Protonated monomers

The singly protonated monomer [HV04]2 can dimerize, forming [V207]4- by elimination of a water molecule. Two protonated forms of the dimer have been found to exist... 

Some ring-opening polymerizations proceed by a different route called activated monomer (AM) polymerization, which typically involves a cationic or anionic species derived from the monomer. For example, cationic AM polymerization proceeds not with monomer, but with protonated monomer that reacts with the neutral functional end group of the propagating polymer... 

Activated monomer polymerization competes with conventional cationic ROP. Initiation in conventional ROP involves the reaction of protonated monomer with unprotonated monomer (Eq. 7-31a), whereas initiation in AM ROP involves the reaction of protonated monomer with alcohol (Eq. 7-3 lb). 

There is competition between conventional and AM ROPs. Initiation in conventional ROP is first-order each in protonated monomer and unprotonated monomer. AM ROP is first-order each in protonated monomer and alcohol. The ratio of the rates of AM-to-conventional ROP depends on [ROH]/[M] and the ratio of the rate constants for the two reactions. Assuming that the two rate constants are comparable, AM ROP becomes the dominant process at high [ROH] and low [M], Thus, AM ROP is carried out under monomer-starved conditions. The instantaneous monomer concentration is very low, but monomer is continuously added to the reactor at a rate equal to its rate of consumption. 

AM propagation continues in a similar manner with nucleophilic attack hy hydroxyl end groups of polymer on protonated monomer (Eq. 7-32). Cyclic oligomer is minimized hy... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

Propagation follows in a similar manner as a nucleophilic attack by the primary amine end group of a growing polymer chain (XXVII) on protonated monomer to yield XXVIII,... 

If desired, the peaks at m/z 154 and m/z 307for the protonated monomer and dimer of the matrix, 3-nitrobenzyl alcohol, can be eliminated by scanning from m/z 310 to 900. 

The direct reaction between alcohols and epoxides is very slow so alkaline catalysts are employed and are presumed to increase the rate by converting the growing chain to the more nucleophilic alkoxide ion. Polymerizations of this kind have recently been carefully investigated by Gee and coworkers (21). The reactions are still not rapid even at elevated temperatures so the molecular weights obtained in this way are normally quite low (< 10,000). The most rapid reactions of epoxides are those catalysed by acids but even they must be regarded as of the nucleophilic type with acceleration due to the increased reactivity of the protonated monomer,... 

Fig. 18. Overlay of plots from DMS-IMS2 response to several nitrotoluenes [2-mononitrotoluene (2-MNT), 3-MNT, 2,4-dinitrotoluene (2,4-DNT), 3,4-DNT, 2,4,6-trinitrotoluene (TNT)] in positive polarity (left). A reactant ion peak (RIP) is seen at 6.5 V and 1.9 ms, whereas protonated monomers are seen near 3 V and 2.2ms, and proton-bound dimers are at 0V and 3.5ms. TNT does not show a response in positive polarity. Overlay of plots from DMS-IMS2 response to several nitrotoluenes (cf. list in positive polarity) in negative polarity (right). A RIN is seen at 7 V and 1.8 ms, whereas molecular adducts or charge-exchanged ions are seen at 0.5 V and 2.5 ms. MNTs do not show a response in negative polarity. Source Neil D. Paz, NMSU...

Finally, interaction between two intramolecular protonated monomer molecules may result in either or both the 3-(3-aminopropyl)glutar-anhydride or its linear dimer. 

For proton affinity determination, the kinetic method involves the formation of the proton bound heterodimer between the two bases whose affinities are to be compared. By tandem mass spectrometry, the appropriate cluster ion [BiHB2]+ is selected and its spontaneous or collisional dissociation is observed. As shown in Figure 4.16, the competitive dissociation leading to the two protonated monomers is analysed and the relative abundances of the monomers [BiH]+ and [B2H]+ are measured. From these abundances, the relative proton affinities of the two bases Bi and B2 can be calculated and the proton affinity of one of the two bases can be determined, if the proton affinity of the other is known. 

