Living polymerization

An important aspect of his work, Szwarc employed the aprotic solvent tetrahydrofuran, which dissolves but is otherwise unreactive toward the organometallic intermediates.The high rate of initiation (together with absence of termination) results in low (or narrow) polydispersity index (PDI), an indication of the broadness in the distribution of polymer chains.These experiments relied on Szwarc's ability to control the levels of impurities which would destroy the highly reactive organometallic intermediates.[12] Owing to the discrete single site on the metallocene catalyst researchers were able to tune and relate how the ancillary ligand (those not directly involved in the chemical transformations) structure and the symmetry about the chiral metal center affect the microstructure of the polymer.The chelate initiators have a high potential for living polymerizations because the ancillary ligands can be designed to discourage or inhibit chain termination pathways.In addition, a weak nucleophile (Nu:) can also be added to reduce the concentration of active species even further, thus keeping the polymer "living".[4][5][17] However, it is important to note that by definition, the polymers described in this example are not technically living due to the introduction of a dormant state, as termination has only been decreased, not eliminated (though this topic is still up for debate).[18] Generally, ROMP reactions involve the conversion of a cyclic olefin with significant ring-strain (>5 kcal/mol), such as cyclobutene, norbornene, cyclopentene, etc., to a polymer that also contains double bonds.The important thing to note about ring-opening metathesis polymerizations is that the double bond is usually maintained in the backbone, which can allow it to be considered "living" under the right conditions.There are two general strategies employed in CRP to suppress chain breaking reactions and promote fast initiation relative to propagation.[20] The first strategy involves a reversible trapping mechanism in which the propagating radical undergoes an activation/deactivation (i.e. Atom-transfer radical-polymerization) process with a species X.Several strategies were employed to minimize monomer-monomer reactions (or self-condensation) and polymerizations with low D and controllable Mn have been attained by this mechanism for small molecular weight polymers.[23] Catalyst transfer polycondensation allows for the living polymerization of π-conjugated polymers and was discovered by Tsutomu Yokozawa in 2004[23] and Richard McCullough.When Yokozawa and McCullough independently discovered the polymerization using a metal catalyst to couple a Grignard reagent with an organohalide making a new carbon-carbon bond.Whereas in a coupling reaction the newly formed alkyl/aryl compound diffuses away and the subsequent oxidative addition occurs between an incoming Ar–Br bond and the metal center.The associative complex is essential to for polymerization to occur in a living fashion since it allows the metal to undergo a preferred intramolecular oxidative addition and remain with a single propagating chain (consistent with chain-growth mechanism), as opposed to an intermolecular oxidative addition with other monomers present in the solution (consistent with a step-growth, non-living, mechanism).This copolymer, upon proper thermal and processing conditions, can form cylinders on the order of a few tens of nanometers in diameter of PMMA, surrounded by a PS matrix.These cylinders can then be etched away under high exposure to UV light and acetic acid, leaving a porous PS matrix.This can be easily tuned due to the easy control given by living polymerization reactions, thus making this technique highly desired for various nanoscale patterning of different materials for applications to catalysis, electronics, etc.
Figure 1: Rate of initiation is slower than the rate of propagation, leading to the formation of active species at different points in time during the polymerization.
Figure 2: Rate of initiation is instantaneous in comparison to the rate of propagation, causing all the active species to form simultaneously, and chain growth to occur at the same rate.
a.) Shows the general form of CpA initiators with one Cp ring and a coordinated Nitrogen b.) Shows the CpA initiator used in the living polymerization of 1-hexene (5)
This is an example of a controlled/living cationic polymerization. Note that the "termination" step has been placed in equilibrium with an "initiation" step in either direction. Nu: is a weak nucleophile that can reversibly leave, while the MXn is a weak Lewis acid M bound to a halogen X to generate the carbocation.
This is an example of a controlled/living cationic polymerization. Note that the "termination" step has been placed in equilibrium with an "initiation" step in either direction. Nu: is a weak nucleophile that can reversibly leave, while the MXn is a weak Lewis acid M bound to a halogen X to generate the carbocation.
The catalytic cycle of a living ring-opening metathesis polymerization with a metal catalyst. Note that the ring can be any size, but should contain some significant ring strain on the alkene.
The catalytic cycle of a living ring-opening metathesis polymerization with a metal catalyst. Note that the ring can be any size, but should contain some significant ring strain on the alkene.
Various common forms of copolymers. Here, the two different colored circles represent two different monomers.
Various common forms of copolymers. Here, the two different colored circles represent two different monomers.
polymer chemistrychain growth polymerizationpolymer chainterminateChain terminationchain transfer reactionschain initiationchain propagationpolymerizationpolydispersity indexblock copolymersmonomermolar massend-groupsmolecular weightdispersityLiving anionic polymerizationLiving cationic polymerizationring-opening metathesis polymerizationLiving free radical polymerizationMichael Szwarcstyrenealkali metalnaphthalenetetrahydrofuranelectron transferradical aniondianionaprotic solventviscositypoly(phthalaldehyde)living cationicliving anionicchain-growth polymerizationinitiationKinetic chain lengthKarl Ziegleranionic polymerizationsodium naphthaleneα-olefinsanionicPolyethylenePolypropylenechiralCrystal structureBeta-Hydride eliminationtransition metalmethylalumoxanemetallocenecatalystssubstituentschelatestereospecificcyclopentadienyldiimineisobutyleneRobert H. GrubbsnorborneneTebbe's reagentRichard R. Schrocktungstengradient copolymerReversible-deactivation radical polymerizationfree radicalcatalytic chain transferatom transfer radical polymerizationreversible addition-fragmentation chain transferAtom-transfer radical-polymerizationNitroxide Mediated Radical PolymerizationReversible addition−fragmentation chain-transfer polymerizationstep-growthLiving chain-growth polycondensationCatalyst transfer polycondensationKumada couplingSonogashira couplingGrignard reagent 1,3-Bis(diphenylphosphino)propane (dppp)oxidative additiontransmetalationreductive eliminationmethacrylatesilyl ketene acetalMichael reactionchain-endOwen WebsterBarry TrostCopolymersPure and Applied ChemistryInternational Union of Pure and Applied Chemistry BibcodeMacromoleculesBielawski, Christopher W.CiteSeerXJ. Am. Chem. Soc.Nealey, Paul F.Gold Book