4/29/20152 Characteristics of Chain-Growth Polymerization 1. Only growth reaction adds repeating units one at a time to the chain time to the chain 2. Monomer concentration decreases steadily throughout the reaction the reaction 3. High Molecular weight polymer is formed at once; polymer molecular weight changes little throughout polymer molecular weight changes little throughout the reaction. the reaction. 4. Long reaction times give high yields but affect molecular weight little. molecular weight little. 5. Reaction mixture contains only monomer, high polymer, and about part of growing chains. polymer, and about part of growing chains.
4/29/20153 The Chemistry of Free Radical Polymerization Radical Generation Initiator Radicals RR 2 R Initiation Monomers R + CCRCC Propagation RCC + CCCCCR Termination RCC + CCCR RCCCCCR Polymer -
4/29/20154 Free Radical Polymerization Mechanisms 1. Overview – Free radical polymerization processes involve at least three mechanistic steps. involve at least three mechanistic steps. A. Initiation 1. Radical Formation (Generation) InIn hv, etc. In + 2. Initiation 2. Initiation InM In + M
4/29/20155 B. Propagation In-M 1. + M 2 In-M 1 M 2. In-M 1 M 2. + M 3 In-M 1 M 2 M 3. In-M 1 M 2 M 3 …M X. + M Y In-M 1 M 2 M 3 …M X M Y.
4/29/20156 C. Termination 1) Radical Coupling (Combination) In + In InIn 2) Disproportionation ( -hydrogen transfer) InM x C H C H HH + In M y C H C H H H H 3 CCH 2 M y In CH 2 CH InM x + In-M X. +. M Y -In In-M X -M Y -In
4/29/20157 D. Chain Transfer (sometimes) – An atom is transferred to the growing chain, terminating the chain growth to the growing chain, terminating the chain growth and starting a new chain. and starting a new chain.P x R P x + R H + P x + P y H P x P y + Chain Transfer to Chain Transfer Agent: Chain Transfer to Polymer: Chain Transfer to Monomer: P x. + H 2 C=CH-(C=O)OR Causes Branching
4/29/20158 E. Inhibition and Retardation – a retarder is a substance that can react with a radical to form products incapable that can react with a radical to form products incapable of reacting with monomer. An inhibitor is a retarder of reacting with monomer. An inhibitor is a retarder which completely stops or “inhibits” polymerization. which completely stops or “inhibits” polymerization. 2. Monomers that are susceptible to free radical addition A. Vinyl Monomers H 2 CCHXH 2 CCH Cl Vinyl chloride H H Y X F FH H Vinylidene fluoride
4/29/20159 B. Allyl Monomers C. Ester Monomers OH O OR O Acrylic Acid Acrylate Esters X Cl Allyl Chloride 1) Acrylates
4/29/ ) Methacrylates OH O OR O Methacrylate Esters 3) Vinyl Esters O O Vinyl Acetate D. Amide Monomers NH 2 O NH 2 O Acrylamide Methacrylamide Methacrylic Acid
4/29/ Monomers that are not susceptible to Free Radical Addition Addition A. 1,2 olefins (Polymerize to oils only) B. Vinyl ethers O R O methyl vinyl ether x C. 1,2-disubstituted Ethylenes H Cl H Cl 1,2-dichloroethylene
4/29/ Initiation – “Getting the thing started!” A. Radical Generators (Initiators) 1. Benzoyl Peroxide C O OOC O C C O O CO 2 (continued)
4/29/ Ph Ph New Active Site Initiator End-Group 2) t-Butyl Peroxide H 3 CC CH 3 CH 3 OOC CH 3 CH 3 CH C H 3 CC CH 3 CH 3 2 (continued)
4/29/ H 3 CC CH 3 CH 3 + O O O O 3) Azobisisobutyronitrile (AIBN) (continued) CH 3 CH 3 CH 3 CH 3 H 3 C – C – N=N – C – CH 3 CNCN CNCN ~60 o C or h or h
4/29/ H 3 CC CH 3 CN + N 2 H 3 CC CH 3 CN C H 2 CH Ph 4) Cumyl Hydroperoxide C CH 3 CH 3 OOH PhO + OH (continued)
4/29/ PhO + O O PhO O O (continued)
4/29/ Hydroperoxides can generate radicals by “induced decomposition” from growing polymer chains: P + HOOR PH + OORROO 2 R-OO-OO-R 2 RO + O 2 What effect does this have on the polymerization process? Acting as a chain-transfer agent, it reduces the degree of polymerization and molecular mass.
