2.4 Methods
2.4.4 Modeling via PREDICI TM
The simulation and modeling of reactions and polymerization processes is important to science and industry. With a good reaction model at hand and kinetic coefficients and thermodynamic data determined by different methods, reactions can be accurately described without starting a real reaction. This permits the prediction of the result-ing product, security problems and an online reactor control. For e.g. CRP the broad spectrum of kinetic data is used to simulate the results of polymerization and effects of solvent before the reaction is applied in a plant. For polymerization reactions the molecular mass distribution is of great importance for the resulting product and its properties. This leads to a complex situation for the simulation, because every polymer chain must be considered. Furthermore, the chain length of the polymer can influence the reaction system, which should be taken into account as well.
A fast method for simulation is necessary, and the program PREDICITMis one of the most commonly used program packages in CRP[254]and RDRP[18,189,250,255–263]simulation. It offers the fast implementation of different reaction steps by an internal catalog of prede-fined reaction sets, the parameter estimation via an implemented tool and the variation of coefficients. The introduced model is solved numercially via discrete Galerkin h-p methods[254,264]and is able to take chain-length dependent reaction steps into account.
The analysis of reaction mechanisms is important for a detailed understanding on a reaction system with look on possible applications. The broad understanding in the RAFT process led to a wide range of new applications and functional polymers.[193–196]
In recent studies in kinetic research for catalytic polymerization often work with major simplifications or approximations which makes their direct conversion into a model for CCG unsuitable. Additionally, the most applied neodymium system needs no additional activator like MAO, which is present within the typical studies. Compared to this system the activation is similar to typical ZN catalysts which are activated by Lewis acids.
Diorganyl magnesium derivatives are ideal candidates for such approaches. On the one hand they show high polymerization activity when applied by neodymium based catalysts. On the other hand they are commercially available or very easy to synthesize.
The system was already found to allow a broad spectrum of possible end group modifi-cations.[23]
For kinetic determination of the process PREDICITM is chosen, because of its broad application in the similar RAFT-process in RDRP. Methods to describe polymer exchange reactions as well different deactivation pathways have already been reported in litera-ture.[189,259–263]Similar exchange reactions are to be expected for CCG systems.[20]In this work six major objectives were planned:
1. The detailed determination of the activation mechanism in absence of monomer by a model system based on Cp*2ZrCl2and dibenzyl magnesium in NMR spectroscopy and the transfer to UV/Vis spectroscopy for later analysis of paramagnetic systems.
2. The detailed analysis of styrene-d8 polymerization in NMR spectroscopy to gain insights in the reaction mechanism with monomer present and for extraction of kinetic information of the occurring processes.
3. Transfer of the mechanistic model to polymerization of styrene analyzed via UV/Vis spectroscopy and modeling of the overall process.
4. Transfer of the determined model to the polymerization of styrene performed in presence of Cp*2Nd2Cl2Li(OEt2)2as catalyst and BuMgOct (BOMAG) as cocatalyst monitored via UV/Vis spectroscopy.
5. Modeling and Simulation of ethylene polymerization in batch reactors with Cp*2Nd2Cl2Li(OEt2)2as catalyst and BuMgOct (BOMAG) as cocatalyst .
6. Synthesis of functional polymers based on polyethylene produced by CCG.
The first point is important to understand the basic reaction steps occurring during the process, which shall be transferred to the polymerization. The main objective is the successful modeling of the CCG process in batch reactors.
Catalyzed Chain Growth
For research of CCG process several catalyst/co-catalyst systems were studied to estab-lish different experimental methods. For this thesis in situstudies which provide the most detailed data of occurring species and monomer conversion were chosen. The most used polymerization system containing Cp*2NdCl2Li(OEt2)2 has the disadvantage of being paramagnetic which leads to problems in characterization in NMR experiments.
In order to gain information about formed complexes during the polymerization process a suitable non-paramagnetic system, i. e. Cp*2ZrCl2, which can be well characterized by NMR was researched. This zirconium catalyst precursor is commercial available and a broad spectrum of literature data is available. In addition the complex has the same set of ligands as the neodymium catalyst and ligand effects can be excluded. The model system is able to polymerize styrene in the presence of different magnesium derivatives (see section 6.1 on page 135) and outgoing from the results the catalyst systems with Cp*2NdCl2Li(OEt2)2were studied.
4.1 Catalysts
Nd Cl Cl Li
OEt2 OEt2
Cp*2NdCl2Li(OEt)2
Zr Cl Cl
Cp*2ZrCl2
Figure 4.1:Catalysts applied for establishment of kinetic models in monomer free CCG activation and CCG polymerization.
Two catalysts were studied (see Figure 4.1) of which Cp*2ZrCl2is commercially available and Cp*2NdCl2Li(OEt2)2was synthesized. The process is a literature known[265]one step procedure and has been slightly modified yielding a blue powder characterized by NMR and ESI-MS (see Scheme 4.1). The catalyst is highly oxygen and moisture sensitive and was therefore stored and handled in a glove box with dry and oxygen free solvents.
NdCl3
Li 2
1. THF reflux, 12 h 2. Et2O
Nd Cl
Cl Li(OEt2)2
Scheme 4.1:Synthesis of Cp*2NdCl2Li(OEt2)2from NdCl3and freshly prepared LiCp* in THF under reflux conditions and work up in Et2O.