Gurkan Hizal

Double click reaction strategies for polymer conjugation and post-functionalization of polymers

Double click reaction strategies, which are a combination of different type of click reactions, allow the preparation of polymers with various topologies and the post-functionalization of polymers, which cannot be easily achieved by using only one click reaction. The most studied click reaction combinations may be listed as the Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC)–Diels–Alder, and the CuAAC–nitroxide radical coupling reactions for polymer–polymer conjugation and the CuAAC–Diels–Alder, or the CuAAC–thiol-ene reactions for post-modification of polymers.

Photosensitized cationic polymerization of cyclohexene oxide: A mechanistic study concerning the use of pyridinium-type salts

Abstract The photoinitiation of the polymerization of bulk cyclohexene oxide (CHO) containing N -ethoxy-2-methyl pyridinium hexafluorophosphate (EMP + PF 6 − ) and either anthracene or thioxanthone (TX) at λ inc> 340 nm was studied. Regarding the action of anthracene it is notable that upon u.v. irradiation of a CHO solution of poly(tetrahydrofuran) bearing terminal anthryl groups a block copolymer, poly(tetrahydrofuran- block -cyclohexane oxide), is formed. The optical absorption spectrum of the block copolymer does not possess bands characteristic for anthracene. Therefore, the following mechanism is postulated: electron transfer from singlet excited anthracene molecules to EMP + ions results in the formation of anthracene radical cations that react with ethoxyl radicals stemming from the decomposition of EMP• radicals. 9-Ethoxy-9,10-dihydroanthryl ions generated in this way react with CHO thus initiating its polymerization. Regarding the action of TX it seems that the polymerization of CHO is essentially initiated by protons. The generation of protons has been evidenced. In propylene carbonate solution protons are generated with Φ(H + = 0.27 whereas Φ(−TX) = 0.028. The postulated mechanism is based on the reaction of triplets, 3 TX * , with both CHO ( k RH= 3 × 10 4 1 mol −1 s −1 ) and EMP + ions ( k ET= 4 × 10 7 1 mol −1 s −1 ). At low concentration of EMP + PF 6 − (6.8 × 10 −4 mol −1 l −1 ) 3 TX * molecules react almost exclusively (93%) with CHO and it appears that ketyl radicals thus formed react with EMP + ions, a process eventually resulting in the formation of protons and the regeneration of TX. At relatively high concentration of EMP + PF 6 − (6.8 × 10 −3 mol l −1 ), thioxanthone triplets are largely (47%) deactivated by electron transfer to EMP + ions. The importance of this reaction with respect to its contribution to the initiation of the polymerization of CHO has not yet been revealed.

Initiation of cationic polymerization via oxidation of free radicals using pyridinium salts

Abstract Pyridinium ions of appropriate reduction potential E red 1 2 are capable of oxidizing carbon-centred free radicals to carbocations that can initiate the polymerization of various compounds. 1-Ethoxy-2-methyl pyridinium ions of E red 1 2 = −0.7 V were found to react with free radicals generated by (a) photolysis or (b) thermolysis of various compounds. Radical generation was achieved in case (a) with benzoinmethylether, diphenyl-2,4,6-trimethylbenzoylphosphine oxide or benzophenone/tetrahydrofuran (BP/THF) and in case (b) with phenylazotriphenylmethane, BP/THF, benzoylperoxide/THF or 2,2-azobisisobutyronitrile/THF. The following monomers were polymerized: 1,2-epoxycyclohexane (cyclohexene oxide) and n-butylvinyl ether.

N-Alkoxy pyridinium ion terminated polytetrahydrofurans. Synthesis and their use in photoinitiated block copolymerization

Abstract Living polytetrahydrofuran was terminated with pyridinium or isoquinolinium N-oxide to yield polymers with the corresponding end groups. Direct and sensitized irradiation of these photoactive polytetrahydrofurans produced alkoxyl radicals at both chain ends capable of initiating the radical polymerization of methyl methacrylate. In this way, triblock copolymers were formed. The block copolymer composition was determined with the aid of g.p.c. and optical and 1H n.m.r. spectroscopy.