In order to protect the proton, and thereby suppress the kinetically favoured proton transfer route, it has been found out that gas-phase addition followed by elimination can be enhanced by reacting the proton bound dimer of the carbonyl compound rather than the protonated monomer [ 134]. In cases where the carbonyl compound has a higher proton affinity than the nucleophile, proton transfer is of course no problem. Alternatively, if the nucleophile already is protonated, as in the reactions between NH] and various carbonyl compounds, proton catalysed addition/elimination is possible as demonstrated experimentally by observation of immonium ion formation [135-137]. Likewise, the hydrazo-nium ion has been found to react with formaldehyde and a wide range of other aldehydes and ketones [138]. 

Fig. 6a-c. Ion mobility spectra of bradykinin ions at m/z=1061 a the two features seen at 441 K are the singly protonated monomer (M+H)+ and the doubly protonated dimer (2M+2H)+ b at a cell temperature of 463 K the dimer dissociates into two monomer units accounting for the fill-in between the two peaks c at 510 K all of the dimer ions have disappeared upon exiting the cell... 

The monomer 2-(N-carbazolyl)ethyl methacrylate (CzEMA) was first prepared by reacting 9H-carbazole-9-ethanol with methacryloyl chloride, using an excess of triethylamine, in dichloromethane at 0°C. The yields of reaction were not very high (typically 60%) due to the presence of the nitrogen atom on the monomer, which can trap a proton during the reaction. This protonated monomer is eliminated while making several aqueous washes to remove the excess of salts in the product. The purity of the final monomer was demonstrated by H-NMR (Fig. 6). 

The reactivity of both species may be quite different. Because the protonation, at least when it involves proton exchange, is fast, the ratedetermining step in initiation may be the reaction of protonated monomer with the next monomer molecule to form tertiary oxonium ion. 

Sawamoto and Higadiimura have continued their investigation of the stop-flow spectroscopic behaviour of the polymerisation of p-methoxystyrene and added triflic acid to the series of catalysts previously tried. Again, they observed the typical absorption around 380 nm, attributed to the protonated monomer and calculated an initiaticm rate constant at 30 °C in ethylene chloride (assuming a bimolecular interaction orders not determined) kj = 5 x 10 s . This value is expectedly much... 

For the sake of simplicity the protonated monomer molecule is further represented by 11b. (See however Addendum p. 131). The acylation of the monomeric lactam results in the formation of an aminoacyUactam ... 

Taking into account the equOibriunuiature of lactam polymerization, there will be always enough monomer avaUable for protonation. However, the proportion of protonated monomer will decrease with conversion. 

The amidine groups are more t sic than tlw primary amino groups and detsease the ptopoe tion of protonated monoma. In this way, the concentration of the active species (proportional to the concentration of protonated monomer) is reduced thus decreasing the overall rate of polymerization. 

According to the activated monomer mechanism, when the polymerization is performed in the presence of alcohol, an attack from the hydroxyl group to the a-carbon of the oxonium ion is possible due to the high nucleophilicty of this functional group. If this process is predominant, a polymer chain is formed resulting from the successive addition of protonated monomer to the terminal hydroxyl of the growing chain (see Equations 11.81 and 11.82 in Scheme 11.29). 

Spectroscopic (NMR, IR) studies show, that protonation (or coordination with Lewis acids) occurs predominantly at the oxygen atom as depicted in Eq. (11-3)5,6). In the absence of other strong bases the positively charged C-atom in the protonated monomer is attacked by the N-atom of another lactam and primary amino- and N-acyl lactam end-groups are formed (11-3) ... 

Although the reaction product between TfOH and D4 (hydrogen-bonded or proto-nated D4) could not be isolated or detected, it seems reasonable to suggest a simplified scheme that includes the protonated monomer ... 

Figure 10.4 The measured angular distributions of scattered ion masses resulting from the electron impact ionization of the scattered neutral methanol clusters. The onset of the H loss fragment ion from the methanol monomer is shown at the limiting angle for the monomer (18.2°). Similarly, the dimer onset, producing the protonated monomer is shown at 9.2°, while the protonated dimer peak produced by reaction (1) is evident with a weak onset at 6.1°. Taken with permission from Buck et al. (1990).

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