ATOMIC TRANSFER RADICAL POLYMERIZATION

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1. Atom Transfer Radical Polymerization GOPI PRAMANIK 19POL206 MTech (polymer engineering & technology)

2. Introduction New methods developed that allow for controlling the radical polymerization To minimize the monomer content and produce very uniform molecules. Shows 1 st order kinetics Easy to get an exact molecular weight Uniformity of polymer distribution much more narrow using controlled living radical polymerization The controlled comes by variety of technique SFRP , ATRP , RAFT Control the number of radicals reacting at any one time 2

3. Dormant species Formation of dormant species from propagating radical is possible by Degenerative Transfer. Reversible Deactivation. Catalytic Reversible Deactivation. RDRP(ATRP,RAFT) based on reversible Deactivation of radicals were the first to be developed. NMP employs stable radical species, such as TEMPO to trap the propagating radicals, thus forming dormant species which can be thermally reactivated by homolyses of the C-T(T=stable species) bond. OMRP uses transition metal complexes as radical trapping agents. 3

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5. Comparison between conventional and Controlled Radical Polymerization Conventional The lifetime of growing chains in RP Initiation is slow Free radical initiator is often left unconsumed Nearly all chains are dead in RP Termination usually occurs between long chains and constantly generated new chains in RP Controlled The lifetime of growing chains is extended to more than 1 hour in CRP Initiation is very fast In CRP the proportion of dead chains is usually of 10%. Polymerization in CR is slower in CRP systems, All chains are short at the early stages of the reaction and become progressively longer; thus, the termination rate significantly decreases with time. 5

6. Why CRP Free radical polymerization essentially could not control MW or MWD Radical polymerization (RP) could not yield block copolymers due to the very short lifetime of the growing chains No pure block copolymers and essentially no Polymers with controlled architecture can be produced by conventional RP 6

7. ATRP It is an example of reversible deactivation radical polymerization ATRP is currently the most widely used CRP technique. ATRP is catalytic reversible deactivation process operating at low catalyst concentration.  A transition metal complex as the catalyst with an alkyl halide as the initiator (R-X). Large range of monomers polymerizable by this technique under a wide range of conditions. ATRP not to be considered as redox free radical polymerization, here the metal catalyst used for only radical generation. In ATRP metal complex play two different role Radical generation from RX/halogen capped chain end(activation) Re-formation of dormant species After the short time radical propagation(deactivation). Various transition metals have been used in ATRP such as Cu, Ru, Fe, Ni, Os , Re, Rh, Pd, Co, Ti , and Mo. 7

8. Components of normal ATRP Five important variable components of atom transfer radical polymerizations Monomer Initiator catalyst ligand Solvent 8

9. Monomer Monomers typically used in ATRP are molecules with substituents that can stabilize the propagating radicals for example, styrenes, (meth)acrylates, (meth)acrylamides, and acrylonitrile Initiator The number of growing polymer chains is determined by the initiator  low polydispersity and a controlled polymerization, the rate of initiation must be as fast or preferably faster than the rate of propagation All chains will be initiated in a very short period of time and will be propagated at the same rate   Alkyl halides    good molecular weight control The shape or structure of the initiator influences polymer architecture For example, initiators with multiple alkyl halide groups on a single core can lead to a star-like polymer shape α-functionalized ATRP initiators can be used to synthesize hetero- telechelic polymers with a variety of chain-end groups 9

10. Catalyst The catalyst is the most important component of ATRP because it determines the equilibrium constant between the active and dormant species There are several requirements for the metal catalyst: There needs to be two accessible oxidation states that are differentiated by one electron The metal center needs to have reasonable affinity for halogens The coordination sphere of the metal needs to be expandable when it is oxidized as to accommodate the halogen The transition metal catalyst should not lead to significant side reactions, such as irreversible coupling with the propagating radicals and catalytic radical termination Most studied catalysts are those that include copper 10

11. Ligand Solubilize the copper halide in whichever solvent is chosen and to adjust the redox potential of the copper Solvents Toluene, 1,4-dioxane, xylene, anisole, DMF, DMSO, water, methanol, acetonitrile, or even the monomer itself (described as a bulk polymerization) are commonly used 11

12. Basic working mechanism of ATRP 12

13. Mechanistic aspects of ATRP ATRP equilibrium between active radicals and dormant species is mediated by the activator( CulL +) and deactivator (X- CullL +)forms of the catalyst, where L=nitrogen based polydentate ligand. The activator CulL + must be sufficiently active to cleave the C-X bond in the alkyl halide initiator (R-X) Similarly , X- CullL + deactivator complex must quickly trap propagating radicals to generate the Pn -X dormant species. The ATRP catalyst ( CulL +) is subjected to both electron transfer and an atom transfer with the halogen atom Pn -X being transferred from dormant species to the catalyst ( CulL + X- CullL + ) and again back to the propagating radicals Pn -X. Copper catalysed ATRP occurs via an inner sphere electron transfer- concerted atom transfer (ISET-AT) mechanism.

