09/04/09 Characteristics of Chain-Growth Polymerization 1. Only growth reaction adds repeating units one at a time to the chain 2. Monomer concentration decreases steadily throughout the reaction 3. High Molecular weight polymer is formed at once; polymer molecular weight changes little throughout the reaction. 4. Long reaction times give high yields but affect molecular weight little. 5. Reaction mixture contains only monomer, high polymer, and about 10 -8 part of growing chains.
09/04/09 The Chemistry of Free Radical Polymerization Radical Generation Initiator Radicals R R 2 R Initiation Monomers R + C C R C C Propagation R C C + C C C C C R Termination R C C + C C C R R C C C C C R Polymer -
09/04/09 Free Radical Polymerization Mechanisms 1. Overview – Free radical polymerization processes involve at least three mechanistic steps.
1. Radical Formation (Generation)
In In h v , etc. In + In 2. Initiation In M In + M
09/04/09 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 .
09/04/09 C. Termination 1) Radical Coupling (Combination) In + In In In 2) Disproportionation ( -hydrogen transfer) In M x C H C H H H + In M y C H C H H H H 3 C CH 2 M y In CH 2 CH In M x + In-M X . + . M Y -In In-M X - M Y -In
09/04/09 D. Chain Transfer (sometimes) – An atom is transferred to the growing chain, terminating the chain growth 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
09/04/09 E. Inhibition and Retardation – a retarder is a substance that can react with a radical to form products incapable of reacting with monomer. An inhibitor is a retarder which completely stops or “inhibits” polymerization. 2. Monomers that are susceptible to free radical addition A. Vinyl Monomers H 2 C CHX H 2 C CH Cl Vinyl chloride H H Y X F F H H Vinylidene fluoride
09/04/09 B. Allyl Monomers C. Ester Monomers OH O OR O Acrylic Acid Acrylate Esters X Cl Allyl Chloride 1) Acrylates
09/04/09 2) 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
09/04/09 3. Monomers that are not susceptible to Free Radical 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
09/04/09 Hydroperoxides can generate radicals by “induced decomposition” from growing polymer chains: P + H O O R PH + O O R R OO 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.
09/04/09 5) Redox Initiator Systems H O O H Fe 2+ HO + OH + Fe 3+ + OR O 3 S O O SO 3 + SO 3 2- SO 4 - + SO 4 2- + S-SO 3 -
09/04/09 6) Photoinitiators (Photocleavage – Norrish I) O HO h v C OH H + C O C OH H + Ph Ph Ph OH H benzoin
09/04/09 (continued) OR C C O O h v C O 2 benzil
09/04/09 7) Photoinitiators (Photo-Abstraction) O h v Ph Ph O * benzophenone excited state C R R H N R R Ph Ph OH + C R R N R R Photosensitizer Coinitiator
09/04/09 5 . Propagation - “Keeping the thing going!” A. The addition of monomer to an active center (free radical) to generate a new active center. R C H 2 CH 2 X X R C H 2 H C X C H 2 CH X X X etc. etc. R C H 2 H C X C H 2 CH X n (continued)
09/04/09 Examples: R C H 2 CH 2 Ph Ph R C H 2 H C Ph C H 2 CH Ph n R C H 2 C H 2 CH C O CH 3 O O CH 3 O R C H 2 C H 2 H C C O CH 3 O C H 2 CH C O CH 3 O Polystyrene Polymethyl Acrylate
09/04/09 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 P C H 2 C H X P H C CH 2 X X X .
09/04/09 2) Radical Stability Considerations Which possible new active center will have the greatest stability? P C H 2 CH 2 P C H 2 CH P C H 2 CH -attack produces resonance stabilized free radical .
09/04/09 P H C CH 2 X No resonance stabilization P ______________________________________________ HC C O O CH 3 CH 2 H 2 C C H C O O CH 3 X P CH CH 2 C O CH 3 O P H C H 2 CH C O O CH 3 P H C H 2 CH C O O CH 3 Secondary radical is resonance stabilized
09/04/09 (more examples) Cl Cl H H H H Cl Cl P X P C Cl Cl CH 2 P C H 2 C Cl Cl P C H 2 C Cl Cl P C H 2 C Cl Cl Tertiary radical is resonance stabilized
09/04/09 3) Steric Hinderance Considerations P HC CH 2 X H 2 C CH X X
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 X X X X Alternating configuration
09/04/09 6. 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)
09/04/09 B. Disproportionation In M x C H C H H H + In M y C H C H H H H 3 C CH 2 M y In CH 2 CH In M x + - Produce one initiator fragment (end-group) per chain - Production of saturated chain and 1 unsaturated chain per termination
09/04/09 C. Factors affecting the type of termination that will take place. 1) Steric factors - large, bulky groups attached directly to the active center will hinder coupling 2) Availability of labile -hydrogens 3) Examples – PS and PMMA + P x C H 2 C H C C H 2 P y H Combination (coupling) Polystyrene (continued)
09/04/09 P y P x C H 2 H C H C C H 2 Ph Ph Ph = CH 3 H 3 C ~~~P X – CH 2 -C . + . C-CH 2 - P Y ~~~ C=O O=C O O CH 3 CH 3 PMMA
09/04/09 CH 3 H 3 C ~~~P X – CH 2 =C + HC-CH 2 - P Y ~~~ C=O O=C O O CH 3 CH 3
Electrostatic Repulsion Between Polar Groups –
Esters, Amides, etc.
09/04/09 ~~~P X – CH 2 -CH . + . HC-CH 2 - P Y ~~~ 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!
09/04/09 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!
09/04/09 7. Chain-Transfer - “Rerouting the thing!”
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 ~~~P X – CH 2 -CH . + CCl 4 ~~~P X – CH 2 -CHCl + Cl 3 C . Y Y B. Chain-transfer to solvent :
09/04/09 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
09/04/09 H H ~~~P X – CH - C . + H 2 C =CH ~~~P X – CH 2 =CH . + H 3 C - C .
09/04/09 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.
09/04/09 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
09/04/09 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.
Inhibition and Retardation - “Preventing the thing
or slowing it down!”
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 .
09/04/09 Kinetics of Free Radical Polymerization 1. Initiation I 2 R . Radical Generation k d R . + M M 1 . Initiation k i 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 ] dt f = efficiency of the initiator and is usually 0.3< f >0.8 (RDS)
09/04/09 2. Propagation M 1 . + M M 2 . M 2 . + M M 3 . M 3 . + M M 4 . . . . M x . + M M x+1 . R p = - d[M] = k p [M . ][M] dt k p k p k p k p We assume that the reactivity of the growing chain is independent of the length of the chain.
09/04/09 3. 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
09/04/09 2fk 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] .
09/04/09 5. Kinetic Chain Length, 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]
2k t [M . ] 2 2k t [M . ]
= __ k p [M] ___ 2(f k t k d [I]) 1/2 will decrease with increases in initiator concentration or efficiency. DP = if termination is exclusively by disproportionation . DP = 2 if termination is exclusively by coupling.
09/04/09 6. 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] Where C x = k tr, x /k p Bottom Line:
09/04/09 7. Qualitative Effects – a Summary Factor Rate of Rxn MW [M] Increases Increases [I] Increases Decreases k p Increases Increases k d Increases Decreases k t Decreases Decreases CT agent No Effect Decreases Inhibitor Decreases (stops!) Decreases CT to Poly No Effect Increases Temperature Increases Decreases
09/04/09 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
09/04/09 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
09/04/09 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.
09/04/09 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: