POLYMEIZATION EATIONS Understand the differences between step-growth and chain-growth polymerization reactions. Predict the products of polycondensation and polyaddition reactions. Explain how reactant ratios and monomer purity may affect the degree of polymerization. Understand the initiation, propagation, and termination processes in free-radical, ionic, coordination (Ziegler- Natta), and ring-opening polymerizations. Explain how the nature of the reaction catalyst influences polymer structure (stereoregularity and tacticity). STEP-GOWT POLYMEIZATION Formerly: ondensation polymerization (arothers, 1931) Monomers are difunctional each has two reactive functional groups hain growth occurs through coupling (condensation, addition) reactions [Monomer] decreases rapidly before any high-mw polymer is formed ate of polymerization is highest at outset, decreases as chain ends are consumed long reaction times Pzn eactions 4-1
Schematic: na nb AB ( ) n Addition: ondensation: na nb AB ) ( n na nb AB ( ) n Y The number average degree of polymerization (X n ) is N 0 Number of molecules at start DP = X n = = N Number of molecules at end The The ratio [A]:[B] strongly influences chain length N 0 /N A B A B A B A B A B A B 12/12 1 A B A B A B A B A B A B 12/11 1.1 A B A B A B A B A B A B 12/10 1.2 A B A B A B A B A B A B 12/9 1.3 A B A B A B A B A B A B 12/8 1.5 A B A B A B A B A B A B 12/7 1.7 A B A B A B A B A B A B 12/6 2 A B A B A B A B A A A B 12/5 2.4 A B A B A B A B A B A B 12/4 3 A B A B A B A B A B A B 12/3 4 A B A B A B A B A B A B 12/2 6 X n A B A B A B A B A B A B 12/1 12 Pzn eactions 4-2
Degree of Polymerization 10 8 X n 6 4 2 2 4 6 8 10 eaction Step Number epresentative Step-Growth eactions ondensation eactions Polyester (Polyethylene terephthalate/pet) O ( 2 ) 2 O OO OO O ( 2 ) 2 O OO 2 O O Polyamide (Nylon-6,6) 2 N ( 2 ) 6 N 2 OO ( 2 ) 4 OO N ( 2 ) 6 N ( 2 ) 4 O 2 O O Addition eactions Polyurethane O ( 2 ) 2 O ON 2 DABO NO O O ( 2 ) 2 ON 2 NO See also Table 3.2, pp 25-26 Pzn eactions 4-3
Predicting Molar Mass in Step-Growth Pzns The extent of reaction (p) is Number of functional groups reacted p = Initial number of functional groups Since the total number of molecules (N) decreases by 1 for each condensation step, N 0 N N 0 1 p = or = N 0 N 1p 1 X n = n 1p 1p arothers equation arothers Equation 150 X n 100 50 0.2 0.4 0.6 0.8 p Pzn eactions 4-4
Step-Growth eactions: MW Distribution N 0 1 X n = = N 1p The corresponding weight-average degree of polymerization is 1p X w = 1p The molecular weight distribution (polydispersity) is therefore X w = 1 p X n as p 1, MWD 2 for any linear step-growth pzn Molecular Weight ontrol na nb AB ( ) n Lowering the temperature before the reaction is complete can reduce product MW. esult: a thermally unstable product. Alternative: use a non-stoichiometric reactant ratio ([A] < [B]). The less abundant reactant is consumed completely. In terms of functional groups (N A < N B ), the reactant ratio is N A r = < 0 N B Substituting gives the general arothers equation: 1 r X n = n 1 r 2rp 2rp Pzn eactions 4-5
Molecular Weight ontrol (cont.) 1 r X n = n 1 r 2rp 2rp X n r p = 0.900 p = 0.950 p = 0.990 p = 0.999 1.000 10.0 20.0 100.0 1000.0 0.999 10.0 19.8 95.3 666.8 0.990 9.6 18.3 66.8 166.1 0.950 8.1 13.4 28.3 37.6 0.900 6.8 10.0 16.1 18.7 Molecular Weight ontrol (cont.) Producing high-mw step-growth polymers requires igh conversions (p > 0.98) Stoichiometric ratios of functional groups igh-purity monomers No side reactions Pzn eactions 4-6
