The sequence structure of a polymer chain refers to the bonding sequence of structural units or monomer units in a polymer chain. Different polymer chains have different sequence structures.

Sequence of a homopolymer

Take homopolymer polypropylene as an example: the *C atoms in its main chain are asymmetric. The molecular chain of polypropylene is completely in the trans conformation (R represents CH3). Assuming that R or H is above the plane of the paper (indicated by a black line) it is a D configuration, and R or H is below the plane of the paper (indicated by a dotted line) it is an L configuration. When two adjacent structural units are both in L or D configuration, it is called m (meso, internal) bonding, abbreviated as m; when L and D configurations are adjacent, it is called r (racemic, racemic) bonding, abbreviated as r. Polypropylene molecular chains that are all connected by mmmm... are called isotactic, those that are all connected by rrrr... are called syndiotactic, and those that are randomly connected by m and r on the molecular chain are called atactic. In fact, polypropylene cannot be 100% isotactic or syndiotactic. There are always a small amount of stereo defects. In the so-called atactic polypropylene chain, the distribution of m and r is not completely subject to Bernoulli's random distribution.

Figure 1 is the spectrum of the CH3 part of 13C-NMR (nuclear magnetic resonance spectrum) of atactic polypropylene. At least 10 peaks can be observed, which correspond to 10 pentads respectively. The area of each peak is equivalent to the relative content. From this, the arrangement of m and r on the molecular chain can be quantitatively observed, and the parameters related to the sequence distribution such as the content, average length and number of intersections of m and r can be deduced. In addition, the normal connection mode of polypropylene structural units is head-to-tail connection, such as: It may also contain a small amount of abnormal structures such as tail-to-tail or head-to-head connection, such as: The sequence structure is closely related to the performance of the polymer. High isotactic polypropylene can crystallize and has excellent mechanical properties. It is an important plastic and fiber material. Atactic polypropylene cannot crystallize and is a waxy semi-solid at room temperature.

Polymer sequence

Take butadiene as an example (diene polymer), the monomer units can be bonded in three ways, namely cis- and trans-1,4-bonding and 1,2-bonding. Polybutadiene with different sequence structures can be obtained by changing the polymerization conditions. Figure 2 is a 13C-NMR spectrum of a high cis-1,4-polybutadiene sample, and the attribution of each peak has been indicated on the figure. It can be seen that in addition to a large number of cis structures, there are also a small amount of trans-1,4 bonding (resonance absorption overlaps on the d peak) and 1,2 bonding structures, the latter of which are isolated and connected between cis-1,4 bonding structures.

Copolymer sequence structure

The sequence of a binary copolymer refers to the arrangement of the two structural units A and B in the polymer chain. According to copolymerization theory and mathematical models, the sequence distribution of A and B units in the molecular chain, the content of A and B, the average number of units and the number of intersections can be derived. NMR spectra are often used to judge the reliability of these derived data. The areas of the three regions a, b, and c in Figure 3 can be used to obtain the concentrations of AA, AB, BA, and BB diads. The peaks a1, a2, a3 and b1, b2, b3, and b4 are equivalent to the concentrations of various tetrads, which can verify the copolymerization theory. If the A and B units contain asymmetric carbon atoms, there are also stereo configuration issues similar to homopolymers.

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