A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

Abstract

Synthesis and characterization of long-chain branched polyolefins has been an important research topic for both academic and industrial researchers for many decades. This is particularly true since the discovery and successful commercialization of the constrained geometry catalyst systems in 1990s. The type of single site catalysts allows control in the synthesis and the precision in the characterization. Long-chain branched polyolefins exhibit improved melt processability, such as higher melt strength and better shear thinning, compared to their linear counterparts having the same molecular weight and distribution. In the previous papers, the catalyst systems and reaction conditions for the controlled synthesis of long-chain branched polyolefins have been reviewed. This paper aims at summarizing the literatures pertinent to the precise characterization of long-chain branched polyolefins. The major methods of long-chain branching characterization include: nuclear magnetic resonance, gel permeation chromatography, rheometry, dynamical mechanical analysis, differential scanning calorimetry, neutron scattering, and Fourier transform infrared spectroscopy. Recent progresses of these characterization methods, as well as their advantages and limitations, have been discussed in details.

Control over Branching Topology by Introducing a Dual Catalytic System in Coordinative Chain Transfer Polymerization of Olefins

“…Therefore, 13 C NMR is only used to study the microstructure regarding short branches, and the formation of LCB by the proposed reaction cycles is primarily investigated by means of rheology due to its high sensitivity to the presence of traces of LCBs. 3,24,28,29 3.1. Rheological Insight into the Microstructure.…”

“…The emergence of long-chain branches has significant effects on different aspects of the rheological properties. 3,30 LCBs can be regarded as long-lasting entanglements, which prolong the reptation of the main backbone beyond their own retraction time. The specific retraction time exponentially depends on the branch length, which considerably delays the terminal relaxation.…”

“…One of the important microstructural aspects of polyolefins, as widely produced commodity materials, is the extent and topology of branching. A great deal of research has been directed toward control of long-chain branching to improve processing conditions without losing the final properties. − In parallel, producing structures with very high branching contents results in pseudo-gel properties and opens a new dimension for application of polymers . Manipulating the catalyst structure and optimizing the reaction conditions are widely studied approaches in academia and industry to provide the ability to tune the microstructure, specifically long-chain branches (LCBs), and consequent properties of polyolefins. , …”

“…The literature on the controlled long-chain branching has been precisely reviewed in a series of publications by Liu et al − Controlled LCB structures are traditionally produced by single-site catalysts, which provide a more homogeneous product with a narrower polydispersity than the traditional multisite Ziegler–Natta catalysts or the less controlled free-radical production of low-density polyethylene. Two strategies have been particularly developed to create and control LCBs in coordination polymerization.…”

Long-Chain Branched Polyethylene via Coordinative Tandem Insertion and Chain-Transfer Polymerization Using rac-{EBTHI}ZrCl2/MAO/Al–alkenyl Combinations: An Experimental and Theoretical Study

“…Yet, because of the radical mechanisms involved in these treatments, ample control over LCB formation is rather impossible; hence, polymers with complex structures are obtained in those cases. In-reactor technologies such as copolymerization of ethylene with either in situ-formed or previously isolated macromonomers, , or with a nonconjugated α,ω-diene − have also been developed. Nevertheless, only a limited number of catalyst systems have been shown to efficiently generate and incorporate macromonomers, while the second approach is limited by the rather scarce availability of α,ω-dienes and by the uneven distribution of LCB in the final polymers.…”