Short-chain branches

We can introduce short chain branching into polymers by three methods copolymerization, backbiting , and chemical modification. The first t vo occur during polymerization, while the last requires a secondary chemical reaction. Short chain branches have well defined chemical structures, the nature of which we can accurately determine via analytical methods or know from the structure of the reactants. 

We can introduce short chain branches by means of many classic organic chemistry reactions. The variety of branches that can be introduced covers the full range that is accessible via standard reactions. 

---

Structure. The physical properties of LDPE depend on the molecular weight, the molecular weight distribution, as well as the frequency and distribution of long- and short-chain branching (2). 

Both propylene and isobutylene ate comonomers that are incorporated along the chain, resulting in additional short-chain branching. One important factor in controlling polymer crystallinity is the choice of chain-transfer agent. Ethane and methane, for example, are inefficient agents whose presence in the monomer feed stream must be considered in reaction control. 

Crystallinity and Density. Crystallinity and density of HDPE resins are derivative parameters both depend primarily on the extent of short-chain branching in polymer chains and, to a lesser degree, on molecular weight. The density range for HDPE resins is between 0.960 and 0.941 g/cm. In spite of the fact that UHMWPE is a completely nonbranched ethylene homopolymer, due to its very high molecular weight, it crystallines poorly and has a density of 0.93 g/cm. ... 

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

It has been shown that intramolecular chain transfer to polymer occurs during the polymerisation of vinyl acetate, lea ding to short-chain branching (81,235—238). The number of short-chain branches has been estimated by nmr to be in the range of 0.12—1.7 mol % (81). The number of short-chain branches increases significantly at low monomer concentration. 

Variation in the degree of short chain branching in the polymer. 

Short chain branching is negligible with Ziegler and Phillips homopolymers although it is possible to introduce deliberately up to about seven ethyl side chains per 1000 carbon atoms in the Ziegler polymers. 

Components of a highly short chain branched (scb) waxymaize and a more long chain branched (Icb) amylomaize were separated on the semipreparative Sephacryl S-1000 system. Both samples contained high dp components that eluted in the exclusion volume, but the percentage of these components was quite different 90% for the scb waxymaize starch and approximately 10% for the Icb amylomaize starch (Fig. 16.6). The degree of polymerization averages for these samples was determined utilizing the previously established linear... 

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

The extent of short-chain branching in PE may be quantitatively determined by a variety of techniques including IR. 1 pyrolysis Ct., 7" and y-radiolysis."84 The most definitive information comes from l3C NMR studies.2S5"2 0 The typical... 

Copolymerization of macromonomers formed by backbiting and fragmentation is a second mechanism for long chain branch formation during acrylate polymerization (Section 4.4.3.3). The extents of long and short chain branching in acrylate polymers in emulsion polymerization as a function of conditions have been quantified.20 ... 

Copolymerization e.g., of 1-butene or 1-hexene with ethylene, gives short-chain branching-, e.g., the branches contain three or five carbon atoms. The random location of the side-chains lowers the crystallinity and density. Long-chain branching refers to branches that are similar in length to the polymer backbone and this type occurs in polyethylene manufactured using the... 

The predominating type of nonlinearity in polyethylene appears to consist of short chain branches three or four chain atoms in length formed by intramolecular chain transfer as follows ... 

---