Lecture 23: End Group Functionalization, Telechelic Oligomers and Novel Architectures using Coupling Techniques

10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 23: End Group Functionalization. Telechelic Oligomers and Novel Architectures using Coupling Techniques From Last Lecture Diene polymerization: 1,2 addition: H2 H C C 1,4 addition: H2 C C H vs n C H H2 C n cis: trans: linkage vs primarily cis: crystallinity suppressed low crystallinity v. low Tg ⇒ used for synthetic rubbers primarily trans: semicrystalline mat w/Tm ↑ than 25oC Living Polymerization - allow intentional introduction of terminating group → ω end-functionalization α − ω functionalization with dianionic initiator - introduce second monomer after first is consumed A + B to yield a AnBm block copolymer - can also have triblock copolymers: ABBBB ABA triblocks ABC triblocks . . . heptablocks (in literature) But what limits it? - we must consider the relative reactivities of the blocks: A must be able to initiate monomer B A must not undergo side reaction with B A + B initiation must be rapid (want narrow polydispersity) all groups get initiated at once → same prop. Rates Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. → uniform blocks “Food Chain” short list of monomers, in increasing electrophilicity most aggressive: • dimethyl amino styrene, p-methoxy styrene, p-methyl styrene, α-methyl styrene • styrene almost the same • butadiene • isoprene • vinyl naphthalene Ease of initiation • p-chlorostyrene A monomer in the list can • vinyl pyridine H C C initiate anything equal or • diphenyl ethylene (DPE) 2 * below it • alkyl methacrylates (MMA) • propiolactones Ex: ethylene oxide cannot initiate • ethylene oxide vinyl pyridine but can initiate DPE cannot O selfvinylidene cyanides propagate propylene sulfide (too bulky) • vinylidene cyanide More reactive monomers → faster polymerization • α-cyanoacrylates Example: Synthesis of a Diblock Copolymer want to make polystyrene–b–poly(methyl methacrylate) PS–b–PMMA A Must start with styrene CH2-CH But this can react with the caroxylic Ester groups of MMA: C O B O CH3 Add DPE DPE CH2 H C CH2 C ← too bulky to undergo any side reactions! Because only one group of DPE, there is no effect on the properties of copolymer 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 23 Page 2 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. C Add MMA H C CH3 CH2 CH2 C x propagates C PS C O PMMA CH3 Another Example: Synthesis of a Triblock Copolymer Styrene–Butadiene–Styrene triblock PS–b–PB–b–PS SBS Can make this polymer at least 3 different ways: • consideration: styrene and butadiene are very close in reactivity So, butadiene can initiate styrene 1. Difunctional initiator: ex: Na naphthalene + butadiene PB Consume B completely and then add styrene (S) Symmetric triblock b/c same prop. rate and time 2. Add S+B+S in sequence (w/monomer fully consumed by each step) 3. First initiate PS a) PS b) add butadiene + styrene together in big aliquot (in anionic polym, do not get alternating copolymers, get blocky behavior) - reactivity rates (anionic) 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond ksb >> kbs rb = 14 rs = 0.03 growing B is preferred Lecture 23 Page 3 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. S + S B B S S B S B S B SB PS PB PS • create block with PB (B preferred over S) ° then transition (both B and S) ° then completely PS (all that is left) • this is done in industry - relatively simple - transitional regime enhances surface properties more diffuse interface → Better properties 4. Use a coupling agent (used for systems with no other options) i) grow diblock of PS–PB PB PS ii) then add reactive coupling agents e.g. phosgene (extremely reactive) O PS Cl C PS Cl O PS C PB PB PS coupled triblock everytime you introduce a monomer or coupling agent, you risk the chance of impurities (can’t get ll-block copolymer or high MW copolymer) other coupling agents: (Quantitative reactions) 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 23 Page 4 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. Br Cl Br H3C Si CH3 Cl O O RO OR Disadvantages of Coupling • often get incomplete reaction (diblock + triblock) • needs perfect stoichiometry (scale is very small) Despite that, you can be creative: Can use coupling to make “stars” SiCl4 Si Or use short oligomer Cl Cl Cl Cl Cl Can make graft copolymers: (or comb copolymers) C O OCH3 C O OCH3 C O OCH3 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond C O C O C O well-defined lengths Lecture 23 Page 5 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.