Living p-quinodimethane Polymerization for the Synthesis of Well-Defined PPV Materials: Progress and Challenges

Transcription

1 Living p-quinodimethane Polymerization for the Synthesis of Well-Defined PPV Materials: Progress and Challenges Thomas Junkers Hasselt University Wetenschapspark 1 BE 3590 Diepenbeek Photovoltaics at the nanoscale 28/10/2011 1

2 PPVs: poly(p-phenylene vinylene) Conjugated polymers oleds Solar cells High level of order and orientation of molecules needed self-assembly Block Specific copolymers end groups Need for controlled polymerization procedure for PPV 2

3 Polymerization routes PPVs can be made via various routes: Wittig Heck ROMP Precursor routes 3

4 Radical Mechanism Monomer Formation Self-Initiation Propagation Termination Transfer 4

5 Radical PPV polymerization No control at all! 5

6 Radical PPV polymerization X X X X X X X X X X X X R R R R R R R R R R R R R R R R R X X X X X X X X X X X X 6

7 Radical PPV polymerization dormant species for good control, k deact must be significantly larger than k p BUT: Active species - high energy gain, low activation barrier - hard to compete with 7

8 Radical PPV polymerization RAFT ATRP CTP NMP ESCP RITP CMRC 8

9 Chain Transfer Polymerization R I 2 Z X Z R R R R X r r 1 t tr,x 1 DP n = + = DP + p r r p Mayo Equation C X c n,0 c X M DP n -1 k C X = k tr p m = C X c X 9

10 Chain Transfer Polymerization Can we see an effect from CBr 4 on the polymerization? MDMO Precursor PPV Conjugated MDMO-PPV NaOtBu 2-BuOH Toluene C CBr4 Conditions Eq. CBr 4 Reaction Time (min) Yield (%) M n (g/mol) PDI 30 C, M C, M ! 10

11 Chain Transfer Polymerization Effective chain length control at excess of transfer agent Good correlation for both eliminated and non-eliminated polymer Transfer activity at least a factor of 1000 lower than in conventional FRP C X =

12 Chain Transfer Polymerization A B C D A B C D Sample Amount CBr 4 app. M n A 1 eq g/mol B 2 eq g/mol C 4 eq g/mol D 8 eq g/mol true molecular weight ~factor 3-4 smaller 12

13 Block Copolymers Good chain length control Control over dispersity Control over functionality ATRP 13

14 Block Copolymers Method of choice for production of block copolymers: Atom Transfer Radical Polymerization Chain extension of precursor polymer with styrene 14

15 Block Copolymers Successful ATRP, clear shift of MWD: 27 kda 23 kda STY/PMDETA/CuBr/pPPV: 200/3/1/1 75 C, 1h 15

16 Achieving living polymerization But how can we achive true living polymerization conditions? k ini >> k p k t 0 LIVING POLYMERIZATION Conversion w log M 3,0 3,5 4,0 4,5 5,0 log M If the radicals are hard to be controlled, is there maybe any other way? 16

17 Anionic PPV polymerization Alternative polymerization pathways exist [B] i /[M] i M w (PM) %PM M w (OM) / g mol 1 / g mol Oligomers Expected polymer 17

18 Anionic PPV polymerization Postulate: A anionic polymerization route exists use aprotic solvent (THF) initiate by using a strong base (LDA or LHMDS) ,4 1 LHMDS M (g/mol) 18

19 Anionic PPV polymerization Pure anionic polymerization, MW changes with monomer concentration [M] 0 (mmol/l) M n (g/mol) PDI Yield (%) , , , ,7 100 Higher monomer concentrations lead to higher M n but also to high PDI Reaction conditions: dry THF; 1.3eq base; 0 C; 15 min 19

20 Anionic PPV polymerization To reach good control over polymerization, dedicated initiators must be used 20

21 Anionic PPV polymerization Does the chain length increase with conversion? RT t / min M n (g/mol) PDI yield (%) 0 8,0 1,8 89 0,5 7,8 1, ,8 1, ,1 1, C t / min M n (g/mol) PDI yield (%) , , ,5 93 Reaction conditions: [M] 0 = 0.05M; dry THF; 1.3eq base; 0,1eq t-bu initiator 21

22 Anionic PPV polymerization (1) Polymerization is extremely fast (2) LHMDS is still in competition with initiator vs. 1 DP n = 1 c M c I Initiator Added (%) M w (g/mol) PDI

23 Anionic PPV polymerization Quo vadis? Anionic polymerization route is promising Reaction is extremely fast (procedure matters) Low polydispersity not yet reached Block copolymers are currently made Other initiators and endcappers are currently developed time M n (g/mol) PDI PPV 15min ,8 1,6 PPV-b-tBuA 1eq tbua; 30 min ,0 2,3 23

24 Conclusion Do we have quinodimethane polymerizations under control? Sort of P almost 24

25 Acknowledgment many thanks to: Dirk Vanderzande Joke Vandenbergh Inge Cosemans 25