AIN-GOWT POLYMEIZATION Formerly: Addition polymerization (arothers, 1931) Polymerization requires an initiator, a substance that starts the reaction hain growth occurs by addition of monomer to a relatively small number of initiation sites (free radicals, anions, cations, transition metal complexes) eaction mixture contains monomer, high polymer, and a low concentration of growing chains [Monomer] decreases steadily as polymer is formed Schematic: Initiator nm M n Initiation: I M IM* Propagation: IM* nm IM n M* Termination: eactions depend on M and M* Moles of monomer consumed DP = X n = Moles of chains produced The The ratio ratio [initiator]:[monomer] strongly influences chain chain length Pzn eactions 4-7
Mol cons X n I A A A A A A A A A A A A 0 I A A A A A A A A A A A A 1 1 I A A A A A A A A A A A A 2 2 I A A A A A A A A A A A A 3 3 I A A A A A A A A A A A A 4 4 I A A A A A A A A A A A A 5 5 I A A A A A A A A A A A A 6 6 I A A A A A A A A A A A A 7 7 I A A A A A A A A A A A A 8 8 I A A A A A A A A A A A A 9 9 I A A A A A A A A A A A A 10 10 I A A A A A A A A A A A A 11 11 I A A A A A A A A A A A A 12 12 Degree of Polymerization 10 8 X n 6 4 2 2 4 6 8 10 eaction Step Number Pzn eactions 4-8
epresentative hain-growth eactions Free-adical Vinyl Pzns Poly(methyl methacrylate) OO 3 OO 3 2 3 K 2 S 2 O 8 2 n 3 arbocationic/anionic Pzns Butyl ubber (II) 3 3 3 3 2 3 2 2 All 3 3 l 2 98n 3 2 2 2n oordination Pzns Polypropylene See also Table 3.2, pp 25-26 2 3 Til 3 /All 3 Al( 2 5 ) 2 l 2 O 2 2 3 2 3 2 3 n 3 eactant ompatibilities Mechanism* Monomer adical ationic Anionic Ethylene α-olefins ( 3 ) 1,1-Dialkylalkenes 1,3-Dienes Styrene, α-me styrene Vinyl chloride Tetrafluoroethylene Acrylate/methacrylate esters Acrylonitrile, methacrylonitrile Vinyl ethers Vinyl esters *Plus () sign = high polymer; minus () sign = no reaction or oligomer. oord n See also Table 5.1, p 124 Adapted in part from G. Odian, Principles of Polymerization, 4 th Ed, 2004, p 200; and F.W. Billmeyer, Jr, Textbook of Polymer Science, 3 rd Ed, 1994, p 83. Pzn eactions 4-9
Vinyl Free adical Polymerization Alkene, 1,3-diene monomers aloethylenes: 2 l Styrene: 2 Acrylic, methacrylic acid derivatives: 2 N 2 O 3 O Free radical initiators Benzoyl peroxide: O Azobisisobutyronitrile (AIBN): K persulfate (peroxydisulfate): O 3 SO OSO 3 Thermal, redox, or photochemical initiation O 2 3 O 2 O 3 N ( 3 ) 2 N 2 2 N N 2 2 3 2 2O 3 SO 2 Initiation O O 2 2 O O = I I 2 I Propagation I 2 I Pzn eactions 4-10
Termination hain Transfer hain Addition Molecular Weight ontrol Most monomer is (or should be) consumed by polymer chain growth, not initiation. Termination involves reactions that stop polymerization. hain transfer to monomer, solvent, or initiator kills the polymer chain but continues the kinetic chain (new chains can be initiated): M i X M i X hain length is proportional to [M] and 1/[I] ½ : [M] [M] X n = n K [I] [I] ½ To maximize DP, increase [M]:[I] in the reaction mixture or increase K by the choice of monomer. Pzn eactions 4-11
Molecular Weight ontrol (cont.) If chain transfer processes to both monomer (M) solvent (S) are important: and 1 1 [S] [S] = M S S X n X n n0 [M] n0 [M] Mayo (Mayo-Walling) eqn where X n0 is the DP you get with no chain transfer. If transfer to monomer is not important, then 1 1 [S] [S] = S S X n X n n0 [M] n0 [M] Simplified Mayo eqn The The reciprocal of of DP DP should be be a linear linear function of of the the ratio ratio of of [S] [S] to to [M]. [M]. The The slope slope would be be a function of of the the rate rate of of chain chain transfer to to solvent. Styrene Pzn: hain Transfer to Solvent 3 3 150 2 3 3 10 5 Xn 100 50 1/X n0 5 10 15 20 [S]/[M] Adapted from.a. Gregg & F.. Mayo, 1947 Disc. Faraday Soc., 2, 328-337 Pzn eactions 4-12
