Branching in LDPE

This technique has been used to investigate the distribution of short-chain branches in LDPE and the relatively higher scission efficiency of ethyl branches would rationalize the yields with 13C NMR measurements of branch frequencies (2). 

Formation of short branches in LDPE is investigated. It is assumed that the probability of an intramolecular rearrangement of the Roedel type is proportional to the probability that the reacting groups are separated by a distance r r and adhere sufficiently closely to "three in a line" geometry. Excluded volume effects are ignored. The calculations rationalize many of the structural features observed in LDPE. For the present purpose, probabilities must be evaluated by using branched-molecule RIS theory. 

Intermediate that can result in formation of 2-ethylhexyl branching in LDPE (See Figure 2.3)... 

Figure 2.3 Mechanism of formation of 2-ethylhexyl branch in LDPE. As in backbiting mechanism for n-butyl group formation, homolytic scission of a CH bond occurs down the chain. is a polymeric alkyl group.

Figure 2.3 Mechanism for formation of 2-ethylhexyl branch in LDPE 28...

L. J. Rose and F. Beer, Characterization of long chain branching in LDPE s using SEC with on-line viscosity and light scattering detectors, MolMass International Conference Proceedings, 1999 (www.chem.leeds.ac.uk/molmass 99). 

A third kind of polyethylene introduced in the late 1970s is called linear low-density polyethylene (LLDPE). It is made by the same metal-catalyzed reactions as HDPE, but it is a deliberate copolymer with other 1-alkenes such as 1-butene. It has some side groups (which reduce the crystallinity and density), but they have a controlled short length instead of the irregular, long side branches in LDPE. LLDPE is stronger and more rigid than LDPE it is also less expensive because lower pressures and temperatures are used in its manufacture. 

ANALYSIS OF POLYOLEFINS 4.3.1 Short-Chain Branching in LDPE... 

TREF was also used as one of the main characterization tools in the lUPAC, Working Party on Molecular Characterization of Commercial Polymers [23]. The study, Long and Short Chain Branching in LDPE , was a collaborative study by five laboratories. The study concluded that indeed TREF did fractionate according to SCB rather than LCB or molecidar weight and the technique was able to provide a good measure of SCB distribution for the LDPE sample. Good reproducibility between laboratories was also noted. 

As a consequence, before 1953, the only possible blends were those of LDPE with other polymers than PO or with elastomers (e.g., chlorosulfonated polyethylene rubber, CSR chlorinated butyl mbber, CBR ethylene/propylene/diene copolymers, EPR, EPDM thermoplastic olefinic elastomer TPE, TPO). However, in addition to the original autoclave polymerization, already in 1938, a tubular reactor was introduced and its product had different properties than that from the autoclave. Also varying the reaction condition affected the degree of short- and long-chain branching in LDPE thus, blending different LDPEs offered a way for optimizing the resin to specific applications. 

It was reported by F. Mirabella, Jr., and E. Ford that the amount of long-chain branching in LDPE samples over a density range of 0.917-0.922 and Melt index values of 0.25-2.5 was about 2.9-3.4 LCB/1000 carbon atoms [18]. 

Note that this describes a line that is different from that for linear polyethylene, ie., if gis set equal to unity, we do not recover the line for HDPE. Thus, LDPE is a distinctly different material than HDPE, and one cannot think of decreasing the level of branching in LDPE so that it approaches HDPE in a continuous manner. 

Nishoika and co-workers [14] determined the degree of chain branching in LDPE using proton NMR, Eourier transform (ET) NMR (ET-NMR) at 100 MHz and ET-NMR at 25 MHz with concentrated solutions at approximately 100 °C. The methyl concentrations agreed well with those of infrared based on the absorbance at 1378 cm (7.25 pm). 

Bugada and Rudin [23] used C-NMR and SEC to determine long chain branching in LDPE. 

---