4/29/ ) Redox Initiator Systems HOOH Fe 2+ HO + OH + Fe 3+ + OR O 3 SOOSO 3 + SO 3 2- SO SO S-SO 3 -
4/29/ ) Photoinitiators(Photocleavage – Norrish I) 6) Photoinitiators (Photocleavage – Norrish I) O HO h v C OH H + C O C OH H + Ph PhPh OH H benzoin
4/29/ (continued) OR CC OO h v C O 2 benzil
4/29/ ) Photoinitiators (Photo-Abstraction) O h v PhPh O * benzophenone excited state C R R HN R R PhPh OH + C R R N R R Photosensitizer Coinitiator
4/29/ Propagation - “Keeping the thing going!” A. The addition of monomer to an active center (free radical) to generate a new active center. to generate a new active center. RC H 2 CH 2 X X RC H 2 H C X C H 2 CH X X X etc. etc. RC H 2 H C X C H 2 CH X n (continued)
4/29/ Examples: RC H 2 CH 2 Ph Ph RC H 2 H C Ph C H 2 CH Ph n RC H 2 C H 2 CH C O CH 3 O O CH 3 O RC H 2 C H 2 H C C O CH 3 O C H 2 CH C O CH 3 O Polystyrene PolymethylAcrylate
4/29/ B. Configuration in Chain-Growth Polymerization 1) Configuration Possibilities - attack - attack P sterically and electronically unfavored favored H 2 C CH X HC CH 2 PC H 2 C H X P H C 2 X X X.
4/29/ ) Radical Stability Considerations Which possible new active center will have the greatest stability? PC H 2 CH 2 PC H 2 CH PC H 2 CH -attack produces resonance stabilized free radical.
4/29/ P H C CH 2 X No resonance stabilization P ______________________________________________ HCC O OCH 3 CH 2 H 2 CC H C O OCH 3 X PCH CH 2 COCH 3 O P H C H 2 CH CO OCH 3 P H C H 2 CH CO OCH 3 Secondary radical is resonance stabilized
4/29/ (more examples) Cl Cl H H H H Cl Cl P X PC Cl Cl CH 2 PC H 2 C Cl Cl PC H 2 C Cl Cl PC H 2 C Cl Cl Tertiary radical is resonance stabilized
4/29/ ) Steric Hinderance Considerations P HC CH 2 X H 2 C CH X X For large X, -substitution is sterically favored is sterically favored 4) Radical Stability 3 o > 2 o > 1 o
4/29/ ) “Bottom Line” Resonance and steric hinderance considerations lead to the conclusion that -substitution (head-to-tail) is strongly preferred in chain-growth polymerization. C H 2 H C C H 2 H C C H 2 H C C H 2 H C XXXX Alternating configuration
4/29/ Termination - “Stopping the thing!” A. Coupling (most common) P x C H 2 C H X + P y C H 2 C X H P y C H 2 C X H P x C H 2 C H X - occurs head-to-head - produces two initiator fragments (end-groups) per chain. per chain.
4/29/ B. Disproportionation InM x C H C H HH + In M y C H C H H H H 3 CCH 2 M y In CH 2 CH InM x + - Produce one initiator fragment (end-group) per chain - Production of saturated chain and 1 unsaturated chain per termination per termination
4/29/ C. Factors affecting the type of termination that will take place. place. 1) Steric factors - large, bulky groups attached directly to the active center will hinder coupling to the active center will hinder coupling 2) Availability of labile -hydrogens 3) Examples – PS and PMMA + P x C H 2 C H CC H 2 P y H Combination (coupling) Polystyrene (continued)
4/29/ P y P x C H 2 H C H C C H 2 PhPh Ph = CH 3 H 3 C CH 3 H 3 C ~~~P X – CH 2 -C. +. C-CH 2 - P Y ~~~ C=O O=C C=O O=C O O O O CH 3 CH 3 CH 3 CH 3 PMMA 1.Sterically hindered 2.5 -Hydrogens 3.Disproportion- ation dominates (continued)
4/29/ CH 3 H 3 C CH 3 H 3 C ~~~P X – CH 2 =C + HC-CH 2 - P Y ~~~ C=O O=C C=O O=C O O O O CH 3 CH 3 CH 3 CH 3 4)Electrostatic Repulsion Between Polar Groups – Esters, Amides, etc.