Styrene Pzn: hain Transfer to Solvent M i X M i X P 2 X P 2 X Function slopes show relative ease of radical (X ) formation: 3 3 > > > 3 arbocationic hain-growth Pzn Alkene, 1,3-diene monomers Isobutylene (2-methylpropene): 3 2 3 Isoprene (2-methyl-1,3-butadiene): 2 2 β-pinene: 2 3 Lewis acid catalysts 3 3 All 3 BF 3 ( 3 2 ) 2 All Brønsted acid or carbocation-donor co-catalysts l or 2 O: BF 3 2 O F 3 B O 2 l: All 3 ( 3 ) 3 l ( 3 ) 3 All 4 Pzn eactions 4-13
Initiation F 3 B O 2 F 3 B O 3 3 F 3 B O 2 3 F 3 B O 3 3 Propagation F 3 B O 3 3 2 3 3 F 3 B O 3 3 2 3 3 3 3 Termination 2 3 3 3 3 3 2 3 3 F 3 B O 2 3 hain Transfer F3 B O 3 2 3 O 3 3 F3 B O Quenching 3 2 3 3 O 3 F 3 B O 2 Pzn eactions 4-14
Molecular Weight ontrol 1 t t = M X n [M] [M] X n Simplified Mayo equation eactions are extremely rapid controlling heat transfer is important. hain transfer to monomer is often significant. DP is independent of initiator concentration. For a typical exothermic polymerization, the reaction rate increases as the temperature is reduced. Net outcome: DP increases at lower temperature Anionic hain-growth Polymerization Alkene, 1,3-diene monomers 1,3-butadiene: Styrene: Methyl methacrylate: Vinyl pyridine: atalysts: Lewis bases, organometallics N 2 2 2 2 2 3 2 2 2 Li (n-butyllithium) Na N 3 O 2 O 3 Pzn eactions 4-15
Initiation 4 9 Li 2 4 9 Li Propagation 4 9 Li 2 4 9 Li Molecular Weight ontrol Anionic polymerizations lack inherent termination processes (ion-pair rearrangements, anion-metal cation reactions). Monomer molecules only react with the anionic end groups on the propagating polymer chains. Polymer chains continue to grow as long as additional monomer is added. These chains are Living Polymers Pzn eactions 4-16
Molecular Weight ontrol (cont) [M] 0 DP = X n = p [I] 0 where p is the fraction of monomer converted. At complete conversion, DP is the monomer:initiator ratio at the outset of reaction: [M] [M] 0 DP 0 DP = X n = n [I] [I] 0 0 The MWD (polydispersity) is therefore : X w 1 = 1 X n n X n n The The MWD MWD of of a living living anionic polymer will will be be extremely narrow, tending toward a value value of of 1 as as X n increases. n Anionic Pzn as a oute to Block opolymers Living polymerization yields polymer chains with reactive end groups: SSS S Li These living chains can react with a second monomer to extend the chain and create a final product with unique chemical and physical properties: SSS S Li nb SSSS BBB B Li Block copolymers comprise two or more homopolymer subunits. These are referred to as n-block copolymers. ommercial block copolymers include SIS (styreneisoprene-styrene) and SBS (styrene-butadiene-styrene) triblocks. Pzn eactions 4-17
ontrolling Stereochemistry Natural polymers have a high level of structural regularity: 3 2 4 3 2 2 1 2 3 3 3 3 2 2 2 2 2 2 2 cis-1,4-polyisoprene (N) Synthetic polymers often contain many structures: 2 3 M j 2 M i M i M j 2 3 2 M i 2 3 2 M j M i 2 3 2 M j 1,2-addition 3,4-addition cis-1,4-addition trans-1,4-addition STEEOEMISTY Properties that that relate relate to to a molecule s 3-dimensional structure. ontrolling Stereochemistry (cont.) onditions I Structure, mol fraction cis-1,4 trans-1,4 1,2 3,4 Free-radical, 20 0.01 0.90 0.05 0.04 Free-radical, 100 0.23 0.66 0.05 0.06 Anionic, Li, 30 0.94 0.01 0.00 0.05 Polymer structures depend on Monomer structure and conformation (cisoid vs. transoid) 2 3 2 Nature of active unit structure (free radical vs. anion) Stability of active unit structure (cis-trans isomerization in 1,4- addition) 2 2 3 Pzn eactions 4-18
