Polymer Synthesis CHEM 421 Chapter 3-6 (Odian)

Polymer Synthesis CHEM 421 Oligomers “Oligomer” – Greek: oligos, “few” mer, “parts” Find commercial application in a variety of fields: Elastomers - poly(ethylene oxide) oligomers in Spandex® Coatings & Adhesives - acrylic oligomers Lubricants - fluorinated oligomers used as lubricants on satellites, disk drives, etc…

Polymer Synthesis CHEM 421 Free Radical Solution Polymerizations Initiation Propagation Termination

Polymer Synthesis CHEM 421 Routes to Oligomers [ M ] o [ I ] o DP = Use large amounts of initiator: Alternative is to use ‘Chain Transfer” Processes… Use low monomer concentrations: Use large amounts of initiator: very expensive high levels of azo-initiators leads to toxic cross-coupling products Use low monomer concentrations: low productivity requires lots of solvents

Polymer Synthesis CHEM 421 Chain Transfer Chain transfer is a chain-breaking step –Decreases size of propagating chain Effect of chain transfer on R p is a function of k a X—A = solvent, monomer, initiator, chain transfer agent… R tr = k tr [P] [XA]

Polymer Synthesis CHEM 421 Aliphatic hydrocarbons with strong C—H bonds show low CT Benzene even lower Alkyl aromatics (benzylic H’s) –t-butyl benzene↓, no benzylic H Butyl iodide (weak C—I bond) Acids, ethers, amines, alcohols >> than aliphatics due to heteroatom stabilization Weak S—S bond Halogenated solvents, weak C—X bond Thiols the largest! Chain Transfer

Polymer Synthesis CHEM 421 Chain Transfer Constants Rate of Polymerization Σ Chain breaking steps DP = ————————————— RpRp + CT to solvent + CT to initiator + CT to CTA ) Σ ( termination + CT to monomer + CT to solvent + CT to initiator + CT to CTA ) DP = ————— —————————— ———————— C M = ———— k tr,monomer kpkp C S = ———— k tr,solvent kpkp C I = ———— k tr,initiator kpkp

Polymer Synthesis CHEM 421 Effect on Rate of Polymerization How does Chain Transfer affect the rate of polymerization (R p )? Two competing processes: Reinitiation vs. Propagation

Polymer Synthesis CHEM 421 Effect on Degree of Polymerization How does Chain Transfer affect the degree of polymerization (DP)? Two competing processes: Transfer vs. Propagation

Polymer Synthesis CHEM 421 Thus, we have three competing processes, all of which affect R p & DP …. Transfer Propagation Reinitiation This leads to four possible scenarios… Competing Processes

Polymer Synthesis CHEM st Case Rate of Propagation >> Rate of Transfer k p >> k tr Rate of Reinitiation ≈ Rate of Propagation k a ≈ k p “ Normal Chain Transfer ” No effect on Rate of Polymerization (R p) ie. same # of monomers consumed / unit time Decrease in Degree of Polymerization (DP)

Polymer Synthesis CHEM nd Case Rate of Propagation << Rate of Transfer k p << k tr Rate of Reinitiation ≈ Rate of Propagation k a ≈ k p “ Telomerization ” Still no effect on Rate of Polymerization (R p) ie. same # of monomers consumed / unit time Huge Decrease in Degree of Polymerization (DP) DP = 1-5 repeat units!!! Unlike 1 st case, transfer (k t ) is more rapid than propagation (k p ) !!

Polymer Synthesis CHEM rd Case Rate of Propagation >> Rate of Transfer k p >> k tr Rate of Reinitiation < Rate of Propagation k a < k p “ Retardation ” Decrease in Rate of Polymerization (R p) R p is decreased b/c reinitiation (k r ) is slower!! Decrease in Degree of Polymerization (DP)

Polymer Synthesis CHEM th Case Rate of Propagation << Rate of Transfer k p << k tr Rate of Reinitiation < Rate of Propagation k a < k p “ Degradative Chain Transfer ” Decrease in Rate of Polymerization (R p) Like Retardation, re-initiation is slow… Large decrease in Degree of Polymerization (DP) Different from Retardation, Transfer (k t ) is rapid

Polymer Synthesis CHEM 421 Chain Transfer Constant (C s ) Mathematical Definition: C s = k tr k p Transfer Propagation The magnitude of C s reflects the activity of the chain transfer agent …

Polymer Synthesis CHEM 421 Determining C s 1 DP [S] [M] Slope = C s The Mayo Equation: 1 DP = 1 DP ( ) o + C s [S] [M] Make a plot… Where: DP = Degree of Polymerization DP o = DP in absence of chain transfer agent [S] = Chain transfer Agent Conc. [M] = Monomer Conc. C s = Chain transfer Constant

Polymer Synthesis CHEM 421 Common Chain Transfer Agents Alkyl Thiols: CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -SH 1-Octanethiol 2-Mercaptoethanol HO-CH 2 -CH 2 -SH Alkyl Halides: Bromotrichloromethane 1,1,2 Trichlorotrifluoroethane (Freon 113)

Polymer Synthesis CHEM 421 Common Chain Transfer Agents Advantages Very reactive Commercially available Some able to functionalize polymer end groups Some are inexpensive Disadvantages Toxicity Stench (Thiols) Non-Catalytic (ie. very low MWs require high conc.)

Polymer Synthesis CHEM 421 Catalytic Chain Transfer (CCT) Advantages: Catalytic - Conc. as low as 100 ppm ! Very low MWs easily achieved Non-Toxic High yields Produces vinyl functional oligomers (macromonomers) Disadvantages: Air Sensitive Need to remove catalyst Only works with Methacrylates and Sytrenes

Polymer Synthesis CHEM 421 Catalytic Cycle: MMA Example Cobalt (III) Hydride Intermediate Propagating Chain Vinyl-terminated Oligomer New Propagating Chain Monomer

Polymer Synthesis CHEM 421 CCT Catalyst Evolution First Generation: 1975 Smirnov and Marchenko use substituted porphyrins to dramatically reduce molecular weight of methacrylate polymers Gridnev, A. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (10), 17;53

Polymer Synthesis CHEM 421 Cobaloxime Catalysts Cobaloximes are the most active CCT catalysts Nonionic Ionic Choice of A - crucial Other species Among the most active By varying ligand substituents one can vary C s over 3 orders of magnitude !!!

Polymer Synthesis CHEM 421 BF 2 Bridging Ligands in CCT Catalysts First reported in 1981 by Nonaka et. al. Current CCT catalyst of choice largely because of decreased sensitivity to oxygen Crucial for CCT on industrial scales Nonaka, T.; Hamada, K. Bull. Chem. Soc. Jpn. 1981, 54 (10), 3185

Polymer Synthesis CHEM 421 Applications of CCT? Polymerizable end group from CCT Graft Copolymer Vinyl-terminated oligomers polymerize well with acrylic monomers Macromonomer route to graft copolymers …

Polymer Synthesis CHEM 421 Chain Transfer No discussion of chain transfer to polymer??? Not easy to determine… Can not simply introduce new term into Mayo equation: since doesn’t lead to decrease in M n Leads to branching… 1 DP = 1 DP ( ) o + C P [P] [M]

Polymer Synthesis CHEM 421 Polyethylene 20 – 30 “short” branches per 10,000 carbons LDPE –50 – 70% x-tal –PDI = 20 – 50 (!) –Density = 0.92 – 0.93 g/mL –Tm ≈ 110 °C

Polymer Synthesis CHEM 421 Polyethylene