1、1Chapter 2 Radical/Chain Polymerization21 Reactions of Radical Polymerization3Elementary Reactions of Radicals1）Addition(加成)2)Transfer(转移)3）Coupling(偶合）4）Disproportionation（歧化）5）Fragmentation（分解）R +RRR+CH3 CH2CH2CH3 CH2CH2+CH4CF3H +CH3 CF3CH3CH2CH3 +CH4CH2CH2+CH3CO CH3 +CO.4Reactions of Polymerizati
2、on where Ri*is a active species of chain length i,I2 is the initiator(引发剂),M is the monomer,S is a transfer agent(链转移剂),and P is polymerfast/slowfastfastslowR*CC*R+Essence of Chain PolymerizationEach addition reproduces the reactive group.(True for every kind of chain polymerization,not just free ra
3、dical.)R*=Rradical polymerizationR*=R-anion polymerizationR*=R+cation polymerizatione.g.coordination polym.ionic polymerization62 Monomers for Chain Polymerization7General for All Chain Polymerizations Vinyl polymerizations work in general because converting a double bond into two single bonds is ex
4、othermic(放热).ThermodynamicsKinetics8 However,in more highly substituted systems,there is steric crowding in the polymer chains,and this decreases the driving force.-2.1 Monomer vs MechanismOOXXHeterolysis(异裂)Homolysis(均裂)ring openingRadical Polym.Ionic Polym.Electron Effect（1）X=Y=Hradical polymeriza
5、tion（2）X=H,Y=OR(electron donating)（3）X=H,Y=CN,COOH,COOR(electron withdrawing)*CH2=CHNO2 can be only polymerized by anionA+CH2CHORACH2CHORCH2CHCNBCH2CH+BCNcation(阳离子阳离子)anion(阴离子阴离子)/radicalVinyl monomers(CH2=CHXY)11Problem with Alkyl Groups The allyl radical(lower structure)is very stable,and forms
6、preferentially.Attempted free radical polymerization of propylene fails！(4)X=H,Y=Cl,F inductive and conjunctive effect,radical polymerization only(5)X=H,Y=aromatic or conjunctive group radical/anion/cation,ca styrene,butadiene(6)X=R,Y=R,1,1-disubstituted if both X and Y are electron donor/acceptor,C
7、H2=CH(CN)2(anion),CH2=CH(CH3)2(cation)if X and Y are different,determined by stronger group CH2CCOOCH3CH3COOCH3CH3,radical and anionTiRMonomer for coordination polymerizationSteric effect of substituteA.1,1-disubstituted vinyl monomer R is too large to be polymerizedB.1,2-disubstituted disfavor for
8、polymerization(electronic and steric)C.tri-and tetra-substituted vinyl monomers normally impossible,except fluoroalkene CH2CPolymerization ability is mainly determined by the electron effect,steric effect should be also considered when the substitute is large.153.Mechanism of Radical Polymerization3
9、.1 Features of RadicalHHCCHHHCl2H2HCl+hvCl22ClhvClH2+HClHH+Cl2HCl+Cl+Chain reactionnot stablestableReactivity of radicalsHCH3C6H5RCH2R2CHCH2=CH-CH2CH2Cvery stable3.2 Elementary Reactions of Radicals1）Addition(加成)2)Transfer(转移)3）Coupling(偶合）4）Disproportionation（歧化）5）Fragmentation（分解）R +RRR+CH3 CH2CH2
10、CH3 CH2CH2+CH4CF3H +CH3 CF3CH3CH2CH3 +CH4CH2CH2+CH3CO CH3 +CO.193.3 Reactions of Radical PolymerizationInitiation(Thermal 热分解)Ea=105-150 KJ/molEa=20-34 KJ/molInitiator breaks down smoothly to generate a low,steady-state(稳态)concentration of radicals.fastAIBN20 Monomer adds quickly to the small number
11、 of growing chains present at any given time.Exothermic(55-95 kJ/mol);low Ea(20-30 kJ/mol)Propagation(增长增长)21Propagation is Fast!Time needed to reach 106 in MW OOClMethyl methacrylateStyreneVinyl Chloride7.6s1.5s0.13s22Mode of addition(加成方式加成方式)Monosubstituted and 1,1-disubstituted vinyl groups pres
12、ent a choice to the incoming radical Attack almost always occurs at the least substituted carbon atom,mostly for steric reasons,but also because of the stabilizing effect of the substituent(s).The result is that head-to-tail addition predominates.The exceptions are small substituents like chlorine o
