Spin capturing with "Clickable" nitrones: Generation of miktoarmed star polymers

A novel nitrone (α-4-(3-(trimetliylsilyl)prop-2-ynyloxy)-N-tert-butyl nitrone) with an alkyne "click" function is synthesized and employed in enhanced spin capturing polymerization (ESCP) as well as in radical coupling reactions between polymers preformed by atom transfer radical polymerization (ATRP) to generate midchain functionalized polymers. Both techniques allow for the facile introduction of chemical functionalities into a polymer midchain position and hence provide an attractive synthetic avenue for the construction of complex macromolecular architectures. Such a strategy is evidenced by the efficient use of polystyrene and poly(isobornyl acrylate) featuring an alkoxyamine midchain function in polymer-polymer conjugation reactions with azide-terminated polymers, yielding 3-arm star (co)polymers via the Cu-catalyzed alkyne/azide cycloaddition reactions. The successful formation of star-block co- and homopolymers is confirmed by conventional size exclusion chromatography (SEC) as well as via hyphenated liquid absorption chromatography under critical conditions (LACCC)-SEC techniques. The synthetic approach reported herein demonstrates that click functional nitrones can efficiently be employed as macromolecular construction agents in modular reactions. © 2010 American Chemical Society

The Kinetics of enhanced spin capturing polymerization: Influence of the nitrone structure

Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number-average molecular weight, Mn, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N-tert-butyl-α- phenylnitrone (PBN), N-methyl-α-phenylnitrone (PMN), and N-methyl-α-(4-bromo-phenyl) nitrone (pB-PMN), addition rate coefficients, kad,macro, in a similar magnitude to the styrene propagation rate coefficient, kp, are found with spin capturing constants C SC (with CSC = kad,macro/kv) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol-1 were deduced for kad,macro, congruent with a decreasing CSC with increasing temperature. Almost constant Mn over up to high monomer to polymer conversions is found when CSC is close to unity, while increasing molecular weights can be observed when the CSC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc

Formation efficiency of ABA blockcopolymers via enhanced spin capturing polymerization (ESCP): Locating the alkoxyamine function

Enhanced spin capturing polymerization (ESCP) constitutes a versatile method for controlling the molecular weights during free radical macromolecular growth. The methodology employs nitrones as controlling agents, which are incorporated as alkoxyamines into the macromolecules in a midchain position (Ri-NO-Rj). It is demonstrated;via both simulations and experiments;that if the radical initiator and the nitrone are judiciously chosen, midchain functionalizations of over 90% can be achieved. Macromolecules with a nitroxide position in the midchain position can be employed in subsequent nitroxide mediated polymerizations to prepare ABA-type block copolymers. It is demonstrated that high yields of midchain macroalkoxyamine are generated as long as the employed nitrone displays low primary radical addition (governed by the addition rate coefficient k ad) in combination with a relatively rapid chain growth initiation rate (characterized by the primary radical initiation rate coefficient, k i). The absolute value of kad appears to be unproblematic for the success of Ri-NO-Rj formation by ESCP. In addition, it is of relatively high importance to employ a large nitrone concentration to achieve high degrees of Ri-NO-R j. The structure of ESCP prepared polystyrenes was confirmed (among other approaches) via thermally cleaving the Ri-NO-R j species and a subsequent quenching of the reaction to obtain a high yield of the individual arm species of half the length of the macroalkoxyamine. © 2009 American Chemical Society

Control of methyl methacrylate radical polymerization via Enhanced Spin Capturing Polymerization (ESCP)

The nitrone mediated polymerization of methyl methacrylate (MMA) via the enhanced (termination) spin capturing polymerization (ESCP) process is made possible via the addition of small amounts of styrene (between 5 and 10 vol.%) to the reaction mixture. Efficient control over the molecular weight between 7000 and 57,000 g mol-1 (at 60 °C) yields macromolecules that feature a mid-chain alkoxyamine functionality and are rich in methyl methacrylate. The collated kinetic and molecular weight data allow for a deduction of the spin capturing constant, CSC, in the range between 0.15 and 0.30. During the ESCP process, the number average molecular weight, Mn, of the formed mid-chain functional polymer is constant up to high monomer to polymer conversions (i.e. 80%). The high degree of alkoxyamine mid-chain functionality present in the generated polymeric material is evidenced via a subsequent nitroxide-mediated polymerization process employing the formed ESCP polymer, indicating a chain extension from 37,700 to 118,000 g mol-1 with a concomitant reduction in polydispersity (from 2.3 to 1.5). © 2010 Elsevier Ltd