Functionality-type Distributions

Due to the fact that different end groups can be formed during the polycondensation, the reaction products may exhibit a functionality-type distribution in addition to the molar mass distribution. Although SEC is suitable to analyze the molar mass distribution, it does not yield information on different end groups. For the determination of the functionality-type distribution, other types of liquid chromatography must be used. 

Other structures like cyclic oligomers can also be formed. For the analysis of the functionality-type distribution of the reaction products, different types of interaction chromatography can be used. 

The problem of molecular weight distribution (MWD) and functionality type distribution (FTD) belongs by definition to an extensive problem of molecular heterogeneity of polymers. In the synthesis of a polymer with the requested properties, e.g. a telechelic polymer, one is always faced with different types of polydispersity the macromolecules can be of different length, they can have a different number of functional groups, i.e. be mono-, bifunctional, etc., they can be branched (star-, comb- or tree-like) and, finally, they can be cyclic. 

In Section 2, the concepts of functionality in the case of macromolecules and of the functionality type distribution are described in greater detail, and some examples are given of early FTD determinations using different chromatographic techniques. 

A vast majority of oligomers with functional groups, used as initial products in various polymerization processes, have not only a molecular weight but also a functionality type distribution. The term FTD — the relative content of macromolecules of different functionality — was for the first time introduced by Entelis1. It is imperative that an efficient synthesis of oligomers with the target functionality should be accompanied by a control of their FTD. 

Fig. 1. Molecular weight distribution (MWD), and functionality type distribution (FTD) of a telechelic oligomer...

To exclude this ambiguity and characterize the width of functionality type distribution it is possible to use, by analogy with average molecular weights, the values of number-average and weight-average, fw, functionality ... 

Let us now examine the application of the above concepts for analyzing the functionality type distribution of some telechelic polymers. 

Let us now formulate the general scheme of choosing the chromatographic system and the separation conditions for the analysis of functionality type distribution. 

Entelis, S. G., Evreinov, V. V., Kuzaev, A. A. Reactive Oligomers. Functionality Type Distribution, in Uspekhi khimii i fiziki polimerov (Progress in Polymer Chemistry and Physics) (ed.) Rogovin, Z. A., p. 201, Moscow, Khimiya, 1973... 

The structural complexity of synthetic polymers can be described using the concept of molecular heterogeneity (see Fig. 1) meaning the different aspects of molar mass distribution (MMD), distribution in chemical composition (CCD), functionality type distribution (FTD) and molecular architecture distribution (MAD). They can be superimposed one on another, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- or bifunctional, copolymers can be block or graft copolymers, etc. In order to characterize complex polymers it is necessary to know the molar mass distribution within each type of heterogeneity. 

Under such chromatographic conditions it is possible to determine the heterogeneities of the polymer chain selectively and without any influence of the polymer chain length. LC-CC has been successfully used for the determination of the functionality type distribution of telechelics and macromonomers [104-109], for the analysis of block copolymers [111-114], macrocyclic polymers [115], and polymer blends [116-118]. 

The analysis of aliphatic polyesters with respect to FTD and MMD was demonstrated by Much, Schulz and Kruger [92,121]. Polyesters from adipic acid and 1,6-hexanediol are manufactured for a wide field of applications with an output of thousands of tons per year. They are intermediates for the manufacture of polyurethanes, and their functionality type distribution is a major parameter in... 

Figure 2.7 shows the distribution of the surface potential ipix, 0) along the plate surface as a function of kx for several values of p/x at (Ti/ffi = 3. Each curve for i/ (x, 0) in Fig. 2.7 corresponds to a curve for cr(x) with the same value of p/x in Fig. 2.5. It is interesting to note that even for the limiting case of p/x = oo, which corresponds to a sharp boundary between two regions of (Ti and (T2, the potential does not show a step function type distribution but varies sigmoidally unlike (j x) in this limit.

Molecular weight distribution information obtained by size-exclusion chromatography on its own is insufficient to characterize the properties of complex polymers, such as copolymers and block and graft polymers [23,514,524]. For these polymers the chemical composition and functionality type distributions are equally important. A major obstacle to the characterization of these materials is that their molecular properties are present as joint distributions. Unlike the mass distribution the composition and functionality distributions can only be determined by separation methods that employ interactions with the stationary phase. To fully characterize a complex polymer it is not unusual to use manual or automated tandem techniques where the sample is fractionated according to its chemical or end group composition for subsequent further separation by size-exclusion chromatography to establish their mass distribution. Chromatographic methods may also be combined with spectroscopic methods to determine microstructural information. 

Dimensional heterogeneity is thus seen to be closely related to all other types of molecular heterogeneity. Not knowing the dimensional distribution, one cannot analyze the chemical composition or functionality type distribution, and not knowing these heterogeneities, one cannot unambiguously interpret SEC data. 

The use of liquid chromatography at the critical point of adsorption (CC, critical chromatography) for the determination of the functionality type distribution of telechelics was demonstrated by Evreinov and co-workers [2,11-15]. Meanwhile, a significant number of investigations on functional polyolefines, oligoethers, polyesters, and epoxy resins were conducted showing the usefulness of this new technique. 

When functional homopolymers are synthesized, in addition to macromolecules of required functionality, functionally defective molecules are formed (see Fig. 4). For example, if a target functionality of f = 2 is required, then in the normal case species with f = 1, f = 0 or higher functionalities are formed as well [7], Deviation of the average functionality from the pre-assigned one may result in a decreased or increased reactivity, cross-linking density, surface activity etc. Each functionality fraction has its own molar mass distribution. Therefore, to fully describe the chemical structure of a functional homopolymer, the determination of the molar mass distribution (MMD) and the functionality type distribution (FTD) is required. 

Heterogeneous or complex polymers are distributed in more than one molecular parameter. For functional homopolymers one has to deal Avith the overlapping effects of molar mass distribution and functionality type distribution, whereas copolymers are distributed at least in molar mass and chemical composition. For many years, detector development and the use of several detectors attached to SEC have been the major thrusts in chromatographic analysis of complex macromolecules. In particular, the combination of a refractive index and an ultraviolet detector has been used extensively, although only a limited number of polymers is UV active. Therefore the application of this technique is certainly not universal. On the other hand, SEC has its merits in the daily routine because it is simple, fast, and very reproducible. 

MALDI analysis of hyperbranched polyesteramides showed rapid dissociation of higher oligomers into lower oligomers, and PSD/CID of mass selected ions was used to discriminate between fragment ions and the iso-baric cyclic oligomers. " The results, as well those obtained by NMR, titration, and FD-MS, indicated that simple MALDI-TOF spectra yielded wrong conclusions about the chemical composition and functionality type distribution for the hyperbranched samples. " ... 

Functionality refers to the moieties chemically distinct from the repeating units of polymer chains. It can be the terminal groups at chain ends or the functional groups deliberately added to a polymer chain. Determination of functionality type distribution of reactive oligomers (telechelics) is an old problem in polymer characterization. The classical method is the combination of SEC separation and functionality sensitive detection. Recent advances of the detection methods such as FT-NMR and MALDl-TOF MS have brought about big improvements in the precision of functionality characterization [12]. 

Bektashi and co-workers [120] studied the effect of Lewis acids and of the medium on the MWD and oligomer functionality type distribution obtained in epichlorohydrin polymerisation by means of HPLC, which was applied to separation of oligo(epichlorohydrins) under critical conditions, in combination with GPC. Chromatography under critical and exclusive separation modes enabled seven types of functionalities, including cyclic and six linear types, to be detected. The low molecular weight fractions of the oligomers corresponded to cyclic and linear tetra-, hexa-, hepta-, and octamers. The maximum content of cyclic oligomers was 50%. 

Despite these obstacles, LC-NMR systems are increasingly available and increasingly applicable to real problems. SEC-NMR canbe used to establish accurate MMDs for relatively low-Mr polymers by online measurement of the number-average molar mass (Hatade etal, 1988 Ute et al, 1998). However, SEC//MALDI seems to be a more attractive option for this application. Determination of the chain regularity is a strong aspect of NMR (see also Section 7.4.2.1) and in combination with LC or SEC, tacticity distributions can be determined (Kitayama et al, 2000 Ute et al, 2001). The chemical heterogeneity of high-conversion poly[styrene-co-ethyl acrylate] (Kramer etal, 1999) and the functionality-type distribution of low-molar-mass polyethylene oxide (Pasch 8c Hiller, 1999) were studied by online SEC-NMR. 

---