MHRs

Mixed hydrocarbon residues (MHRs) are obtained by reevaluation of some unsaturated fractions with a wide compositional spectrum (aliphatic or aromatic) [135-137]. Introduction of aliphatic structures along with the aromatic ones in HRs generally has a favorable effect on the products obtained [138-140]. 

In obtaining MHRs one can also produce products with higher compatibility than various other materials, or with an increased capacity of reaction for chemical modifications. 

In general MHRs are obtained by radical oligomerization initiated thermally or with organic peroxides and naphthenates or metal stearates [ 141 ], or by cationic oligomerization initiated with Lewis acids [142]. 

HRs of all types can be chemically modified by various reactions—either polymer-homologous transformations or copolymerization processes (grafted or block). Except for the hydrogenation, these reactions transform HRs from neutral, nonpolar materials to polar, basic or acid products capable of participating in various subsequent reactions. These modifications determine essential changes of HR physical and chemical properties. These resins become water soluble, thermoreactive, and compatible with polar materials. 

There are various types of modification reactions. Carboxyl groups introduction, imidization, hydrogenation, oxidation, and epoxidation are (only) some examples. Also known is modification by simple mixing, without chemical reactions. This latter process is characteristic of HR applications, not HR synthesis. 

---

MHR = maximum stiffness during cure minus minimum stiffness, both in a rheometer. To convert /cm to Ibfin., divide by 0.0175. Too = Scorch time. 

The mononuclear heterocyclic rearrangement (MHR) of isoxazole-3-amidoxime 108 in the presence of a base and hydroxylamine with concomitant removal of the amide moiety affords furazan acetaldoxime 109 (Scheme 56) (91CHE651, 91KGS827). 

An elegant synthesis of a wide range of carbonyl derivatives of furazan from di-3,3 -isoxazolyl ketone with MHR as key step was developed in 1946 by Quil-ico and Freri (46G3). The intermediate acetaldoxime derivative was produced in 85% yield. It was converted regioselectively into the functionalized acetylfurazans (Scheme 57). 

Nitroethylfurazan 143 has been synthesized by base-promoted mononuclear heterocyclic rearrangement (MHR) of oxime 142 (Scheme 82) (36G819,82G181). 

MHR of dioxime 144, derived from dibenzoylfuroxan, with an acid catalyst afforded furazanylfuroxan 145 (Scheme 83) (33G159). 

The thermal recyclization of 4,4 -bis(acetamido)-3,3 -azofuroxan 195 to (nitrotriazol-2-yl) furoxan 196 has been shown to involve two consecutive MHR (99MC17) (Scheme 128). Intramolecular nucleophilic attack initiated the process. 

Comparison of the MHR with non-modified pectic hairy regions of apple cell wall, isolated in a mild and defined way, revealed great resemblance indicating that the modifications of the MHR during enzymic liquefaction were only minor. Analogous MHR fractions could be isolated from potato fibre, pear, carrot, leek, and onion tissue. 

Finally, an adapted model is presented for the prevailing population of apple MHR having the highest molecular weight. The universal validity of this model for pectic hairy regions from other plant sources is discussed. 

Structures of identified oligomers, obtained after degradation of apple MHR-S by RGase. 

Figure 1. Elution profile on HPAEC of apple MHR-S after treatment with RGase at 30°C and pH 5.0 for 24h [37],...

RGase was only active on MHR after removal of the acetyl groups either chemically or enzymatically. This indicates that these acetyl groups are positioned in the alternating rhamnose-galacturonic acid sequences from which the oligomers are released. 

Analysis of the MHR by high-performance size-exclusion chromatography (HPSEC) revealed three distinct populations to be present, which were isolated on a preparative scale. The composition of the populations differed mainly in the relative proportion of rhamnose, xylose and the amount of methyl esters and acetyl groups, although the general characteristics were rather similar (Table 2). However, degradation studies with RGase showed that, in contrast... 

Sugar composition (mol%) of the fractions of MHR isolated by chromatography over Sephacryl S200 and S500 [34],... 

Figure 2 - Size-exclusion chromatography on Sephacryl S200 of MHR population A after degradation with RGase ------------, uronic acids —, neutral sugars [39].

Sugar composition (mol%) of MHR population A and fractions I-IV, obtained after Sephacryl S200 size-exclusion chromatography of the digest of MHR population A with RGase [39]. 

Table 4. Sugar composition (mol%) of the xylogalacturonan fractions obtained after DEAE Sepharose anion-exchange chromatography of the Sephacryl S300 fractions I originating from RGase degraded saponified apple MHR population A (1-a and 1-Z>) and from RGase/RGAEase treated non-saponified apple MHR population A (2-a, 2-b, and 2-c).

---