Chapter 5. Ionic Polymerization. Anionic.
Anionic Polymerization Dr. Houston S. Brown Lecturer of Chemistry UH-Downtown brownhs@uhd.edu
What you should know: What is anionic polymerization? What is MWD, and why is it so important to high tech polymers? What are the experimental conditions to make anionic polymers? What kind of polymers can we make? Properties?
Anionic Polymerization Anionic polymerization is a special type of reaction to make polymers which: Have a very narrow Molecular Weight Distribution (the molecules look almost exactly alike). Have special properties vs. identical polymers without those narrow MWDs. Have a block structure, where one block is unique, and very similar to the block in the next polymer molecule.
Molecular Weight Distribution (mwd) Chain length is often expressed in terms of the molecular weight of the polymer chain, related to the relative molecular mass of polymers, and the number of polymers connected in the chain. All synthetic polymers are polydisperse to some degree in that the length of the polymer chains are not exactly identical. Hence, the molecular weight of the polymer is not a single value, but rather a distribution of molecular weights (and therefore, chain lengths).
How does one determine the molecular weight and the mwd? This is usually done by gel permeation chromatography (also, hint-hint, called size exclusion chromatography). How does this work? Uses size exclusion for separation. The stationary phase contains solvent filled pores of certain sizes that allow smaller molecules to enter while excluding larger molecules. Technique is mostly used to characterize the MW distribution of polymer materials or separation of high MW weight proteins.
Gel Permeation Chromatography
Number Average Molecular Weight, M n
Weight Average Molecular Weight, M w
Higher Average Molecular Weight, M z, M z +1
Molecular Weights, compared
polydispersity
Example Three molecules, with Molecular weights all 100, vs. Molecular weights 80, 100, 120 First set, Mn = 100, Mw = 100 so q = 1.0 Second set Mn = 100, Mw = 100.7, q = 1.03 [Mw = (80 2 /1 + 100 2 /1 + 120 2 /1)/300 = 102.7]
iclicker Question 1 We want to make a very narrow mwd polystyrene. a. Wow! That would be a small polydispersity index b. M z+1 / M n is 1.05 c. Add a little water to the styrene d. Use a gel permeation chromatography column e. All of the above
So how does one make this narrow polydispersity? First, some chemistry!
Block Copolymers - Review Polymer A-A-A-A-A-A-A-A-A-A-A-A-A (examples PS, PE, PP) Alternating co-polymer A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B (Example PET) Block Copolymer AAAAAAA-BBBBBBB-BBBBBBB-AAAAAAA (example: Kraton Polymers, SBS) Random Copolymer A-B-B-B-A-B-A-A-B-A-A-B-A-B-B-B-B-A-B-A-A Notice the difference: The blocks are not copolymers.
Anionic Polymerization Produced by chain-reaction polymerization Initiator (anion) adds to a carbon carbon double bond of an unsaturated substrate (a vinyl monomer) to yield a reactive intermediate that reacts with a second molecule of monomer
Production of living polystyrene Li H 3 C CH 3 C H 3 CH - C H 2 CH 3 C H 3 CH 3 CH - + C H 2
Living polystyrene, continued C H 3 CH 3 CH - + C H 2 H 3 C CH 3 Ph Ph Ph Ph CH - Ph
How do you do this in the lab? Solvent? Sec-butyllithium Styrene monomer Reaction conditions
iclicker Question 2 This is called a living anion because: a. If you do the reaction correctly, you will still be living b. Things for better living (like baby-diapers parts) are made with this process c. The anion is still present at the end of the reaction d. The anion never terminates due to impurities e. The anion is extinguished, and is not really living
iclicker Question 3 We want to make a very narrow mwd polystyrene. The order to do stuff is: a. Flame dry (to remove water), cyclohexane, styrene, sec-butyllithium b. Cyclohexane, flame dry (to remove water), sec-butyllithium, styrene c. Flame dry (to remove water), sec-butyllithium, cyclohexane, styrene d. Flame dry (to remove water), cyclohexane, sec-butyllithium, styrene e. None of the above
Now, we add the second monomer H 3 C CH 3 Ph Ph Ph Ph CH - Ph + CH 2 H 2 C 1,4 Addition Ph Ph Ph Ph CH 2 - H 3 C Ph CH 3
Note: We showed 1,4 addition 1,2 addition can occur with changes in solvent H 3 C Ph Ph Ph Ph CH - Ph + CH 2 H 2 C 1,2 Addition CH 3 CH 2 CH - C H 3 H 3 C Ph Ph Ph Ph Ph
Summary of the Living Polymerization
Living Polymerization
Conditions of the anionic polymerization Chains are initiated all at once (fast initiation) Little or no termination (except purposeful). Little or no depolymerization. All chains grow under identical conditions. The result is that the monomers get divided evenly among chains. Narrow mwd (PD approaches 1.0, typically 1.05-1.2). The molecular weight is predictable (unlike other polymerizations). For monofunctional initiators, the chain length is simply x = [monomer] / [initiator] For difunctional initiators (electron transfer), the chain length is twice as large.
Living (anionic) vs. free radical polymerization
Types of initiators Organometallic compounds Organolithium compounds Stable (hello, Aldrich?) Li-R bond is covalent The initiator is soluble in organic solvents Polymerization is a homogeneous (single phase) process Organosodium or organopotassium compounds More ionic Less soluble in organic solvents Polymerization is heterogeneous (two phase).