14. Equillibrium of ATRP: From a thermodynamic point of view, the equilibrium of ATRP can be formly expressed as the combination of following four contributing reaction. C-X bond homolyses(BH) of the alkyl halide initiator or dormant chain, K BH Electron affinity (EA)of the halogen radical, K EA Reduction of the CullL2+ complex, K ET Association of the halide anion to the CullL2+complex, KXll These formly the equilibrium constant of ATRP K ATRP =( K BH K EA K X ll )/K ET 1 and 2 depends upon the nature of RX. 3 only concerns the environment of the copper center (ligand and solvent) 4 depends on the nature of catalytic complex, halogen atom and solvent . More the polar solvent and high temperature results in larger K ATRP.

15. Polydispersity index is expressed as: D: Deactivating agent. p: fractional conversion of monomer at any time in the reaction. The molecular weight distributions are narrower with lower initiator concentrations, higher conversions, rapid deactivation (higher values of kd and [D]), and lower kp values. When these conditions are fulfilled the molecular weight distribution follows the Poisson distribution. 15

16. Advantages Increased control of molecular weight Molecular architecture Polymer composition Maintaining a low polydispersity (1.05-1.2) The halogen remaining at the end of the polymer chain after polymerization allows for facile post-polymerization chain-end modification into different reactive functional groups. The use of multi-functional initiators facilitates the synthesis of lower-arm star polymers and telechelic polymers. The effect of inhibitor and retarder, solvent, chain transfer agent all remain same as in convention radical polymerization. The regioselectivity, stereoselectivity, and copolymerization behavior remain same as of above. 16

17. Disadvantages Drawback of ATRP is the high concentrations of catalyst required for the reaction The removal of the copper from the polymer after polymerization is often tedious and expensive limiting ATRP’s use in the commercial sector A final disadvantage is the difficulty of conducting ATRP in aqueous media Polymers synthesized through ATRP Polystyrene Poly (methyl methacrylate) Polyacrylamide ATRP polymer have green color due to the presence of copper in it which can be reduced by extraction with water if the ligands impart water solubility to Cu2+. Cu2+ can be removed by treatment of the polymerization reaction system with alumina. 17

18. Standard reduction potentials of halogen atom in different solvents at T=25ᵒC

19. Block Copolymer Statistical( random) copolymer Gradient copolymer Block & Graft copolymer Other Architectures: Hyperbranched Brush & star Functionalized polymers Block copolymer synthesized via ATRP possible by two methods: One-pot sequential Isolated Macroinitiator methods

20. BLOCK COPOLYMER All Blocks via ATRP An AB diblock copolymer is produced in the One pot sequential method by when polymerizing monomer A first and then Monomer is added when most of the A has reacted. In the macroinitiator method the halogen terminated monomer A ( RA n X ) is isolated and than used as an initiator (macroinitiator) together with CuX t o polymerize monomer B. RA n X is usually isolated by precipitation with a nonsolvent or by other techniques. Both methods requires that the polymerization of 1 st monomer not to be carried to completion ,usually 90% conversion is the maximum conversion, because the extent of bimolecular termination increases as the concentration decreases. This would result in loss of halogen terminated chain and the corresponding loss of the ability to propagate when the second monomer is added. The final product will be a block copolymer contaminated with some homopolymer A. Similarly in isolated macroinitiator case requires isolation of RA n X prior to complete conversion so that there is minimum loss of functional group for initiation. Loss of functionality can also be minimized by adjusting the choice and amount of component of the reaction system(activator, deactivator, solvent ligand) and reaction condition (temperature, concentration) to minimize normal termination. The ono pot sequential method has some disadvantage that the propagation of the second monomer involves a mixture of second monomer plus unreacted first monomer. The second block is actually a random copolymer.

21. A symmetric block copolymer such as ABA and CABAC can be made efficiently by using a difunctional initiator , such as α , α dichloro-p-Xylene or dimethyl 2,6- dibromoheptanedioate instead of monofunctional initiator. Blocks via combination of ATRP and non-ATRP One approach is to use an initiator in the non-ATRP polymerization to produce a polymer with a halogenated end group either by initiation or termination. The halogen terminated end capped is than used as macroinitiator in ATRP. For example cationic polymerization of styrene with 1-phynyl ethyl chloride and SnCl4, The anionic ring opening polymerization of caprolactone with 2,2,2-tribromoethanol and triethyl aluminium and the convention radical polymerization of vinyl acetate with a halogen containing azo compound. Another approach is to use an initiator for ATRP that produces a polymer with functional group capable of initiating a non- ATRP polymerization. ATRP polymerization of methyl-methacrylate with 2,2,2-tribromoethanol produces a hydroxyl terminated poly(MMA). The OH-PMMA act as initiator in presence of triethyl aluminium for the ring opening polymerization of caprolactone.

22. Other polymer architecture: A star polymer contains polymer chains as arm emanating from a branch point. Star polymers can be synthesized via ATRP by using an initiator containing 3 or more halogens for example 3arm polymer is obtained by using a tribromo initiator: ATRP produces a graft copolymer when the initiator is a polymer with one or more halogen-containing side groups: When the polymer initiator containing many halogen , there will be many grafted side chains, and the product is called a comb or brush polymer