Lecture 27: Polysiloxanes, Lactams, Etc

10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 27: Polysiloxanes, Lactams, Etc. Issues of Equilibria (Ring-Opening Polymerization) -particularly relevant with long lived polymerization processes - “living” systems: step growth → revert to equilibrium Ch 7 Odian Effects of Ring Strain (CH2)n • 3-4 atoms → angle strain • 5,7 atoms → torsional strain b/c of eclipsed conformations on neighboring atoms (not a planar condition, → chair conformation (6)) • 6 atoms → perfect situation (no angle, torsional strain, have chair conformation) • 8-11 atoms → transangular strain (due to repulsive interactions w/in ring from H atoms) → much more pronounced when things are bigger then H-atoms • 13 atoms → eliminate strains but difficult to make ends meet for polymerization In general, 5 and 6 membered rings are more likely to undergo depropagation due to cyclic nature, or in non-ring opening polym, it’s a temperature effect O ⇒ → Equilibrium monomer conc for corresponding T (e.g. Tc) THF → PTMO O O O trioxane CH2 O PMO depropagates into formaldehyde O HCH highly volatile vaporizes at high T 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. Because of possible depropagation, - for “Quasi-living” systems → living behavior to approach of [M]e But for more strained systems, have full conversion e.g. Ethylene oxide oxitanes O O R R Siloxanes • Accessible by anionic or cationic means R Si O n R' Step growth is also possible CH3 CH3 Cl + Si Cl H2O + H Cl CH3 Si O HO Si OH CH3 CH3 CH3 HO Si O CH3 CH3 Si CH3 OH CH3 CH3 HO -HCl -H2O Si O CH3 n H Siloxanes form stable ring compounds → alleviate ring strains → also form SiO n n=3-5 most common (favored in equilibrium) Note: most stable cycle is n=4 (8 atoms) for Si-O bond Anionic Ring Opening R O R Si O Si R R R trimer e.g. R=CH3 ⇒ dimethyltrisiloxane “D3” R Si O 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond D3 and other trimers like it can be initiated by - alkyl metal hydroxide CH3CH2O- M+ or HO- M+ metal hydroxide or CH3-Si-O- M+ silanolate anion Lecture 27 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. (of course, can use butyl Li, strong initiators) A + O Si Si O O Si A Si O Si O Si O M M propagate - R group can be alkyl, cannot be H (because Si-H is acidic → termination) - R group can be vinyl group * with appropriate initiator or other non-reactive group Cationic Propagation Can use D3 or D4 ← too stable to be made by anionic NOTE: D4 is cheaper commercially only D3 can be made by anionic Si O Si O Si O Si O H protonate with acid initiator (protonic acid like H2SO4 or use Lewis acid) SiO Lactams form polyamids via “activated monomer” mechanism (cyclic amide) O C (CH2)m NH most common: m=5 (ε-caprolactone) can go by anionic or cationic mechanisms 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 27 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. Anionic - Use metal or metal hydride as initiator (M) (MH) O C H2C H2C NH CH2 CH2 CH2 Initiation O O Step 1 + C (CH2)5 M (CH2)5 NH O + C Step 2 (CH2)5 N slow step M in initiation process + C O C C (CH2)5 2 H2 M N O HN 1/ (CH2)5 O N C (CH2)5 NH attacks carbonyl group can be sped O up by introducing C a more reactive group (CH2)5 N (acyl lactam) M very reactive rapid H exchange O O C + (CH2)5 C NH2 (CH2)5 N active monomer O C (CH2)5 + NH R O O C C Cl (CH2)5 goes very fast in initiation step O N C R Ri >> Rp ⇒ living system Can be used to create block copolymers Can have well-defined blocks with other vinyl monomers. 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 27 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. Polyesters as Biodegradable Polymers Poly(α-hydroxy acids) O H from OCHR C O R n O H R O O R=H ⇒ polyglycolic acid (PGA) (polyglycolide) R=CH3 ⇒ polylactic acid → poly (L-lactide) (PLLA) poly (D-L-lactide) (PDLLA) copolymers: “PLGA” (vary composition to change Tm) and other mechanical properties - can proceed by anionic or cationic means most common: anionic initiated by oxyanions O RO O O + O O RO CH2 O propagate O O Details - can achieve living polymerization - commonly add catalyst that accelerates esterification e.g. stannous octanoate - can also change conditions to achieve step growth-like conditions (e.g. initiator, solvent conditions) condensation nature → control MW with alcohol endcappers Polylactones O O O O (CH2)m C (CH2)m n Anionic or cationic Initiate with CH3O m=3,4,5 (5 most common) ε-caprolactone 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 27 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.