4/29/ ~~~P X – CH 2 -CH. +. HC-CH 2 - P Y ~~~ C N N C C N N C Polyacrylonitrile (PAN) One might assume electrostatic repulsion in this case. BUT, how about electrostatic attraction from the nitrogen to the carbon? Also, steric hindrance is limited. At 60 o C, this terminates almost exclusively by coupling!
4/29/ D. Primary Radical Termination ~~~P X – CH 2 -CH. +. In X ~~~P X – CH 2 -CH-In X More Likely at High [In. ] So molecular mass can be controlled using chain-transfer agents, hydroperoxide initiators, OR higher levels of initiator!
4/29/ Chain-Transfer - “Rerouting the thing!” The transfer of reactivity from the A.Definition – The transfer of reactivity from the growing polymer chain to another species. An atom is transferred to the growing chain, terminating the chain and starting a new one. ~~~P X – CH 2 -CH. + X-R ~~~P X – CH 2 -CHX + R. Y Y Y Y ~~~P X – CH 2 -CH. + CCl 4 ~~~P X – CH 2 -CHCl + Cl 3 C. Y Y B. Chain-transfer to solvent:
4/29/ C. Chain-transfer to monomer: ~~~P X – CH 2 -CH. + H 2 C =CH ~~~P X – CH 2 -CH 2 + H 2 C =C. OR
4/29/ H H H H ~~~P X – CH - C. + H 2 C =CH ~~~P X – CH 2 =CH. + H 3 C - C.
4/29/ Propylene – Why won’t it polymerize with Free Radicals? ~~~P X – CH 2 -CH. + HCH=CH CH 3 CH 3 ~~~P X – CH 2 -CH 2 -CH 3 + CH 2 =CH-CH 2. H 2 C-CH-CH 2 Chain-transfer occurs so readily that propylene won’t polymerize with free radicals.
4/29/ D. Chain-transfer to polymer: ~~~P X – CH 2 -CH 2 -CH 2. + ~~~CH 2 -CH 2 -CH 2 ~~~ ~~~P X – CH 2 -CH 2 -CH 3 + ~~~CH 2 -CH-CH 2 ~~~ Increases branching and broadens MWD! E.Chain-transfer to Initiator (Primary Radical Termination): ~~~P X – CH 2. + R-O-O-R ~~~P X – CH 2 -OR +. OR
4/29/ The transfer of reactivity from the Definition – The transfer of reactivity from the growing polymer chain to another species. An atom is transferred to the growing chain, terminating the chain and starting a new one. F. Chain-transfer to Chain-transfer Agent: Examples: R-OH; R-SH; R-Cl; R-Br ~~~P X – CH 2 -CH 2. + HS-(CH 2 ) 7 CH 3 ~~~P X – CH 2 -CH 3 +. S-(CH 2 ) 7 CH 3. CXH-CH 2 - S-(CH 2 ) 7 CH 3 etc., etc., etc.
4/29/ Inhibition and Retardation - “Preventing the thing or slowing it down!” Compounds that slow down or stop poly- Definition – Compounds that slow down or stop poly- merization by forming radicals that are either too stable or too sterically hindered to initiate poly- merization OR they prefer coupling (termination) reactions to initiation reactions. ~~~P X – CH 2 -CH. + O= =O para-Benzoquinone ~~~P X – CH 2 -CH 2 -O- -O. Will Not Propagate ~~~P X – CH 2 -CH. + O=O ~~~P X – CH 2 -CH-O-O.
