648:
termination step, but is in this case reversible. Because of the high rate of coupling of the nitroxide to the growing chain end, there is little coupling of two active growing chains, which would be an irreversible terminating step limiting the chain length. The nitroxide binds and unbinds to the growing chain, protecting it from termination steps. This ensures that any available monomer can be easily scavenged by active chains. Because this polymerization process does not naturally self-terminate, this polymerization process is described as “living,” as the chains continue to grow under suitable reaction conditions whenever there is reactive monomer to “feed” them. Because of the PRE, it can be assumed that at any given time, almost all of the growing chains are “capped” by a mediating nitroxide, meaning that they dissociate and grow at very similar rates, creating a largely uniform chain length and structure.
644:(PRE). The PRE is a phenomenon observable in some radical systems which leads to the highly favored formation of one product to the near exclusion of other radical couplings due to one of the radical species being particularly stable, existing in greater and greater concentrations as the reaction progresses while the other one is transient, reacting quickly with either itself in a termination step or with the persistent radical to form a desired product. As time goes on, a higher concentration of the persistent radical is present, which couples reversibly with itself, meaning that any of the transient radical still present tends to couple with the persistent radical rather than itself due to greater availability. This leads to a greater proportion of cross-coupling than self-coupling in radical species.
682:
solvent lends itself better to C-O homolysis, so polar solvents which cannot bind to a labile nitroxide are the most effective for NMP. It is generally agreed that the structural factor that has the greatest effect on the ability of a nitroxide to mediate a radical polymerization is steric bulk. Generally speaking, greater steric bulk on the nitroxide leads to greater strain on the alkoxyamine, leading to the most easily broken bond, the C-O single bond, cleaving homolytically.
26:
673:
groups contribute stability, but only if there is no resonance provided by allyl or aromatic groups α to the N. These result in decreased stability of the nitroxide, presumably because they offer less sterically hindered sites for radical coupling to take place. The resulting inactivity of the radical makes hemolytic cleavage of the alkoxyamine quite fast in more sterically hindered species.
796:. This generates a carbon-centered radical which couples with the nitroxide to generate the desired alkoxyamine. This method has the disadvantage of being relatively inefficient for some species, as well as the inherent danger of having to work with extremely toxic hydrazine and the inconvenience of having to run reactions in inert atmosphere.
715:
under most conditions. TEMPO derivatives with even bulkier groups at the positions α to N have a rate of homolysis great enough to induce NMP of butyl acrylate, and the bulkier the α groups, the faster polymerization occurs. This indicates that the steric bulk of the nitroxide fragment can be a good
690:
In the case of cyclic nitroxides, five-membered ring systems have been shown to cleave more slowly than six-membered rings and acyclic nitroxides with t-butyl moieties as their R groups cleaved fastest of all. This difference in the rate of cleavage was determined to result not from a difference in
672:
stability, nitroxides used in NMRP always contain bulky, sterically hindering groups in the R1 and R2 positions. The significant steric bulk of these substituents entirely prevents radical coupling in the N-centered resonance form while significantly reducing it in the O-centered form. These bulky
681:
The choice of a specific nitroxide species to use has a large effect on the efficacy of an attempted polymerization. An effective polymerization (fast rate of chain growth, consistent chain length) results from a nitroxide with a fast C-O homolysis and relatively few side reactions. A more polar
724:
Because TEMPO, which is commercially available, is a sufficient nitroxide mediator for the synthesis of polystyrene derivatives, the preparation of alkoxyamine initiators for NMP of copolymers is in many cases a matter of attaching a nitroxide group (TEMPO) to a specifically synthesized alkyl
647:
In the case of a nitroxide-mediated polymerization reaction, the persistent radical is the nitroxide species and the transient radical is always the carbon radical. This leads to repeated coupling of the nitroxide to the growing end of the polymer chain, which would ordinarily be considered a
716:
indicator of the strength of an alkoxyamine initiator, at least up to a point. The equilibrium of its homolysis and reformation favors the radical form to the extent that recombination to reform an alkoxyamine over the course of NMP occurs too slowly to maintain control of chain length.
667:
is a result of their unique structure. In most diagrams, the radical is depicted on the oxygen, but another resonance structure exists which is more helpful in explaining their stability in which the radical is on the nitrogen, which has a double bond to the oxygen. In addition to this
691:
C-O bond lengths, but in the difference of C-O-N bond angle in the alkoxyamine. The smaller the bond angle the greater the steric interaction between the nitroxide and the alkyl fragment and the more easily the initiator species broke apart.
627:
for polymerization to occur successfully. NMP allows for excellent control of chain length and structure, as well as a relative lack of true termination that allows polymerization to continue as long as there is available
575:
844:. This technique has the advantage of requiring only the appropriate alkyl bromide to be synthesized without requiring inconvenient reaction conditions and extremely hazardous reagents like Braso et al.’s method.
