Abnormal groups

Although it is not part of the biosynthesis pathway, stricto sensu, it is worth mentioning that after termination the linear isoprene chain may undergo further chemical modifications. Indeed, abnormal groups such as aldehyde, epoxy or lactone have been detected on the polymer chain. 

B. Sekhar, Abnormal groups in rubber and microgel, Proc. 4th Rubber Technol. Conf., London, 1960. 

Identification of the abnormal groups in a polymer may not be sufficient it is often necessary also to determine the numbers of such groups for, say, 1000 normal units. NMR techniques can give the required information but precautions are needed if reliable results are to be obtained. The problems can be illustrated by considering the use of NMR to compare the number... 

It should be mentioned that the ratio between the intensities of signals from abnormal groups derived from a reagent, such as an initiator, and... 

When these compounds are chemically bonded to the rubber molecules, they are collectively known as abnormal groups. Based on the biosynthesis mechanism of other naturally occurring cw-polyisoprenes, there are at least two abnormal groups on the main-chain rubber molecule, namely the co-terminal i.e. initiating end) and a-terminal (terminating end) groups. 

Table 3.10 Abnormal groups of natural rubber reported in the literature.

Abnormal groups Fraction Cone. (mmollkg) Method Reference... 

The analysis of fractionated natural rubbers indicated that these abnormal groups were concentrated in the high molecular weight fractions. ... 

NR molecule chain representing scan abnormal group... 

Figure 19.15 Possible interaction via hydrogen bonding between the carbonyl group of PET with abnormal groups such as hydroxyl function in NR.

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