Segmented Copolymers. Segmented Polyurethanes (Prof. Hammond s thesis)

Transcription

1 0.569 Synthesis of Polymers Prof. Paula ammond Lecture 0: Introduction to Radical Polymerization Segmented opolymers Segmented Polyurethanes (Prof. ammond s thesis) soft segment ends in groups - oligomer - low T g (liquid-like at 25 o ) ( 2 ) 4 oligomeric diols simple polyether MW: ,000 2 Si Diisocyanate: (-N==) e.g. N-R-N N-( 2 ) 6 -N N 2 N 3 hain extender - onnector between different units - Almost always short diol ex: -( 2 ) n - To get segmented polyurethane:. Endcap soft segment w/diisocyanate: 2 N R N + (rapid) N R N N R N no byproduct itation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT penourseware ( Massachusetts Institute of Technology, Date.

2 2. or N N + N R N + stoichiometric proportion soft isocyanate chain extender ard Segment 4 3 an be made longer by adding diol and diisocyanate in equal proportions [ ] [ ] 4 N R N N R N hard segment hard domains App: Nike shoe soles biodegradable scaffolds physical network - held together by hydrogen bonds - some deg of crystallinity T m (flow temp) Step Growth Polymerization Polymers at high π 2 nd order kinetics MW linearly with time ( p n = + [a] o kt ) MW π All species in rxn bath are reactive Need high π for high MW hain Growth (Addition) monomer activation required for polymerization only activated monomer/polymer growing chains are active in rxn (v. small fraction at given time) growing chains get large rapidly then terminate, deactivates chain new monomer is activated Prof. Paula ammond Page 2 of 6 itation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT penourseware ( Massachusetts Institute of Technology, Date.

3 R* + M RM* RM* + M RMM* RMn* monomer high MW hig h π chains + monomer have only monomer high MW polymer growing chains MW f(π) unless living system although it is f[m] o Addition monomers are: R vinyl groups (=) ketones (=) aldehydes heterocyclic ring monomers (strained) Propagating (active) species: anionic - cationic + free radical Processes in Addition Polymerization:. Initiation 2. Propagation 3. Termination 4. Transfer of charge or active species from one chain to another Prof. Paula ammond Page 3 of 6 itation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT penourseware ( Massachusetts Institute of Technology, Date.

4 (but not always present) Free Radical Polymerization. Initiation: Kinetics: I 2R 2 R + 2 R initiation fragment 2. Propagation Step: RM + M RM 2 RM n + M RM n R + 2 R 3. Termination: appens one of 2 ways: a. coupling RM n + RM p RM n+p R doubling size of polymer b. disproportionation ktd hain Transfer: Prof. Paula ammond Page 4 of 6 itation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT penourseware ( Massachusetts Institute of Technology, Date.

5 + R" 2 + R" 2 2 propagating species impacts MW Kinetic Rate Expression Initiation: I k d 2R dissociation (rate determining) R + M RM d [ RM ] d R = f [ ] efficiency factor d[] I = k d [] Propagation d[ R ] I = 2 create 2 fragments d [ RM ] d = f [ R ] = 2 fk d [] I k d l mol sec RM n + M RM n+ d M [ ] R p = = k p [ M ][ M ] [ M ] [ Mn ] any active monomer (assume equal reactivity for all M species) k p l mol sec Termination M i + M j M i+j Prof. Paula ammond Page 5 of 6 itation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT penourseware ( Massachusetts Institute of Technology, Date.

6 d [ M ] R t = = 2k[ M ] 2 assume same disproportionation: let k t = k tc + k td k t l mol sec ow fast are you creating polymer? Polymerization rate d[ M ] = R p = k p [ M ][ M ] Assume steady state free radical concentration [M ] R i = R t 2k t Solve for [M ]: [ M ] 2 = 2k [ ] d f I 2 f I M = k d k t [ ] [] plug into R p expression k f I 2 R p = k p M k t d [] [ ] Generic Form: R p p = 2 k R M i 2 k t 2 [ ] Prof. Paula ammond Page 6 of 6 itation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT penourseware ( Massachusetts Institute of Technology, Date.