Constitutional repeat units

Table 4. CAS Registry Numbers for Constitutional Repeat Unit...

Mention should be made of the nomenclature for the polymer. Industrially the materially is invariably known in the English-speaking world as polypropylene. However, the lUPAC name for the monomer is propene and until 1975 the recommended lUPAC name was polypropene, a term very rarely used. The latest lUPAC rules base the name of a polymer on the constitutional repeating unit, which in this case is a propylene unit (c.f. a methylene unit for polyethylene) and this leads to the name poly(propylene) (i.e. with brackets). In this volume the more common, unbracketed but still unambiguous name will be used. 

It is well known, for instance, the case of isotactic polybutene (i-PB), in which the three known crystalline forms (referred to as I, II, and III) contain helices with a number of constitutional repeating units per turn in the range 3-4 (3/1, 11/3, 4/1 helices, respectively) all corresponding to regular sequences of nearly trans and nearly gauche dihedral angles [3,21]. 

Constitutional repeating unit in a regular macromolecule, a regular oligomer molecule, a regular block, or a regular chain, the configuration of which is defined at least at one site of stereoisomerism in the main chain. 

Note In a regular polymer, a eonfigurational base unit eorresponds to the constitutional repeating unit. 

For the polymer -[-CH(COOR)CH(CH3)-hp, if only the ester-bearing main-chain site in each constitutional repeating unit has defined stereochemistry, the configurational repeating unit is (7) and the corresponding isotactic polymer is (8). 

Note As the definition above indicates, a regular polymer, the configurational base units of which contain one site of stereoisomerism only, is atactic if it has equal numbers of the possible types of configurational base units arranged in a random distribution. If the constitutional repeating unit contains more than one site of stereoisomerism, the polymer may be atactic with respect to only one type of site if there are equal numbers of the possible configurations of that site arranged in a random distribution. 

The configurational sequence and stereosequence coincide in this particular case because there is only one site of stereoisomerism in each constitutional repeating unit (compare Definitions 2.1.3 and 2.1.4). 

In these two cases, the constitutional unit -CH(CH3)-CH2- can be called the constitutional repeating unit of the homosequence. 

For a regular polymer containing double bonds in the main chain of the constitutional repeating units, these are the fractions of such double bonds that are in the cis and trans configurations, respectively. 

Statistical co-crystallization of different constitutional repeating units, which may either belong to the same copolymer chains (copolymer isomorphism) or originate from different homopolymer chains (homopolymer isomorphism). 

The formation of a systematic name for a polymer requires the identification and naming of a preferred constitutional repeating unit (CRU). This basic name is then modified by prefixes, which convey precisely the structural identity of the polymer in question. Such names are referred to as structure-based names. However, polymers can also be named as being derived from a monomer (or precursors), named according to lUPAC rules. Such names are referred to as source-based names. Over the years, rules for determining polymer nomenclature under these two systems have developed in parallel. An example of the modification of the lUPAC name of an organic molecule to lUPAC structure-based and source-based names of a polymer is illustrated below. 

The principal deficiency of source-based nomenclature is that the chemical structure of the monomeric unit in a polymer is not identical with that of the monomer, e.g., -CHX-CH2- versus CHX=CH2. The structure of the constitutional repeating unit (CRU) may also not be clearly identified in this scheme for example, the name polyacrylaldehyde does not indicate whether (i) the vinyl group or (ii) the aldehyde group was the locus of polymerization. 

Nevertheless, it is useful to think of the macromolecules of a polymer as being represented by a single structure that may itself be hypothetical. To the extent that the structure can be portrayed as a chain of structural repeating units (SRUs) or constitutional repeating units (CRUs) (the terms are synonymous), the structure can be named by the rules in this report in addition, provision has been made for including end-groups in the name. 

The largest main-chain (backbone) segment of the constitutional repeating unit that can be named as a single unit under organic nomenclature rules [4, 5], This may be a ring or ring system, a heteroatom or a heteroatom chain or an acyclic carbon chain. 

This nomenclature method rests upon the selection of a preferred constitutional repeating unit (CRU) of which the polymer molecule is a multiple. Wherever possible, the CRU and subunits are named according to the lUPAC-recommended nomenclature of organic chemistry [4, 5]. 

Write the structure of the polymer chain. A sufficient portion of the chain should be written to show structure repetition. The portion that repeats is a constitutional repeating unit (CRU). 

In 1976, the lUPAC Commission on Macromolecular Nomenclature published rules on the nomenclature of regular single-strand organic polymers. These were later revised [1]. A regular single-strand polymer is one which can be described by constitutional repeating units with only two terminals composed of one atom each. 

Note Partial ladder (imperfect ladder, block ladder) polymers [5], in which the sequence of rings is interrupted and a divalent constitutional repeating unit can be identified, are not double-strand polymers. They are named as single-strand polymers. 

This nomenclature rests upon the selection of a preferred constitutional repeating unit (CRU) [1, 4] of which the polymer is a multiple the name of the polymer is the name of this repeating unit prefixed by poly . The unit itself is named wherever possible according to the established principles of organic nomenclature [7]. For double-strand polymers, this unit usually is a tetravalent group denoting attachment to four atoms. Since some of these attachments may be double bonds, the unit may be hexavalent or octavalent. 

Examples given in Section 2 above have illustrated the repeating nature of the constitutional units within each polymer structure. Some of the ways to break the same structures into constitutional repeating units are illustrated below. 

To allow construction of a unique name, a single preferred constitutional repeating unit must be selected. 

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