1、1Chapter 2 Radical/Chain Polymerization21 Reactions of Radical Polymerization3Elementary Reactions of Radicals1）Addition(加成)2 ) Transfer(轉(zhuǎn)移)3）Coupling (偶合） 4）Disproportionation （歧化）5）Fragmentation （分解）.4Reactions of Polymerization where Ri* is a active species of chain length i, I2 is the initiator

2、(引發(fā)劑), M is the monomer, S is a transfer agent (鏈轉(zhuǎn)移劑), and P is polymerfast/slowfastfastslowEssence of Chain PolymerizationEach addition reproduces the reactive group. (True for every kind of chain polymerization, not just free radical.)R* = Rradical polymerizationR* = R-anion polymerizationR* = R+c

3、ation 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 exothermic(放熱). ThermodynamicsKinetics8However, in more highly sub

4、stituted systems, there is steric crowding in the polymer chains, and this decreases the driving force. -2.1 Monomer vs MechanismHeterolysis(異裂)Homolysis(均裂)ring openingRadical Polym.Ionic Polym.Electron Effect（1）X=Y = Hradical polymerization（2）X=H, Y=OR (electron donating) （3）X=H, Y=CN, COOH, COOR

5、(electron withdrawing) * CH2=CHNO2 can be only polymerized by anioncation(陽離子)anion(陰離子)/radicalVinyl monomers (CH2=CHXY)11Problem with Alkyl Groups The allyl radical (lower structure) is very stable, and forms preferentially. Attempted free radical polymerization of propylene fails！(4) X=H, Y= Cl ,

6、 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, CH2=CH(CN)2 (anion), CH2=CH(CH3)2 (cation)if X and Y are different, determ

7、ined by stronger group COOCH3CH3, radical and anionMonomer for coordination polymerizationSteric effect of substituteA. 1,1-disubstituted vinyl monomer R is too large to be polymerizedB. 1,2-disubstituted disfavor for polymerization (electronic and steric)C. tri- and tetra-substituted vinyl monomers

8、 normally impossible, except fluoroalkene Polymerization ability is mainly determined by the electron effect, steric effect should be also considered when the substitute is large.153. Mechanism of Radical Polymerization3.1 Features of RadicalChain reactionnot stablestableReactivity of radicalsvery s

9、table3.2 Elementary Reactions of Radicals1）Addition(加成)2 ) Transfer(轉(zhuǎn)移)3）Coupling (偶合） 4）Disproportionation （歧化）5）Fragmentation （分解）.193.3 Reactions of Radical PolymerizationInitiation (Thermal 熱分解)Ea =105-150 KJ/molEa =20-34 KJ/molInitiator breaks down smoothly to generate a low, steady-state (穩(wěn)態(tài))

10、concentration of radicals. fastAIBN20Monomer adds quickly to the small number of growing chains present at any given time. Exothermic (55 - 95 kJ/mol); low Ea(20 -30 kJ/mol )Propagation (增長)21Propagation is Fast! Time needed to reach 106 in MW 22Mode of addition (加成方式)Monosubstituted and 1,1-disubst

11、ituted vinyl groups present 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 s

12、ubstituents like chlorine or especially fluorine.Regioselectivity(區(qū)域選擇性)Stereoselectivity(立體選擇性)23Disproportionation: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 methacrylateCoupling(Combina

13、tion):Two radicals at the chain termini simply join to form a single bond, as shown here in an example with styrene:Termination (終止)kt,ckt,dEa=821KJ/molDisproportionation Vs CouplingTermination mode depends largely on the structure of the monomer unit, the reaction temperature and pressure.The mode

14、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 （穩(wěn)態(tài)）Initiation is relatively slow but continuous. Termination speeds up as active radical concentration builds. Terminati

15、on 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. The result must b

16、e at least one radical species. Decrease the MWImpact on polymerization rate ?Chain Transfer (鏈轉(zhuǎn)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 general. T

17、he 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 dange

18、rous 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 per

19、oxide (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 (分子間)30Chain transfer to polymerThe most convenient site is a H that is

20、a short 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).Intramolecular (分子內(nèi))SummaryElementary Reactions of radical polymerizationinitiation：Ea = 80 120 kJ/mol;