816:
forming formic acid and the desired alkyl radical, which couples with tempo to produce the target alkoxyamine. The reaction appears to give fairly good yields and tolerates a variety of
663:
are effective mediators of well-controlled radical polymerization because they are quite stable, allowing them to act as persistent radicals in a reaction mixture. This
756:
proceeds by a radical addition mechanism, which can be taken advantage of by introducing the radical TEMPO group into the reaction mixture. After treatment with a mild
568:
574:
539:
77:
72:
82:
982:
304:
119:
166:
768:
addition of nitroxide to the alkene. Jacobsen's catalyst is fairly mild, and a wide variety of functionalities on the alkene
532:
289:
87:
319:
114:
109:
699:
The efficiency of polymerization increases more and more with increased steric bulk of the nitroxide up to a point.
314:
143:
611:
of the C-O bond can occur, yielding a stable radical in the form of a 2-center 3-electron N-O system and a carbon
67:
641:
525:
615:
which serves as an initiator for radical polymerization. For the purposes of NMP, the R groups attached to the
282:
904:
Volodarsky, L.B., Reznikov, V.A., Ovcharenko, V.I. Synthetic
Chemistry of Stable Nitroxides. CRC Press, 1994.
784:
An alternative method is to react a substrate with a C-Br bond at the desired location of the nitroxide with
591:(NMP) are a family of compounds referred to as alkoxyamines. An alkoxyamine can essentially be viewed as an
384:
379:
153:
829:
733:
588:
265:
769:
608:
479:
299:
294:
765:
789:
148:
42:
788:, generating an alkyl substituted hydrazine which is then exposed to a nitroxide radical and a mild
660:
612:
592:
489:
57:
813:
761:
664:
452:
435:
394:
218:
812:, which adds to the carbonyl group. The resulting species rearranges in situ in the presence of
817:
809:
773:
445:
409:
228:
223:
836:
to yield a nucleophilic nitroxide. The nitroxide nucleophile is then added to an appropriate
604:
399:
369:
309:
124:
745:
560:
472:
414:
364:
191:
757:
712:
620:
506:
359:
102:
25:
711:
derivatives fairly easily, but is not sufficiently labile to induce polymerization of
976:
837:
404:
213:
208:
186:
895:
Hawker, C.J., Barclay, G.G., Dao, J. J. Am. Chem. Soc., 1996, 118 (46), 11467–11471.
841:
793:
496:
440:
389:
374:
198:
181:
704:
511:
457:
245:
233:
600:
564:
354:
334:
176:
725:
fragment. Several methods have been reported to achieve this transformation.
805:
785:
737:
669:
657:
556:
329:
260:
250:
203:
47:
741:
624:
616:
484:
462:
324:
623:
and the R group in the O- position forms a stable radical, generally is
753:
749:
708:
629:
62:
52:
931:
Siegenthaler, K.O., Studer, A. Macromolecules, 2006, 39(4), 1347–1352.
776:
are not necessarily as high as those reported by Dao et al., however.
833:
703:((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) is capable of inducing the
501:
940:
Dao, J., Benoit, D., Hawker, C.J.J. Poly. Sci., 1998, 36, 2161–2167.
578:
A chain growth step in a nitroxide mediated polymerization process.
804:
Yet another published alkoxyamine synthesis involves treatment of
700:
596:
828:
A synthesis has been described by Moon and Kang consisting of a
467:
255:
129:
573:
24:
967:
Moon, B., Minjyuk, K. Macromol. Res., 2005, 13(3), 229–235.
922:
Moad, G., Rizzardo, E. Macromolecules, 1995, 28, 8722–8728.
868:
Moad, G., Rizzardo, E. Macromolecules, 1995, 28, 8722–8728.
555:
is a method of radical polymerization that makes use of an
958:
Schoening, K.U., et al. J. Org. Chem. 2009, 74, 1567–1573.
949:
Braslo, R., et al. Macromolecules, 1997, 30, 6445–6450.
913:
Bertin, D., et al. Chem. Soc. Rev., 2011, 40, 2189–2198
877:
Bertin, D., et al. Chem. Soc. Rev., 2011, 40, 2189–2198
859:
Nicolas, J., et al. Prog. Polym. Sci., 2013, 38, 63–235
886:
Fischer, Hanns. Chem. Rev., 2001, 101 (12), 3581–3610.
559:
initiator to generate polymers with well controlled
824:Electrophilic bromination and nucleophilic attack
587:The initiating materials for nitroxide-mediated
569:reversible-deactivation radical polymerization
800:Treatment of aldehydes with hydrogen peroxide
632:. Because of this it is said to be “living".