Charge-transfer complexes of pyridinium ions and methyl- and methoxy-substituted benzenes as photoinitiators for the cationic polymerization of cyclohexene oxide and related compounds

Abstract The cationic photopolymerization of cyclohexene oxide and 4-vinylcyclohexene dioxide was achieved by using charge-transfer (CT) complexes of pyridinium salts and aromatic electron donors (hexamethylbenzene and 1,2,4-trimethoxybenzene) as initiators. Irradiation of the CT complexes with light of relatively long wavelength produces radical cations of the aromatic electron donors capable of initiating cationic polymerization. N-Vinylcarbazole and n-butyl vinyl ether are spontaneously polymerized in CH2Cl2 solution in the dark at room temperature upon addition of the CT complexes mentioned above. The molar extinction coefficients ect and equilibrium constants Kct of the CT complexes have been determined.

Synthesis of an ABCD 4-Miktoarm Star Quaterpolymer Through a Diels-Alder Click Reaction

An ABCD 4-miktoarm star quaterpolymer with A = poly(ε-caprolactone) (PCL), B = poly(tert-butyl acrylate) (PtBA), C = polystyrene (PS) and D = poly(methyl methacrylate) (PMMA) arms was prepared using the Diels–Alder reaction strategy. Firstly, PCL with anthracene (PCL-Anth) and PtBA with furan-protected maleimide (PtBA-MI) end-functionalities were synthesized separately via ring-opening polymerization of ε-CL and atom transfer radical polymerization of tBA, respectively. These homo-polymers were linked via the Diels–Alder click reaction in toluene at 100°C in order to give PCL-b-PtBA co-polymer. Next, this block co-polymer is utilized successively as macroinitiator in nitroxide-mediated radical polymerization of styrene and in free radical photo-polymerization of MMA in order to achieve the PCL-PtBA-PS-PMMA 4-miktoarm star quaterpolymer.

Synthesis of asymmetric difunctional initiators and their use in the preparation of block copolymers via ATRP and SFRP

Abstract Novel asymmetric difunctional initiators 2-phenyl-2-[(2,2,6,6-tetramethylpiperidino)oxy]ethyl 2-bromo-2-methyl propanoate and of 2-phenyl-2-[(2,2,6,6-tetramethylpiperidino)oxy]ethyl 2-bromo propanoate were synthesized in a three-step reaction sequence and used in atom transfer radical polymerization (ATRP) of methyl methacrylate or tert-butyl acrylate leading to corresponding polymer with tempo moiety as chain end. These polymers were found to be efficient initiators for stable free radical polymerization (SFRP) of styrene. 1H NMR and g.p.c. studies of the obtained polymers show that block copolymers are readily formed as a result of combination of ATRP and SFRP mechanisms.

An easy way to the preparation of multi-miktoarm star block copolymers via sequential double click reactions

As a new synthetic route, we here employed sequential double click reactions involving azide–alkyne and Diels–Alder reactions for the preparation of multi-miktoarm star block copolymers by using the arm-first approach.

Synthesis of hydroxy-terminated polytetrahydrofuran by photoinduced process

SummaryThis paper describes the preparation of hydroxy-functional telechelics by photoinduced decomposition of polytetrahydrofuran possessing terminal pyridinium ions in THF solution. Hydroxyl functionality was evidenced by end capping and polycondensation with isocyanates.

Synthesis of block copolymer by combination of living cationic and iniferter polymerization systems

Abstract Functional polytetrahydrofurans containing N , N ′-diethyl dithiocarbamates were synthesized by the living cationic polymerization of tetrahydrofuran using trifluorosulfonic acid anhydride and N , N ′-diethyl dithiocarbamic acid sodium salt as an initiator and terminator, respectively. The molecular weight distribution was narrow and the number average molecular weights of the polymer were very close to the theoretical one, M n,th . N , N ′-diethyl dithiocarbamoyl polytetrahydrofuran was used as a macroiniferter in the photopolymerization of methyl methacrylate and block copolymers were obtained.