1、1,Chapter 2 Radical/Chain Polymerization,2,1 Reactions of Radical Polymerization,3,Elementary Reactions of Radicals,1）Addition(加成) 2 ) Transfer(转移) 3）Coupling (偶合） 4）Disproportionation （歧化）5）Fragmentation （分解）,.,.,4,Reactions of Polymerization,where Ri* is a active species of chain length i, I2 is t
2、he initiator (引发剂), M is the monomer, S is a transfer agent (链转移剂), and P is polymer,fast/slow,fast,fast,slow,Essence of Chain Polymerization,Each addition reproduces the reactive group. (True for every kind of chain polymerization, not just free radical.),R* = R radical polymerization,R* = R- anion
3、 polymerization,R* = R+ cation polymerization e.g.coordination polym.,ionic polymerization,6,2 Monomers for Chain Polymerization,7,General for All Chain Polymerizations Vinyl polymerizations work in general because converting a double bond into two single bonds is exothermic(放热).,Thermodynamics,Kine
4、tics,8,However, in more highly substituted systems, there is steric crowding in the polymer chains, and this decreases the driving force.,-,2.1 Monomer vs Mechanism,Heterolysis (异裂),Homolysis (均裂),ring opening,Radical Polym.,Ionic Polym.,Electron Effect （1）X=Y = Hradical polymerization（2）X=H, Y=OR (
5、electron donating)（3）X=H, Y=CN, COOH, COOR (electron withdrawing)* CH2=CHNO2 can be only polymerized by anion,cation(阳离子),anion(阴离子)/radical,Vinyl monomers (CH2=CHXY),11,Problem with Alkyl Groups,The allyl radical (lower structure) is very stable, and forms preferentially. Attempted free radical pol
6、ymerization of propylene fails！,(4) X=H, Y= Cl , Finductive 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-disubstitutedif both X and Y are electron donor/acceptor, CH2=CH(CN)2 (anion), CH2=CH(
7、CH3)2 (cation)if X and Y are different, determined by stronger group,COOCH3CH3, radical and anion,Monomer for coordination polymerization,Steric effect of substitute A. 1,1-disubstituted vinyl monomerR is too large to be polymerizedB. 1,2-disubstituteddisfavor for polymerization (electronic and ster
8、ic) C. tri- and tetra-substituted vinyl monomers normally impossible, except fluoroalkene,Polymerization ability is mainly determined by the electron effect, steric effect should be also considered when the substitute is large.,15,3. Mechanism of Radical Polymerization,3.1 Features of Radical,Chain
9、reaction,not stable,stable,Reactivity of radicals,very stable,3.2 Elementary Reactions of Radicals,1）Addition(加成) 2 ) Transfer(转移) 3）Coupling (偶合） 4）Disproportionation （歧化）5）Fragmentation （分解）,.,.,19,3.3 Reactions of Radical Polymerization,Initiation (Thermal 热分解),Ea =105-150 KJ/mol,Ea =20-34 KJ/mol
10、,Initiator breaks down smoothly to generate a low, steady-state (稳态) concentration of radicals.,fast,AIBN,20,Monomer 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 (增长),21,Propagation is Fast!,Time needed
11、to reach 106 in MW,22,Mode of addition (加成方式),Monosubstituted and 1,1-disubstituted 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
12、 result is that head-to-tail addition predominates. The exceptions are small substituents like chlorine or especially fluorine.,Regioselectivity(区域选择性),Stereoselectivity(立体选择性),23,Disproportionation: The radical at the end of one chain attacks a hydrogen atom at the second-to-last carbon atom in the
13、 second chain, as shown here in an example with methyl methacrylate,Coupling(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),kt,c,kt,d,Ea=821KJ/mol,Disproportionation Vs Coupling,Termination mode depends la
14、rgely on the structure of the monomer unit, the reaction temperature and pressure. The mode of termination has no influence on the rate of radical polymerization process The molecular weight distributions are strongly influenced by the termination mode.,25,The Steady State （稳态）,Initiation is relativ
15、ely slow but continuous. Termination speeds up as active radical concentration 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.,