ontrolling Stereochemistry (cont.) P 2 2 3 2 2 3 P 2 2 3 2 2 3 1,4-trans addition Free-radical Anionic P 2 2 3 3 2 2 1,4-cis addition 3 P 2 Li 2 2 3 2 3 P 2 2 Li 2 2 3 P 3 2 2 Li 2 2 3 1,4-cis addition Ziegler-Natta atalysis: oordination hain-growth Polymerization Alkene, cycloalkene, 1,3-diene monomers Propylene (Propene): 1,3-Butadiene: 5-Ethylidene-2-norbornene: Group 4B-8B transition metal catalysts Til 4 2 2 3 2 2 3 Vl 4 Ti, Zr metallocenes Ti l l Group 1A-3A metal co-catalysts/activators ( 3 2 ) 3 Al (with Til 4 ) [Al( 3 )O] n (with titanocene dichloride) Pzn eactions 4-19
Ziegler-Natta atalysts Active catalyst produced by reaction of transition metal component with Group 1A-3A co-catalyst: Til 4 ( 2 5 ) 3 Al l 3 Ti 2 5 ( 2 5 ) 2 All The concentration of active catalyst [* p ] is (0.01-10)n% of [transition metal component]. Termination reactions are relatively unimportant. DP tends to be high eactions must be quenched by active sources ( 2 ): l 3 TiM n 2 5 [] l 3 Ti M n 2 5 Kinetics of oordination Polymerization (cont.) l l l l l Ti l Et 3 l l Ti l Et 3 l l Ti l 3 2 Et Specific stereochemistry Adapted in part from E.J. Arlman, Jr, & P. ossee, 1964 J. atal., 3, 99-104. Pzn eactions 4-20
Stereochemistry of Polymerization The stereochemistry of polymer chain propagation controls the relative stereochemistry (tacticity) of the polymer. In ionic polymerizations, high stereoregularity can be achieved if there is strong coordination of the counter-ion with the terminal unit on the growing chain. (Promoted by low reaction T, polar substituent groups) Stereoregularity influences chain-chain interactions, chain stiffness, crystallizability. TATIITY The The relative stereochemistry of of adjacent chiral chiral centers within a macromolecule ATATI random configurations Pzn eactions 4-21
ISOTATI identical configurations SYNDIOTATI alternating configurations Pzn eactions 4-22
Stereoselective Polymerizations Monomer Iso 4 vinyl ether Me methacrylate Propylene onditions Et 2 O BF 3 in propane, 80 to 60 n- 4 9 Li in toluene, 78 to 0 n- 4 9 Li in TF, 78 Til 4 -Et 3 Al in heptane, 50 Vl 4 -Et 3 Al PhO 3 in toluene, 78 Structure isotactic isotactic syndiotactic isotactic syndiotactic Property Atactic Polypropylene Isotactic Polypropylene Syndiotactic Polypropylene Melting point, 160-171 130-160 rystallinity, % 0 55-65 50-75 Adapted from G. Odian, Principles of Polymerization, 4 th Ed, 2004, p 640. omparing Step-Growth & hain Growth Pzn Monomer addition eaction rate MW growth Process type Examples Step-Growth Any two species can combine (monomer or polymer) Much slower than chain growth Increases continuously as monomers react Batch Nylon, PETE hain-growth apid addition to small number of active centers apid and constant igh MW present even at low conversion ontinuous, semibatch B, I, SB, II, EPM, EPDM Adapted from ExxonMobil hemical Polymer Technology ourse 2009. Pzn eactions 4-23
ING-OPENING POLYMEIZATION hain-growth reactions. Proceed by relieving Strain in small-ring monomers Steric crowding in large-ring monomers eactions are catalyzed by Free radicals Anions ations Transition metal-carbene complexes Example: ( 6 11 ) 3 P l u l ( 6 11 ) 3 P ING-OPENING PZN (cont.) Polymerizations follow two general pathways: ing opening of cyclic monomers: ethers (epoxides), esters (lactones), amides (lactams), etc. n ( 2 ) x Y ( 2 ) x Y ing-opening metathesis polymerization (OMP) of olefins: cycloalkenes, bicycloalkenes n Y = O, OO, ON M ( 2 ) x M ( 2 ) x n ( 2 ) x M ( 2 ) x n1 METATESIS The The exchange or or transposition of of bonds between two two reactants Pzn eactions 4-24