13、r especially fluorine.Regioselectivity(区域选择性)Stereoselectivity(立体选择性).+.head to tailhead to head?+isotacticsyndiotactic23Disproportionation:The radical at the end of one chain attacks a hydrogen atom at the second-to-last carbon atom in the second chain,as shown here in an example with methyl methac
14、rylateCoupling(Combination):Two radicals at the chain termini simply join to form a single bond,as shown here in an example with styrene:Termination(终止终止)kt,ckt,dEa=821KJ/molDisproportionation Vs CouplingTermination mode depends largely on the structure of the monomer unit,the reaction temperature a
15、nd pressure.The mode of termination has no influence on the rate of radical polymerization processThe molecular weight distributions are strongly influenced by the termination mode.25The Steady State（稳态）（稳态）Initiation is relatively slow but continuous.Termination speeds up as active radical concentr
16、ation builds.Termination removes(kills)active radicals.a steady-state concentration of radicals is established early in the reaction.The concentration of radicals is very small(ca.10-8 M)and nearly constant throughout.26Chain transfer occurs when a radical species reacts with a nonradical species.Th
17、e result must be at least one radical species.Decrease the MWImpact on polymerization rate?Chain Transfer(链转移)X-Y can bemonomer;solvent;polymer;other chemicals27Types of Chain TransferIn many cases,a chain transfer agent is added deliberately to the reaction mixture.Mercaptans(thiols)are the most ge
18、neral.The sulfur-centered radical reinitiates very efficiently.The result is a diminution of the molecular weight without changing the overall rate of conversion of monomer to polymer.Using more initiator is another way to decrease MW,but the reaction rate would increase proportionally,a possibly da
19、ngerous situation.Chain transfer agent28Naturally,there are many even-electron species present in the reaction mixture(i.e.,monomer,initiator,solvents,etc.),and all of these may participate in transfer reactions.Here is an example of transfer to initiator featuring acrylonitrile and benzoyl peroxide
20、(BPO)Chain transfer to others29Chain transfer to polymerThis reaction happens very commonly during the free radical polymerization of polyethylene.greater stability of the secondary radical.Long branches.Intermolecular(分子间)H.+.30Chain transfer to polymerThe most convenient site is a H that is a shor
21、t distance back on the same chain,Short branches are very common,typical:3-6%short(=6 carbons)branchesBranches disrupt chain packing,and decrease degree of crystallinity,creating low density polyethylene LDPE is much softer than the version that is linear(made by coordination polymerization).Intramo
22、lecular(分子内)H.back biteSummaryElementary Reactions of radical polymerizationinitiation：Ea=80 120 kJ/mol;slowpropagation：Ea=20 34 kJ/mol;fasttermination：Ea=8.5 20 kJ/mol;fast chain transfer：Ea=8 60 kJ/mol;fast324 Initiation334.1 Thermal initiators1 initiatorA.azo compounds(-N=N-，偶氮化合物)Release of nitr
23、ogen34B.Peroxides(-O-O-，过氧化合物)The rate of decomposition of peroxide initiators is a function of their chemical structure:(R COR 100-150 C;Ri M3Pure product；H+Diels-Alderdimerizationmolecular-assistedhomolysisAlder ene-reactionradical recombination1,3-H shift414.3 Photoinitiation(光引发光引发)424.4 Ionizing radiation(辐射引发辐射引发)X-ray(=100-0.1A),gamma-ray(=0.5-0.001A).Random destruction leads to radical formation.Used only in very special cases,e.g.solid phase polymerization434.5 Plasma(等离子体引发等离子体引发)4.6 Microwave(微波引发微波引发)=1m-1mm accelerate the polymerization rate
