Polymer dormant

In some systems most of the polymers potentially capable of growing are inert and do not contribute to propagation. The polymerization involves a small fraction of active polymers remaining in dynamic equilibrium with the inert ones. We refer to the latter as the dormant polymers in equilibrium with the living ones. A few examples illustrate this behavior  

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The mechanism of anionic polymerization of styrene and its derivatives is well known and documented, and does not require reviewing. Polymerization initiated in hydrocarbon solvents by lithium alkyls yields dimeric dormant polymers, (P, Li)2, in equilibrium with the active monomeric chains, P, Li, i.e. 

Our approach to polymer chain growth modeling is based on population balances for the various polymer species participating in and resulting from chain growth and transfer [34], The kinetics scheme is written below in mathematical fashion and is a precursor to the derivation of population balances. Monomer units are represented as M, and growing polymer chains are represented by the symbol Pn, where n is the number of repeat units attached to the active catalyst. Dormant polymer is represented by An where n is the number of repeat units attached to the CTA. Dead polymer chains, which arise from chain termination events such as hydrogenolysis... 

Bulk polymer moments Growing polymer moments Dormant polymer moments... 

D. J. Worsfold, NRC, Ont. I was glad to see that you were able to identify and measure the amount of dormant polymer present in the THF polymerization. The amount of the dormant chain end... 

A brief historical review of the concept of living polymers and its ramifications is followed by consideration of various mechanisms of anionic polymerizations. The pertinent papers presented in this meeting are surveyed. Special attention is devoted to polymerizations involving Li counterions proceeding in hydrocarbon solvents. It is stressed that living and dormant polymers participate in such reactions and the consequences of their presence are deduced. 

However a slow re-initiation of polymerisation, giving rise to a second slower stage, occurs on formation of a three membered cyclic sulphonium ion salt by intramolecular nucleophilic attack of the penultimate sulphur in the dormant polymer ... 

PpClaMeSt PPVL PVL PSt -St-IB-Cl triblock copolymer Poly(p-chloro-a-methylstyrene) Poly(pivalolactone) Pivalolactone Polystyrene Dormant polymer end with styrene as the penultimate and isobutylene as the ultimate monomer unit... 

In processes based on reversible termination, like NMCRP and ATRP (Sect. 4.4.2), a species is added which minimizes bimolecular termination by reversible coupling. In NMCRP this species is a nitroxide. The mechanism of nitroxide-mediated CRP is based on the reversible activation of dormant polymer chains (Pn-T) as shown in Scheme 1. This additional reaction step in the free-radical polymerization provides the living character and controls the molecular weight distribution. 

Scheme 1 Reversible activation of dormant polymer chains...

This living mechanism consists in the reversible combination of the growing radical chains, Rn, and the so-called persistent radical species , X (the nitroxide radical group), to form dormant polymer chains, R -X ... 

The dormant polymer is living in the sense that it grows until the monomer is depleted, and that it can grow on after additional monomer feed as in an ionic living polymerization.3 The final degree of polymerization is determined by the initial concentrations of the monomer and of the radical precursor Ro Y, and the formation of block copolymers is possible. 

A series of a-halopropionates (1-21 and 1-22, X = Cl, Br), model compounds of the dormant polymer terminal of acrylates, are suitable for not only acrylates but also styrenes and acrylamides. Ethyl 2-chlo-ropropionate (1-21, X = Cl) was employed for the controlled radical polymerizations of MA and styrene catalyzed by CuCl/L-1 to afford relatively narrow MWDs (MwIMn 1.5).84 A better controlled polymerization of MA is achieved with the bromides 1-21 and 1-22 (X = Br) in conjunction with CuBr/L-1 to give narrower MWDs (MJMn 1.2).84 A similar result was obtained with the combination of 1-23 and CuBr/L-1 for the polymerization of styrene.166 A nickel-based system with Ni-2 and 1-21 (X = Br) gave another controlled polymerization of nBA.134 The iodide compound 1-21 (X = I) is specifically effective in conjunction with an iodide complex such as Re-1 to induce controlled polymerization of styrene.141... 

Side reactions such as termination and transfer were investigated in the polymerizations of styrene,291 acrylates,292 and methacrylates.293 The occurrence of thermally initiated radical polymerizations was observed in the copper-catalyzed styrene polymerization, while the resulting polymer chain can be converted into the dormant polymer terminal via abstraction of halogens from the persistent metal radical in higher oxidation states.294... 

Allyltri-n-butylstannane (EC-2) similarly terminates the copper-catalyzed polymerization of MA to give allyl-functionalized polymers via elimination of the stannyl group accompanying the bromine originated from the dormant polymer terminal.346 Allyl a -end PMMA was obtained also by the copper-catalyzed reaction between allyl bromide (EC-3) and the isolated bromine-capped PMMA, although the functionalization was 57%.226 Another allyl derivative (EC-4) similarly leads to methacrylate-based macromonomers quantitatively in the presence of Cu(0).347... 

Like RAFT, ITP is a degenerative transfer polymerization using alkyl halides [10,11]. ITP was developed in the late 1970s by Tatemoto et al. [226-229]. In ITP, a transfer agent RI reacts with a propagating radical to form the dormant polymer chain P -1. The new radical R can then reinitiate the polymerization. In ITP, the concentration of the polymer chains is indeed equal to the sum of the concentrations of the transfer agent and of the initiator consumed. The newly formed polymer chain P- can then propagate or react with the dormant polymer chain P -1 or R -1 [230]. The mechanism of ITP with alkyl iodide is shown in Scheme 41. 

The presence of dormant polymers does not affect the character of propagation perpetuated by the active polymers. However, the concentration of the active end-groups depends on the equilibrium established between the active and dormant groups, and consequently the observed rate of propagation is affected accordingly. Whenever the lifetime of the dormant polymer is short compared with the time between two consecutive monomer additions, this phenomenon does not affect molecular weight distribution, otherwise it leads to its broadening. 

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