Polymerization diradical

Several studies have demonstrated the successful incoriDoration of [60]fullerene into polymeric stmctures by following two general concepts (i) in-chain addition, so called pearl necklace type polymers or (ii) on-chain addition pendant polymers. Pendant copolymers emerge predominantly from the controlled mono- and multiple functionalization of the fullerene core with different amine-, azide-, ethylene propylene terjDolymer, polystyrene, poly(oxyethylene) and poly(oxypropylene) precursors [63,64,65,66,62 and 66]. On the other hand, (-CggPd-) polymers of the pearl necklace type were fonned via the periodic linkage of [60]fullerene and Pd monomer units after their initial reaction with thep-xy y ene diradical [69,70 and 71]. 

Thiirene intermediates in the photolysis of 1,2,3-thiadiazoIes readily undergo ring opening to a diradical which can be trapped by reaction with an alkyne (Scheme 25) (79CB1769). Significant polymerization is not observed. 

The polymerization is photosensitive, involves diradicals and leads to chain lengths that exceed 200000 S atoms at -I80"C before dropping slowly to "-1000 S at 400" and -100 S at 600". 

Polymeric Sn is dark yellow with an absorption edge at 350 nm (cf. p. 683) but the colour is often obscured either by the presence of trace organic impurities or, in pure S, by the presence of other highly coloured species such as the dark cherry-red trimer S3 or the deeper-colouied diradicals S4 and S5. 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

Dimer and trimer byproducts have been isolated from MMA polymerizations and these are suggestive of 1,4-diradical intermediates.323 28 Lingnau and Mcycrhoff523 found that rates of spontaneous polymerization of MMA were substantially higher in the presence of transfer agents (RH). They were able to isolate the compound (98) that might come from trapping of the biradical intermediate (Scheme 3.65). 

Hall112,329 has proposed a unifying concept based on tetramethylen.es (resonance hybrids of 1,4-diradical and zwitterionic limiting structures - Scheme 3.67) to rationalize all donor-acceptor polymerizations. The predominant character of the tetramethylenes (zwitterionic or diradical) depends on the nature of the substituents. 12" 40 However, more evidence is required to prove the more global application of the mechanism. 

In eq. 8, the rate of polymerization is shown as being half order in initiator (T). This is only true for initiators that decompose to two radicals both of which begin chains. The form of this term depends on the particular initiator and the initiation mechanism. The equation takes a slightly different form in the case of thermal initiation (S), redox initiation, diradical initiation, etc. Side reactions also cause a departure from ideal behavior. 

Using the first-principles molecular-dynamics simulation, Munejiri, Shimojo and Hoshino studied the structure of liquid sulfur at 400 K, below the polymerization temperature [79]. They found that some of the Ss ring molecules homolytically open up on excitation of one electron from the HOMO to the LUMO. The chain-like diradicals S " thus generated partly recombine intramolecularly with formation of a branched Sy=S species rather than cyclo-Ss- Furthermore, the authors showed that photo-induced polymerization occurs in liquid sulfur when the Ss chains or Sy=S species are close to each other at their end. The mechanism of polymerization of sulfur remains a challenging problem for further theoretical work. 

In the search for a plausible mechanism for initiation in thermal polymerization, it is necessary to reject unimolecular processes such as the opening of the double bond to form a monomeric diradical... 

Thermal initiation makes an appreciable contribution to the polymerization rate for styrene at very low initiator concentrations, as we have pointed out earlier. Since the rate Rp includes contributions from thermal as well as from catalytic initiation, the second term in Eq. (36) remains valid provided the thermal initiation involves monoradicals. Diradical initiation, if it occurred, would introduce a deviation, since it produces no chain ends. 

When the polymerization of St was carried out with 51 under conditions identical to those in Fig. 3, i.e., [7]/4=[8]/2=51=2X 10-3 mol/1, the formation of benzene-insoluble polymers was observed from the initial stage of the polymerization. Although 7 and 8 induced living radical mono and diradical polymerization similar to that previously mentioned, benzene-insoluble polymers were formed in the polymerization with 51, and the molecular weight of the soluble polymers separated decreased with the reaction time. This suggests that a part of the propagating polymer radicals underwent ordinary bimolecular termination by recombination, leading to the formation of the cross-linked polymer, which was prevented by the addition of 13. 

But the comparable reaction of organic radicals with olefins does not necessarily mean that the reacting olefins are first excited to a diradical state. In fact, as will be discussed later, there is some reason to believe that diradicals are not involved in the purely thermal polymerization of olefins either. 

Diradicals have been postulated as the growing entity in the thermally or photochemically initiated polymerization of olefins. 

Although a patent claims the synthesis of the quinone shown below, the method is ambiguous and it is doubtful that a substance of this precise structure has been prepared.108 It should exist as a polymeric peroxide or as a diradical. 

Because of the weight of 43, the system 41/43 undergoes an easier rotation about its formally Ge = C double bond than about its formally C-C single bond. The diradical character of 41 should favor any addition with radical intermediates and polymerization reactions.45... 

The linear prodnct is obvionsly formed according to the seqnence 2,5-di(thiocyano)thio-phene potassinm 2-mercaptido-5-thiacyanonothiophene —> tristhiomaleic anhydride thio-phene-2,5-disulfenyl biradical (the diradical valence tantomer) —> the depicted (Scheme 2.18) linear polymer in which the thiophene rings are connected by dusulfide bridges. It was recently confirmed that tristhiomaleic anhydride is nnstable and polymerizes just at the moment of its formation (see Paulssen et al. 2000, Ref. 15 therein). 

Wunderlich, 1975 Surendran et al., 1987]. The reaction is carried out by pyrolyzing/7-xylene at temperatures of up to 950°C (in the absence of oxygen) to yield di-/7-xylylene (LIU) (also named [2,2]-paracyclophane), which is isolated as a stable solid. Subsequent polymerization of di-/ -xylylene to poly(/ -xylylene) involves vaporization of di-/ -xylylene by heating to about 160°C at 1 torr (1.3 x 102 Pa) followed by cleavage to /7-xylylene diradical (LIV) by heating at 680°C at 0.5 torr. The diradical nature of /7-xylylene was shown by ESR and... 

Polymerization is initiated by coupling of two p-xylylene radicals to form a diradical (LV), which then grows by the addition of p-xylylene at both of its radical centers ... 

One aspect of the polymerization that is well established is the initiation step when di-p-xylylene is pyrolyzed. An alternate initiation mode involving the direct formation of the diradical LV from LIII by cleavage of only one of the two CH2—CH2 bonds is ruled out from experiments with monosubstituted di-p-xylylenes. When acetyl-di-p-xylylene is pyrolyzed and the pyrolysis vapor led through successive condensation surfaces at temperatures of 90 and 25°C, respectively, the result is the formation of two different polymers neither of which is poly(acetyl-di-p-xylylene). Pyrolysis yields acetyl-p-xylylene and p-xylylene... 

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