10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 10: Introduction to Radical Polymerization Segmented Copolymers Segmented Polyurethanes (Prof. Hammond’s thesis) 1 “soft segment” → ends in –OH groups - oligomer - low Tg (liquid-like at 25oC) HO OH HO (CH2)4 O H oligomeric diols simple polyether MW: CH3 ∼ 1000 – 10,000 HO CH2O Si O CH2OH CH3 2 Diisocyanate: (-N=C=O) OCN-R-NCO e.g. OCN-(CH2)6-NCO H2OCN NCOC 3 Chain extender - Connector between different units - Almost always short diol ex: HO-(CH2)n-OH To get segmented polyurethane: 1. Endcap soft segment w/diisocyanate: OO H H OCN R N CO OC N R NCO2 OCN R NCO + HO OH (rapid) no byproduct 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.→2. or + OCN R NCO stoichiometricOCN NCO proportion+ HO R' OH soft isocyanate chain extender C H N R H N C O O O O R' OC O H N R H N C O 1 2 1 1 3 2 Hard Segment 1 4 3 Can be made longer by adding diol and diisocyanate in equal proportions - - - [ ]3 - - - [ ]4 hard segmenthard segment→ hard domains→ hard domainsApp: → Nike shoe soles → biodegradable scaffolds Step Growth Polymerization physical network - held together by hydrogen bonds - some deg of crystallinity → Tm (flow temp) Polymers at high πPolymers at highπ• 2nd order kinetics • MW↑ linearly with time ( pn = 1 +[a]o kt ) 1 • MW ∝ 1 −π • All species in rxn bath are reactive • monomer activation required for • Need high π for high MW polymerization • only activated monomer/polymer growing chains are active in rxn Chain Growth (Addition) (v. small fraction at given time) • growing chains get large rapidly then terminate, deactivates chain • new monomer is activated 10.569, Synthesis of Polymers, Fall 2006 Lecture 10 Prof. Paula Hammond Page 2 of 6 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.R* + M → RM* RM* + M → RMM* RMn* • have only monomer high MW polymer growing chains • MW ≠ f(π) monomer monomerhigh MW higWh πhhigh Mhig π unless “living” system chains +chains + although it is f[M]o monomermonomerAddition monomers are: vinyl groups (C=C) R O ketones (C=O) H aldehydes O OO heterocyclic ring monomers (strained) Propagating (active) species: --CCCanionic ++CCCcationic free radical C•CC•Processes in Addition Polymerization: 1. Initiation 2. Propagation 3. Termination 4. Transfer of charge or active species from one chain to another 10.569, Synthesis of Polymers, Fall 2006 Lecture 10 Prof. Paula Hammond Page 3 of 6 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.(but not always present) Free Radical Polymerization Kinetics: I → 2R• 1. Initiation: H H2R + H2C CH RCC initiation fragment R' R' 2. Propagation Step: RM + M RM2 RMn + M RMn+ 1 H H H H2H2H2RCC RCCC C+ H2C CHR' R' R'R' 3. Termination: Happens one of 2 ways: a. coupling RMn RMn+ p Rdoubling size of polymer+ RMp b. disproportionation ktd C CH C CH + C CH2H2 C CH + HH2 H2 R' R' R' R' 4. Chain Transfer: 10.569, Synthesis of Polymers, Fall 2006 Lecture 10 Prof. Paula Hammond Page 4 of 6 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.H C C C CH2 + R" C+ HC R"H2 H2 R' R' propagating species impacts MW Kinetic Rate Expression Initiation: kd dissociation (rate determining) 2RI R + M RM [ ]= fd R ⋅d RM ⋅ [] dt dt efficiency factor −d[]I =kd []I = []⋅1 d R dt 2 dt create 2 fragments d RM ⋅[][ ]=fd R ⋅= 2 fkd []I kd ∼ 10-4 – 10-6 l dt dt mol ⋅sec Propagation RMn + M RMn+ 1 d MRp = −[]= kp []M ⋅[]Mdt [ ][ ]≡ Mn ⋅ any active monomer M ⋅ (assume equal reactivity for all M⋅ species) kp ∼ 102 – 104 l mol ⋅sec Termination Mi + Mj Mi+j 10.569, Synthesis of Polymers, Fall 2006 Lecture 10 Prof. Paula Hammond Page 5 of 6 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.d MRt = −[ ] ⋅= 2k[]M ⋅2 dt assume same disproportionation: let kt = ktc + ktd kt ∼ 106 – 108 l mol ⋅sec How fast are you creating polymer? Polymerization rate d M− [ ] ⋅= Rp = kp [ ][]M ⋅ Mdt Assume steady state free radical concentration [M⋅] ⇒ Ri = Rt [] M ⋅2 = 2k [] 2kt df I Solve for [M⋅]: 1 ⎛ f I[] M ⋅ =⎜⎜kdk []⎞⎟⎟2 plug into Rp expression ⎝ t ⎠ 1 ⎛k f I ⎞2 d [] []Rp =kp ⎜⎜⎟⎟M ⎝ kt ⎠ Generic Form: 1 R ⎜ p ⎟ Mp = ⎜⎛ 2 kk 2 Ri ⎟⎞2 [] ⎝ t ⎠ 10.569, Synthesis of Polymers, Fall 2006 Lecture 10 Prof. Paula Hammond Page 6 of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology,
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