pubs.acs.org/Macromolecules Published on Web 02/23/2010 r 2010 American Chemical Society 2780 Macromolecules 2010, 43, 2780–2788 DOI: 10.1021/ma902297b Molar Mass and Molecular Weight Distribution Determination Of UHMWPE Synthesized Using a Living Homogeneous Catalyst Saeid Talebi, † Rob Duchateau, † Sanjay Rastogi,* ,†,§ Joachim Kaschta, ) Gerrit W. M. Peters, ‡ and Piet J. Lemstra † † Department of Chemical Engineering and Chemistry, and ‡ Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 560 0MB Eindhoven, The Netherlands, § Department of Materials, Loughborough University, Leicestershire LE11 3TU, Loughborough, England, United Kingdom, and ) Institute for Materials Science, University of Erlangen-N€ urnberg, Erlangen, Germany Received October 15, 2009; Revised Manuscript Received January 28, 2010 ABSTRACT: Understanding of the physical characteristics of a polymer requires molar mass determina- tion. For the commercially available polymers, having average molar mass below 1 000 000 g/mol, chromatography is the method that is often applied to determine the molar mass and molar mass distribution. However, the application of conventional chromatography techniques for polymers having molar mass >1 000 000 g/mol becomes very challenging, and often the results are disputed. In this article, melt rheometry based on the “modulus model” is utilized to measure the molar mass and polydispersity of ultra high- molecular-weight polyethylenes (UHMWPEs) having molar mass >1 000 000 g/mol. Results are compared with the chromatography data of the same polymer samples and the boundary conditions where the chromatography technique fails, whereas the rheometry provides the desired information is discussed. The rheological method is based on converting the relaxation spectrum from the time domain to the molecular weight domain and then using a regularized integral inversion to recover the molecular weight distribution curve. The method is of relevance in determining very high molar masses (exceeding 3 000 000 g/mol) that cannot be ascertained conclusively with the existing chromatography techniques. For this study, UHMWPEs with various weight-average molar masses, where the number-average molar mass exceeds >1 000 000 g/mol, are synthesized. Catalyst used for the synthesis is a living homogeneous catalyst system: MAO-activated bis(phenoxy imine) titanium dichloride. The rheological behavior of the thus synthesized nascent reactor powders confirms the disentangled state of the polymer that tends to entangle with time in melt. Introduction The polyolefin synthesis is dominated by the use of multisited heterogeneous Ziegler-Natta catalysts. However single-site cat- alysts are finding an exceptional role in the polymer industry. 1 The advantage of single-site catalysts over classical Z-N cata- lysts is their capability to produce tailormade polymers with controlled molecular weight, specific tacticity, and a narrow molecular weight distribution. 2 The ultimate control is obtained with a living catalyst because it allows us to fine-tune the molecular weight, form block copolymers, or form end-functio- nalized polyolefins while maintaining a very narrow polydisper- sity (PDI). Group 4 metal phenoxyimine complexes, invented by scientists at Mitsui, 3 provide an interesting class of living olefin polymer- ization catalysts. These catalysts have been used in different laboratories, and progress in the area of stereoselective as well as living olefin polymerization has been reported. 4,5 Fujita et al. have demonstrated that when activated with MAO, the bis- (phenoxyimine) titanium complex, [2-(t-Bu)-C 6 H 3 O(CHNC 6 F 5 )] 2 - TiCl 2 , is producing an ethylene polymerization catalyst that is living at room temperature. The reported highest molar mass that they obtained in 1 min of polymerization was ∼4.5  10 5 g/mol. 6 Carrying out the same polymerization at room temperature for 5 min, Sharma 7 reported synthesis of polyethylene with molar mass >1 000 000 g/mol. However, because of the limited reliability of the applied GPC method, the mentioned molecular weight remained disputable. Using melt rheometry as a characterization method, in this article it will be demonstrated that the titanium complex is capable of producing ultra high-molecular-weight polyethylene (UHMWPE) with molar mass exceeding 9  10 6 g/ mol, still in a controlled manner. The use of homogeneous polymerization catalysts provides a novel route to produce disentangled polymer crystals directly in a reactor. Upon polymerization at low temperatures, the growing chains experience a “cold” environment and crystallize individu- ally into folded-chain crystals, ultimately forming “monomole- cular crystals”, viz. one long chain forms one crystal. In fact, this is an easy and direct approach toward highly disentangled chains. 8,9 Upon melting, such a disentangled semicrystalline polymer provides a disentangled polymer melt, which is a thermodynamically unstable and relatively low viscous state. To achieve the thermodynamically stable melt state, chains tend to entangle with time. The presence of fewer entanglements in the initial stages of the disentangled melt lead to higher molar mass between entanglements and thus a lower elastic modulus of the melt. Figure 1 shows a typical time sweep experiment performed on two different disentangled polyethylenes having average molar masses of (2.0 and 5.3)  10 6 g/mol, respectively. From here it is evident that the time required for the modulus build-up increases with the increasing molar mass. Ultimately, the initially disentangled chains entangle to form the thermodynamically stable melt state. Stress relaxation and frequency sweep measure- ments are performed on the fully entangled samples. Because of *Corresponding author. E-mail: s.rastogi@lboro.ac.uk.