characterize the reaction rate.nHalflife is the time needed for the initiator to decompose to half its original concentration.Arrhenius formulation nThe dependence of the rate constant for initiator decomposition on tempe
11、rature is in conformity to Arrhenius formulation.nThe plot of lnkd to 1/T should be a straight line.nFrequency factor-Ad can be worked out from the intercepting.nThe decomposition free energy-Ed can be worked out from the slope.Table 3-6 Kinetic Parameter of Some Typical Initiators Initiators solven
12、t T/oC kd/S-1 t1/2/h Ed/kj/molAIBN 50 2.64*10-6 73 128.4 60.5 1.16*10-5 16.6 69.5 3.78*10-5 5.1 BPO benzene 60 2.0*10-6 96 124.3 80 2.5*10-5 7.7cumine hydroperoxide toluene 125 9*10-6 21.4 139 3*10-5 6.4 potassium Persulphate 0.1mol/60 3.16*10-6 61 140.2 L.KOH 70 2.33*10-5 8.3.（2）Efficiency of Initi
13、ationnOnly a part of primary radicals are effective in initiating monomers polymerizing.nSome initiators are exhausted,because of the side reactions accompanying with the induced decomposition,and/or the cage effect.nThe efficiency of initiation is defined as the ratio of initiators which are used t
14、o initiate polymerization to initiators that decompose in the whole process of polymerization,and is denoted as f.Induced Decomposition of InitiatorsnThe chain transfer reaction from free radicals to initiator molecules.nThe total number of free radicals are not increased,however,one initiator is ex
15、hausted,which lowers the efficiency of initiation.Factors Impact on the Efficiency of InitiationnThe efficiency of initiation depends on the initiator type.nThe induced decomposition of AIBN is very little.nHydroperoxide easily undergoes induced decomposition,which makes the efficiency of initiation
16、 lower than 0.5.nThe type of monomers also affects the efficiency of initiation.nMonomers with high activity,such as styrene,and acrylonitrile,can react with the free radicals rapidly,and reduce the induced decomposition.nIn contrast,the ability of capturing free radicals is relatively poor for viny
17、l acetate type monomers,so the value of f is fairly low.Decomposition Rate of InitiatorsnInduced decomposition usually increases Rd,and decreases t1/2.nWhen accompanied by induced decomposition,the decomposition rate of initiators can be expressed byNormal first order decomposition rateInduced decom
18、position rateBetween 1 and 2.Cage EffectnThe initiator concentration is very low in polymerization system,so the primary radicals are besieged by the cage of solvent molecules.nIn order not to react with each other,primary radicals must get out of the cage.nThe life expectancy of free radicals in th
19、e cage is about 1011109 S.For examplenSide reactions of cage effect of AIBN.Side reactions of cage effect of BPO.Table 3-7 Efficiency of Initiation of AIBNnThe efficiency of initiation depends on the initiators,monomers,solvent,system viscosity,and other factors.nThe value of f of AIBN to different
20、monomersMonomersf,%Monomersf,%AN100VC7077Styrene80MMA52VAC6882.The Choose of InitiatorsnFirstly,according to the polymerization methods,nInitiators with oleiferous solubility are OK for bulk,solution,and suspension polymerization.nInitiators with aqueous solubility are OK for emulsion polymerization
21、.nSecondly,according to the halflife,The chosen initiator should have a halflife comparative to or of the same order with the time of polymerization.nThirdly,the dosage of initiator should be reasonable,nThe initiator concentration I affects not only the rate of polymerization,but also the molecular
22、 weight of the products,further,it has a opposite effect（aftermentioned）nThe proper initiator concentration depends on an amount of experiments.nIn addition,such as price,source,toxicity,stability,the effect on the color shade of polymers,and so on,should also be counted into consideration.3.2.3 Oth
23、er Initiating AbilitynThermal-initiated polymerizationnMonomers can polymerize under the function of heat,without initiator,which is called thermal-initiated polymerization,or for short,thermal polymerization.nPhoto-initiated polymerizationnUnder light irradiation,many olefinic monomers can produce
24、free radicals,which can initiate polymerization.We call this light initiated polymerization.nHigh energy radiation initiated polymerization.(1)Thermal-Initiated PolymerizationStyrene is used most to study the mechanism of thermal polymerizationThe rate of initiation is .Styrene Thermal Polymerizatio
25、nnIf the conversion of styrene thermal polymerization reaches 50 percent n400 days are needed at 29.n235 minutes are needed at 127.nOnly 16 minutes are needed at 167.(2)Photo-Initiated PolymerizationnLight quantum energy Planck constant The speed of lightThe frequency of light The wavelength of ligh
26、tnPhoto-initiated polymerization can be divide into direct light initiated polymerization and photosensitive polymerization.Direct Light Initiated Polymerization Light quantumMonomer moleculeExcited state Free radical.Example.Photosensitive PolymerizationnPhotosensitizers are used to bring about the
27、 homolysis of a monomer or initiator which does not undergo sufficient excitation at the available frequencies of light.nRadicals are subsequently produced to initiate polymerization.AIBN+MInitiating Polymerization.Methyl Vinyl Ketone and Benzoin.Characteristics of hotopolymerizationnControl of phot
28、oinitiation is most easyjust light on and light off，the generation and quenching of radicals are within rather short time，Light intensitys control and measurement is easy，good experimental repeatability，product is pure.Photopolymerization is always used in the determination of radical life,constants
29、 for chain propagation,termination.nThe overall activation energy for photoinitiation is low，polymerization can take place at comparatively low temprature.There are few applications in industry，but in printing plate and integrate circuit，photosensitive resin has been used widely.(3)High Energy Radia
30、tion Initiated PolymerizationnInitiating Polymerization by High Energy Radiation，for short,Radiation-Initiated polymerizationnHigh Energy Radiation includes-ray,x-ray,-ray（electron flow）,-ray（high speed 2He flow）,neutron flow ectnRadiation Initiation is rather complicatednTemperature effect on polymerizing rate is comparatively small.