533:
8:
540:
526:
15:
553:Nitroxide-mediated radical polymerization
135:Nitroxide-mediated radical polymerization
852:
840:, yielding the alkoxyamine by a simple
640:The living nature of NMP is due to the
18:
7:
832:of a nitroxide radical in metallic
607:is that under certain conditions,
14:
120:Controlled radical polymerization
764:, this yields the product of a
1:
83:Flory–Huggins solution theory
772:can be tolerated. Practical
621:sterically hindering groups
149:Condensation polymerization
115:Free-radical polymerization
110:Chain-growth polymerization
999:
144:Step-growth polymerization
705:polymerization of styrene
642:persistent radical effect
636:Persistent radical effect
983:Polymerization reactions
728:
154:Addition polymerization
88:Coil–globule transition
830:one-electron reduction
744:commonly used for the
603:. The utility of this
589:radical polymerization
583:Alkoxyamine Initiators
579:
266:Self-healing hydrogels
29:
595:bound to a secondary
577:
480:Cookware and bakeware
432:Industrial production
300:X-ray crystallography
28:
820:in the alkyl chain.
734:Jacobsen's catalyst
729:Jacobsen's catalyst
720:Preparation methods
652:Nitroxide stability
453:Protective Coatings
68:Mark–Houwink theory
762:sodium borohydride
619:are always bulky,
580:
567:. It is a type of
30:
818:functional groups
810:hydrogen peroxide
656:As stated above,
550:
549:
463:Consumer products
990:
968:
965:
959:
956:
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947:
941:
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905:
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869:
866:
860:
857:
677:Nitroxide choice
605:functional group
542:
535:
528:
446:Applied coatings
283:Characterization
16:
998:
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867:
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826:
802:
790:oxidating agent
782:
748:epoxidation of
746:stereoselective
731:
722:
697:
688:
679:
654:
638:
585:
563:and a very low
561:stereochemistry
546:
517:
516:
428:
420:
419:
350:
342:
341:
285:
275:
274:
192:Polyisobutylene
173:Functional type
169:
159:
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105:
95:
94:
38:
19:Polymer science
12:
11:
5:
996:
994:
986:
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849:
846:
825:
822:
801:
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781:
778:
758:reducing agent
730:
727:
721:
718:
713:butyl acrylate
696:
693:
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684:
678:
675:
653:
650:
637:
634:
584:
581:
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530:
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507:Plastic bottle
504:
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490:Food Container
487:
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214:Vinyl polymers
211:
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167:Classification
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63:Phase behavior
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839:
838:alkyl bromide
835:
831:
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209:Polycarbonate
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187:Polypropylene
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34:
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27:
23:
22:
17:
963:
954:
945:
936:
927:
918:
909:
900:
891:
882:
873:
864:
855:
842:SN2 reaction
827:
803:
794:lead dioxide
783:
732:
723:
698:
689:
680:
655:
646:
639:
586:
552:
551:
497:Vinyl record
441:Blow molding
427:Applications
199:Polyurethane
182:Polyethylene
134:
43:Architecture
766:Markovnikov
754:epoxidation
695:Steric bulk
601:single bond
512:Plastic bag
458:3D printing
246:Homopolymer
234:Polystyrene
58:Degradation
848:References
599:by an N-O
565:dispersity
473:Whitewalls
395:Staudinger
365:MacDiarmid
349:Scientists
335:Viscometry
177:Polyolefin
53:Morphology
37:Properties
806:aldehydes
786:hydrazine
780:Hydrazine
770:substrate
738:manganese
686:Ring size
670:resonance
665:stability
658:nitroxide
609:homolysis
557:nitroxide
436:Extrusion
415:Braconnot
405:Baekeland
385:de Gennes
370:Shirakawa
330:Rheometry
261:Hydrogels
251:Copolymer
242:Structure
204:Polyester
103:Synthesis
48:Tacticity
977:Category
792:such as
760:such as
742:catalyst
661:radicals
625:benzylic
617:nitrogen
485:Bakelite
400:Goodyear
325:Rheology
752:. This
750:alkenes
740:-based
709:styrene
630:monomer
613:radical
593:alcohol
410:Hayward
390:Ziegler
380:Edwards
834:sodium
774:yields
502:Kevlar
360:Heeger
808:with
736:is a
701:TEMPO
597:amine
468:Tires
375:Natta
355:Flory
814:CuCl
707:and
295:FTIR
256:Gels
229:PVAc
130:RAFT
125:ATRP
78:LCST
73:UCST
320:DMA
315:TGA
310:NMR
305:DSC
290:GPC
224:PVA
219:PVC
979::
571:.
541:e
534:t
527:v
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