epresentative ing-opening Pzns ing-opening Pzn of Epoxides Polyethylene glycol (PEG) from ethylene oxide 2 n O O 2 2 O O 2 2 O 2 2 O 2 2 O 2 KO n1 ing-opening Pzn of Lactams Nylon-6 from ε-caprolactam 2 n 2 2 O N 2 2 260 OMP of Bicycloalkenes Polynorbornene ubber (PN) N 2 O 2 2 2 2 2 N n n ul 3 /l 4 9 O n n OPOLYMEIZATION Simultaneous polymerization of two or more monomers yields copolymers with unique properties unlike those of homopolymers from each monomer. opolymers with irregular (random) monomer sequences can have rubber-like properties because of reduced crystallinity. Appropriate reaction conditions can provide a range of tailored structures: andom (statistical) ABBABAABABBAABAB Alternating ABABABABABABABAB Block AAAABBBBAAAABBBB Graft AAAAAAAAAAAAAAAA BBBB BBBB Pzn eactions 4-25
omposition ontrol Monomers differ in their ability to copolymerize. The relative rates of monomer reaction are a function of the monomer concentrations ([M 1 ],[M 2 ]) and the monomer reactivity ratios (r n ): k 11 r 1 = k 22 r 2 = k12 k21 where k ij is the rate constant for the reaction of monomer i with monomer j. eactivity ratios determine copolymer composition and structure. Free adical Pzn: eactant atios Monomer 1 r 1 Monomer 2 r 2 T, Ethylene 0 Acrylonitrile 7 20 0.79 Vinyl acetate 1.4 130 0.58 1,3-Butadiene 1.4 50 Styrene 0.29 Acrylonitrile 0.020 60 55 Vinyl acetate 0.01 60 0.78 1,3-Butadiene 1.4 5 Me acrylate 0.84 Acrylonitrile 1.5 50 6.4 Vinyl acetate 0.03 60 Adapted from G. Odian, Principles of Polymerization, 4 th Ed, 2004, pp 491-492. Pzn eactions 4-26
omposition ontrol (cont.) Limiting (or idealized) reactivity ratios: r 1 = r 2 = 0 Neither chains end adds its own monomer. esults: 1. a perfectly alternating copolymer and 2. an azeotropic copolymer composition (the ratio of monomers in the feed = the ratio of monomer units in the copolymer). r 1 = r 2 = Each chain adds only its own monomer. esult: a mixture of homopolymers. r 1 r 2 = 1 or r 1 = r 2 = 1 All chain ends add either monomer with equal probability in an ideal copolymerization. esult: a random copolymer. omposition ontrol (cont.) In most cases r i is not exactly 0, 1, or ; polymers tend toward a particular composition and type of structure. omonomer sequences can be inferred from r 1 r 2 values: r 1 r 2 < 1 Tendency toward alternating structure r 1 r 2 > 1 Tendency toward block structure r 1 r 2 >> 1 Tendency toward homopolymer structure When r 1 >> 1 >> r 2 there is copolymer composition drift. At low conversions, the copolymer is enriched in the more reactive monomer. At high conversions, the copolymer is enriched in the less reactive monomer that remains. omposition drift can be reduced by delayed or (semibatch) continuous addition of the more reactive monomer. Pzn eactions 4-27
omposition ontrol (cont.) In terms of the mole fraction (ƒ) of each monomer in the co-monomer mixture, [M 1 ] ƒ 1 = [M 1 ] [M 2 ] [M 2 ] ƒ 2 = 1 ƒ 1 = [M 1 ] [M 2 ] the copolymer equation, expressed as the mole fraction (F) of each monomer unit in the copolymer chain, is: r 1 ƒ 12 ƒ 1 ƒ 2 F 1 = r 1 ƒ 12 2ƒ 1 ƒ 2 r 2 ƒ 2 2 F 2 = 1 F 1 These equations can be used to calculate the relationship of reactant feed composition to instantaneous polymer composition for various reactant ratios. opolymer omposition: Impact of r 1 r 2 F 1 0.8 0.6 0.4 0.2 Styrene Vinyl l 17 0.02 r 1 r 2 = 0.34 alternating random composition drift Stilbene Styrene Butadiene 0.78 1.39 r 1 r 2 = 1.1 Maleic Anydride 0.03 0.03 r 1 r 2 = 0.0009 0.2 0.4 0.6 0.8 ƒ 1 Pzn eactions 4-28
Exercise The graph shows the feed and product compositions for the free-radical reaction of styrene (monomer 1) and acrylonitrile (monomer 2). What feed composition do you need to get a copolymer that is 50% styrene? a) About 55% styrene b) About 55% acrylonitrile c) About 25% styrene 0.8 0.6 F 1 0.4 0.2 What kind of copolymer will you get? a) One that tends to have an alternating structure. b) One that is enriched in styrene. c) One that tends to be a mixture of copolymers. 0.2 0.4 0.6 0.8 ƒ 1 Polymerization 4-29