H 3 C C -Na+ + O O CH 3 O O Na + CH - CH 2 methyl prop-2-enoate (methyl acrylate) CH 2 - K + + H 2 C O - N + CH - O N + O - Benzyl potassium O
H 2 C H 3 C O - K + + NR O CH 3 O H 3 C O - K + + H 2 C O H 3 C O C - O CH 3 N Methyl cyanoacrylate N If the electron withdrawing capability is strong enough, even water (OH-) is a strong enough base to initiate a polymerization.
Types of initiators Electron Transfer Initiatitors Note: This is now a di-anion initiator.
iclicker Question 4 We want to make a B-S-B polymer. The best initiator would be: a. n-butyllithium b. sec-butyllithium c. A dianion initiator d. Sodium metal e. None of the above
Solvent Effects A. Useful Solvents: B. Non-useful solvents: C H 3 O Benzene, cyclohexane, toluene, THF, diethylether H 3 C O CH 3 Cl Ph Ph Ph Ph CH - + CH H 3 C Ph 3 CH 3 C. Effects of solvent polarity Non-polar (benzene, cyclohexane) H 3 C Ph Ph Ph Ph CH 3 Polar (THF) CH 3 Ph Ion Pair Solvent Separated Fully Solvated
Termination When carried out under the appropriate conditions, termination reactions do not occur in anionic polymerization. One usually adds a compound such as water or alcohol to terminate the process. The new anionic species is too weak to reinitiate. The "Dark Side:" Compounds such as water, alcohols, molecular oxygen, carbon dioxide, etc. react very quickly with the carbanions at the chain ends, terminating the propagation. Therefore, one must scrupulously dry and de-aerate the polymerization ingredients to be able to get a truly living system. This is not easy to do, and adds to the potential costs of the process.
Termination Functionalization of the Chain Ends: The beauty of anionic polymerization lies in the lack of termination reactions when carried out under the appropriate conditions (living polymerization). This means that the propagating species remains unchanged at the chain end when the monomer is consumed, so subsequent chemical reactions can be carried out. (The chain end is a carbanion, and the organic chemistry of carbanions is diverse.) Here are a few examples among many possible:
Termination Carboxylation of end groups: Alcohol end groups via ethylene oxide: Coupling agents: (Method of making block copolymers)
Anionic Synthesis - 2 Manufacturing Process *For SBS & SEBS Styrene monomer + Initiator S Butadiene monomer S-B Coupling reaction (S-B)n (S-E/B)n Hydrogenation by catalyst S-B-S Sequential step Features by styrene monomer 1. Precise control of block size and formation 2. Precise control of mol. wt. 3. Narrow molecular distribution 4. Precise control of micro-structure of the midblock 5. High hydrogenation ratio S-E/B-S MA functionalized S-E/B-S
Block Copolymer Chemistry Initiator Styrene Isoprene/Butadiene Coupling Reinitiation Isoprene/ Butadiene Styrene Isoprene/ Butadiene Styrene Coupling Coupling, n >2 Coupling, n = 2 Full sequential Sequential Coupling, n >2 39
Block Structure Diblock Radial Block (Coupled) D1118 (80%),... G1726 (70%), G1701 (100%),... Triblock (Coupled) D1184, D4274... D1101, D1102, D1107, D1112,... G1657,... Star Block (Coupled) Triblock (Linear Sequential) 40 D KX405, D1155, G1651, G1650, G1652,... G1780
Coupling Effect on Properties CA Why are some Kraton polymers coupled? 1. Easy to precisely control molecular size of each block 2. 2-arm, 3-arm, and 4-arm variation available 3. Faster production rate by eliminating step-3 process 4. Diblock gives excellent tackiness to adhesive compounds Increased diblock content leads to: Flow Increase - Sol. Visc. Decrease - Melt Visc. Decrease Hardness Decrease Strength Decrease Tackiness Increase Heat resistance Decrease CHEM 4364 41 Dr. Houston Brown - 2016 Tensile Strength (MPa) 30 20 10 0 0 25 50 75 100 Diblock Content (%)
Thank You Special thanks to: Dr. Byron Christmas, UHD Dr. John E. Flood (Kraton polymers, and former colleague at Shell).
References 1. Dr. Byron Christmas, Polymer Chem Lecture Notes, UHD. 2. http://www.chem.agilent.com/library/technicaloverviews/public/5990-7890en.pdf 3. http://inside.mines.edu/~dknauss/dendriticpolymers.html&h=398&w=510&sz=6&tbnid=pfhlpfcdkg2onm:&tbnh=93&tbnw=119&zoom=1&u sg= ojbvafwpowj-it_nshhjmwcgtoe=&docid=echxhrhoa_6im&hl=en&sa=x&ei=p1squdjrc8jq2qxoi4hwba&ved=0chaq9qewew&dur=418 4. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ceeqfjaa&url=http%3a%2f%2fchemistry.unt.edu%2f~tgolde n%2fcourses%2flecture16%2520sec.ppt&ei=lmaqufjrhqwe2gwov4hydq&usg=afqjcnhsowutr1lq8pwkzkzp2_evr_wia&bvm=bv.42768644,d.b2i&cad=rja 5. McMurray, John e., Organic Chemistry, Eighth Edition, Centgage, 2012 6. http://chem.chem.rochester.edu/~chem421/anionic.htm 7. Dr. John E. Flood, Kraton polymers. Personal communication. 8. http://www.usm.edu/polymerkinetics/ch13.pdf 9. http://zeus.plmsc.psu.edu/~manias/plmse406/chapter2-3.pdf