16、26,Chain transfer occurs when a radical species reacts with a nonradical species. The result must be at least one radical species. Decrease the MW Impact on polymerization rate ?,Chain Transfer (链转移),X-Y can be monomer; solvent; polymer; other chemicals,27,Types of Chain Transfer,In many cases, a ch
17、ain transfer agent is added deliberately to the reaction mixture. Mercaptans (thiols) are the most general. 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 i
18、nitiator is another way to decrease MW, but the reaction rate would increase proportionally, a possibly dangerous situation.,Chain transfer agent,28,Naturally, there are many even-electron species present in the reaction mixture (i.e., monomer, initiator, solvents, etc.), and all of these may partic
19、ipate in transfer reactions. Here is an example of transfer to initiator featuring acrylonitrile and benzoyl peroxide (BPO),Chain transfer to others,29,Chain transfer to polymer,This reaction happens very commonly during the free radical polymerization of polyethylene. greater stability of the secon
20、dary radical. Long branches.,Intermolecular (分子间),30,Chain transfer to polymer,The most convenient site is a H that is a short distance back on the same chain, Short branches are very common, typical: 3-6% short (=6 carbons) branches Branches disrupt chain packing, and decrease degree of crystallini
21、ty, creating low density polyethylene LDPE is much softer than the version that is linear (made by coordination polymerization).,Intramolecular (分子内),Summary,Elementary Reactions of radical polymerization initiation： Ea = 80 120 kJ/mol; slow propagation： Ea = 20 34 kJ/mol; fast termination： Ea = 8.5
22、 20 kJ/mol; fast chain transfer： Ea = 8 60 kJ/mol; fast,32,4 Initiation,33,4.1 Thermal initiators,1 initiatorA. azo compounds ( -N=N-，偶氮化合物) Release of nitrogen,34,B. Peroxides (-O-O-，过氧化合物)The rate of decomposition of peroxide initiators is a function of their chemical structure: ( R COR ROCO) The
23、nature of the solvent may also affect the rate of decomposition: tert-butyl peroxide higher in acetonitrile than in cyclohexane,inorganic,35,C. Redox initiators （氧化还原引发剂）Usually two components low Ea for polymerization at low temperature,36,2. Kinetics of decompositionUnimolecular decomposition and
24、first order kinetics for azo and peroxides.Integration leads to an expression that describes the decreasing initiator concentration as a function of time. Two parameters: kd and t 1/2 = ln2 / kd (half life of initiator),kd(s-1, min-1, or h-1 ) is the rate coefficient of initiator decomposition,Rd (m
25、ol/Ls ) is the rate of initiator decomposition,37,3 Initiation Efficiency ( f )Cage effect As an initiator molecule decomposes, the resulting geminate radicals can either react with one another inside the solvent cage or diffuse out of the cage to participate in other reactions, mainly addition to m
26、onomer. This so called cage effect is the main reason that the initiator efficiency f is lower than unity (another reason is primary radical termination) .,38,Efficiency of AIBN as initiator (solution styrene:toluene 50% v/v, 70 oC) Moad et al., Makromol,Chem.Rapid.Commun 1984, 5, 793, cumulative O
27、instantaneous,39,4 Selection of initiator,According to the polymerization method Bulk, solution polymerization organic Emulsion, aqueous polymerization water-soluble or redox According to polymerization temperature kd or t1/2,40,4.2 Self-initiation/Thermal-initiation (热引发)Only proven to occur with s
28、tyrene. Mechanism involves preliminary dimer formation (via Diels-Alder reaction of two monomers), followed by hydrogen atom transfer to a third monomer. Tp100 - 150 C ; Ri M3 Pure product；,41,4.3 Photoinitiation (光引发),42,4.4 Ionizing radiation (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 polymerization,43,4.5 Plasma (等离子体引发)4.6 Microwave (微波引发) = 1m-1mm accelerate the polymerization rate,