4/29/ Kinetics of Free Radical Polymerization 1. Initiation I 2 R. Radical Generation kdkdkdkd R. + M M 1. Initiation kikikiki Assuming that k i >>k d and accounting for the fact that two Radicals are formed during every initiator decomposition, The rate of initiation, R i, is given by: R i = d[M i ] = 2fk d [ I ] R i = d[M i ] = 2fk d [ I ] dt dt f = efficiency of the initiator and is usually (RDS)
4/29/ Propagation M 1. + M M 2. M 2. + M M 3. M 3. + M M M x. + M M x+1. R p = - d[M] = k p [M. ][M] R p = - d[M] = k p [M. ][M] dt dt kpkpkpkp kpkpkpkp kpkpkpkp kpkpkpkp We assume that the reactivity of the growing chain is independent of the length of the chain.
4/29/ Termination M x. +. M y M x -M y (Combination) M x. +. M y M x + M y (Disproportionation) k tc k td Since two radicals are consumed in every termination, then: R t = 2k t [M. ] 2 4. Steady State Assumption Very early in the polymerization, the concentration of radicals becomes constant because R i = R t 2fk d [ I ] = 2k t [M. ] 2
4/29/ fk d [ I ] = 2k t [M. ] 2 Solve this equation for [M. ]: [M. ] = (fk d [I]/k t ) 1/2 Substituting this into the propagation expression : R p = k p [M. ][M] = k p [M](fk d [I]/k t ) 1/2 Since the rate of propagation, R p, is essentially the rate of polymerization, the rate of polymerization is proportional to [I] 1/2 and [M].
4/29/ Kinetic Chain Length, 5. Kinetic Chain Length, The average number of monomer units Definition – The average number of monomer units polymerized per chain initiated. This is equal to the Rate of polymerization per rate of initiation: R p /R i = R p /R t under steady state conditions. k p [M][M. ] = k p [M] k p [M][M. ] = k p [M] 2k t [M. ] 2 2k t [M. ] = __k p [M]___ 2(f k t k d [I]) 1/2 2(f k t k d [I]) 1/2 will decrease with increases in will decrease with increases in initiator concentration or efficiency. DP = if termination is exclusively by disproportionation. DP = 2 if termination is exclusively by coupling.
4/29/ When Chain-transfer is Involved When chain-transfer in involved, the kinetic chain length must be redefined. 1/ tr = 1/ C m [M] + C s [S] + C I [I] [M] [M] Where C x = k tr, x /k p Bottom Line:
4/29/ Qualitative Effects – a Summary FactorRate of RxnMW [M]IncreasesIncreases [I]IncreasesDecreases k p IncreasesIncreases k d IncreasesDecreases k t DecreasesDecreases CT agentNo EffectDecreases InhibitorDecreases (stops!)Decreases CT to PolyNo EffectIncreases TemperatureIncreasesDecreases
4/29/ Thermodynamics of Free Radical Polymerization G p = H p - T S p H p is favorable for all polymerizations and S p is not! However, at normal temperatures, H p more than compensates for the negative S p term. The Ceiling Temperature, T c, is the temperature above which the polymer “depolymerizes”. At T c, G p = 0. H p - T c S p = 0 H p = T c S p T c = H p / S p
4/29/ Thiol-ene Polymerization: A Brief Introduction hiols (mercaptans) can react with any “-ene”; any double bond. After all, they ARE chain-transfer agents! They serve as a “bridge” between step-growth and chain-growth polymerization processes because they use free radicals in a step-growth polymerization process. HS-R-SH + H 2 C=CH-R’-CH=CH 2 HS-R-S-CH 2 -CH-R’-CH=CH 2 UV
4/29/ If either thiol or ‘ene’ is only monofunctional, no polymerizations will take place. The thiol will serve as a chain-transfer agent and a standard free radical polymerization of the ‘ene’ will take place. If the If the mole ratio of thiol to ‘ene’ is close to one, no Effective polymerization will take place. If both are difunctional and in stoichiometric balance, a linear polymer will form. In order to get a crosslinked thiol-ene polymer, the thiol must be at least trifunctional.
4/29/ The process begins with a hydrogen abstraction from the thiol – a very rapid process – to form a ‘thiyl’ radical: (HS) 2 -R-SH +. In (HS) 2 -R-S. + H-In (HS) 2 -R-S. + H 2 C=CX – R’ (HS) 2 -R-S-CH 2 -CX – R’ (HS) 2 -R-SH + (HS) 2 -R-S-CH 2 -CX – R’ etc. The thiyl radical attacks a double bond: This radical then abstracts a hydrogen atom: