Short Chain Branching Distribution

Short chain branches are frequently introduced into polymers by copolymerization. The chemical structure of the comonomer controls the type and length of the short chain branch. The polymerization catalyst, reaction conditions, and comonomer content in the reaction medium determine the probability of finding a branch at any particular location along a drain. Comonomers, and hence the short chain branches derived from them, can be introduced at random or as blocks. 

By using two or more polymerization catalysts simultaneously, polymer chemists can produce copolymers with a bimodal composition distribution. This is made possible by the feet that no ttvo catalysts incorporate monomers at exactly the same rate. The net result is that short chain branches may be preferentially incorporated into either the higher or lower molecular weight fractions. Polymer manufacturers can obtain a similar result by operating two polymerization reactors in series. Each reactor produces a resin with a different copolymer distribution, which are combined to form a bimodal product. Copolymers with a bimodal composition distribution provide enhanced toughness when extruded into films. 

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Figure 2 Short chain branching distribution in polyethylenes. Source Ref. 31.

Due to their multi-sited nature, Ziegler-Natta and chromium catalysts produce structurally heterogeneous ethylene homo- and copolymers. This means that the polymers have broad MWD and broad composition (short-chain branching) distribution (Fig. 9). Catalyst active sites that produce lower molecular weights also have a tendency to incorporate more comonomer... 

New types of LLDPEs based on the metallocene catalyst technology have been introduced recently in the market place. Such LLDPEs are characterized by narrower molecular weight and homogeneous short-chain branching distribution. Some of the metallocene catalyst based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to have LCB. For the properties of metallocene LLDPE see the entry Polyethylene, metallocene linear low density, in this handbook. 

A scheme of the extended chain molecular population and branching in an LLDPE resin is shown in Fig. la, where it is seen that the larger the molecule the lower chances of comonomer incorporation (lower branch content). Most interesting in LLDPE is the bimodality of the CCD, sometimes referred to as the short-chain branching distribution (SCBD), as shown in Fig. lb, due to the population discontinuity observed between (1) the fraction of linear molecules, practically excluding the comonomer incorporation in certain catalyst sites, and (2) the remaining fractions with increasing amounts of comonomer incorporated. Catalyst sites, where the bulkier and less reactive comonomer can be incorporated. 

Fig. 9. Calibration curve for calculating short-chain branching distributions from analytical TREF data (9). Reprinted with permission...

TREF of EVA copolymers was first reported by Wild, Ryle, Knobeloch and Peat [9]. They subjected EVA copolymers of VA content of 9, 14 and 18 wt% to TREF analysis and obtained the short-chain branching distribution shown in Fig. 22. Included with the data for EVA was a TREF analysis of an ethylene-ethyl acrylate copolymer (EEA) containing 18 wt% ethyl acrylate. The EVA copolymers exhibit relatively narrow SCB distribution at increasingly higher SCB branch... 

The SCBDI = wt% of macromolecules having a comonomer content within 50 % of the median total molar comonomer content, calculated from TREF (temperature rising elution fractionation) data. The elastic, substantially linear C2-Cg copolymer has 0.01 < LCB/IOOOC < 3, M /Mn = 1.5-2.5, 2 < SCB (CH3/IOOOC) < 30), and short-chain branch distribution index SCBDI > 50 %. The homogeneously branched copolymer may he produced as described in C. T. Elston (DuPont Canada Ltd.) patent. Films produced from the bimodal MWD new copolymers show good impact and tensile properties... 

T. Usami, Y. Gotoh, S. Takayama, Generation mechanism of short-chain branching distribution in linear low-density polyethylenes. Macromolecules 19(11), 2722-2726 (1986)... 

Fig. 2. Comparison of the short-chain branching distribution of ZN-LLDPE as obtained by TREE, CRYSTAF, and DSC. A calibration curve for the DSC data is plotted on the right axis. Reprinted from Ref 101, Copyright (2000), with permission from Elsevier Science.

For ethylene/1-olefin copolymers, chain crystaUizabihty is mainly controlled by the fraction of noncrystalhzable comonomer imits in the chain. Consequently, the differential Crystaf profile shown in Fig. 1, together with an appropriate cahbration curve, can be used to estimate the copolymer chemical composition distribution (CCD), also called the short-chain branch distribution. The CCD of a copolymer describes the distribution of the... 

Table 2. Precise short chain branch distribution and its effect on thermal behavior...

Chu K-J, Soares IBP, Penlidis A (2000) Variation of molecular weight distribution (MWD) and short chain branching distribution (SCBD) of ethylene/1-hexene copolymers produced with different in-situ supported metallocene catalysts. Macromol Chem Phys 201 340-348... 

In 1982, Wild and coworkers [17] reported the short-chain branching distribution of LDPE produced in each type of reactor design by utilizing the temperature rising elution fractionation (TREF)analytical technique. The autoclave LDPE sample had a 0.924 g/cc density and a 3.0 Melt Index. The tubular LDPE sample had a 0.921 g/cc density and a 2.2 Melt Index. Ihe branching distribution data from each sample is summarized in Figure 5.7. 

Figure 5.7 Comparison of the short chain branching distribution of LDPE produced from an autoclave reactor (O) and from tubular reactor (A) using TREE method. Reprinted from [17] with permission from John WUey and Sons.

Shan, C. L., Chu, K-J., Soares, J. B. R, Penlidis, A. Structure-property characteristics of ethylene/ 1-hexene copolymers with tailored short chain branching distributions. Soc. Plastics Engrs. ANTBC Proceedings (2000), pp. 1616-1619... 

Zhang S, Zhao N, Wu Y, et al Short chain branches distribution characterization of ethyl-ene/l-hexene copolymers by using TREF - - C-NMR and TREF + SC methods, Macromol Symp 312(1) 63—71, 2012a. 

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