Limitations of the conventional, statistically occurring coordination-insertion polymerization are high-dispersed products (for heterogenous Ziegler-Natta catalysts: Ð = 8-30, for homogenous catalysts: Ð = ~2) and on the other hand strongly limited functionalizations of polyethylene.3,73 One technique to overcome these limitations is the coordinative chain transfer polymerization that is based on a transition metal catalyst in combination with a chain transfer agent (CTA) (Scheme 2-6).13
Scheme 2-6. Simplified mechanism of coordinative chain transfer polymerization.
Basic Mechanism of CCTP and CCG
The mechanism of the coordinative chain transfer polymerization (CCTP) differs from the conventional polymerization in one major element. As described in chapter 2.2, the polymer chain is growing on the active catalyst in a typical coordination-insertion polymerization. In an additional step, the growing chain is reversible exchanged with another polymer that is bound on a catalytically inactive chain transfer agent (dormant species, see Scheme 2-6).74 One special feature of this method is the possible usage of two catalysts which are selective to different monomers in combination with only one suitable CTA. By implementing this approach block copolymers are accessible via CCTP.75
Within this field the related concept of catalyzed chain growth (CCG) was established by Gibson and coworkers.12 The first system introduced by this group is based on a bis(imido)pyridyl iron catalyst in combination with diethyl zinc as chain transfer agent.76,77 It was demonstrated that in the absence or suppression of termination processes and under consideration of an efficient and completely reversible transfer between the CTA and the catalyst, all chains have the same probability to grow.77,78 In addition, by using efficient CTAs working also as activators, multiple chains grow on one catalyst molecules parallelly whereas in a conventional system only one chain per catalyst can grow at the same time.79 Furthermore, after the CCG polymerization process occurred, nearly all polymer chains are bound to the main group cofactor. Based on the resulting metal-organic group, CCG provides access towards end-group functionalization as well as the possibility to synthesize block copolymers by the subsequent addition of another monomer.13,74,79 The
basic concept of catalyzed chain growth is comparable to RAFT (reversible addition-fragmentation chain transfer polymerization) polymerization, a well-established controlled radical polymerization technique.80 For the reasons mentioned, catalyzed chain growth can be considered as living-type polymerization.12,81
In CCG and CCTP various transition metal catalysts e.g. chromium82, iron76, different lanthanides like lanthanocene83, zirconium84, cerium85, neodymium86 or hafnium87 were applied. Established chain transfer agents are commonly based on aluminium74,87, magnesium88 or zinc77. A suitable combination of the catalyst/cocatalyst depends on e.g. the sterically hinderance. Furthermore, the binding energy of the polymer to the CTA has to be comparable to the stability of the polymer−catalyst bond in order to achieve a fast and full reversibility of the transfer reaction.13,78
Catalyzed Chain Growth based on Neodymium and Magnesium One highly active precatalyst that is suitable for the polymerization of ethylene is [(cp*)2NdCl2Li(OEt)2].89 In combination with a dialkylmagnesium compound, that works both as CTA and activator, the resulting system is able to polymerize ethylene in a controlled fashion.
This well-established and investigated system was applied in this work and therefore will be presented herein.88 The catalyzed chain growth mechanism of ethylene using a [Nd] catalyst is shown in Scheme 2-7.
Scheme 2-7. CCG polymerization mechanism of ethylene using dialkylmagnesium and [(cp*)2NdCl2Li(OEt)2].
Analogous to the discussions made before, the first mechanism-step is the activation of the precatalyst via a diorganomagnesium compound forming an active, unsaturated complex (Scheme 2-7 upper site).90,91 Proceeding from this species the ethylene polymerization occurs in a typical coordination-insertion mechanism (chapter 2.2, Scheme 2-4). As described above, within the CCG system the diorganomagnesium compound exhibits an additional function and acts as a chain transfer agent via a bimetallic intermediate.88,91 Based on the efficient and completely reversible exchange of growing polymer chains a uniform molecular weight distribution is achievable under those conditions.83,90 In addition, the polymerization degree is linearly proportional to the reaction time and monomer conversion.
End-group Functionalization of Polyethylene
Besides the mentioned improvements, the major advantage of CCG is the formation carbon−magnesium bonds at all chains (PE−Mg−PE) after propagation. This terminated polyethylene intermediate provides a strong electrophilic behavior and can react in Grignard-type reactions.
Based on this highly reactive species various end groups has been introduced (Scheme 2-8).14
For instance, elemental iodine reacts efficiently with PE−Mg−PE resulting in iodo terminated polyethylene (PE-I) with excellent yields up to 95 %.15 PE-I can be converted into an azide end-group (PE-N3) by the reaction with NaN3 that can be applied for instance in a Huisgen cycloaddition.15,92 Another pathway based on PE-N3 is the quantitative reduction providing an amine end-functional polyethylene (PE-NH2) that can be subsequently converted e.g. into an macroinitiator for the synthesis of well-defined block copolymers via controlled radical polymerization techniques.14,15 Based on the Grignard type intermediate also macroalkoxyamines are accessible for the application in nitroxide-mediated radical polymerization.90,93 Also PE-based macroinitiators for further polymerization methods were successfully synthesized via a CCG procedure in combination with multiple step end-group modifications.16 Furthermore, by using prefunctionalized diorganomagnesium compounds even --functional polyethylene bearing different end-groups are accessible.17,18,94 In addition, by the reaction with sulfur or oxygen14 respective thiol or hydroxyl functional groups can be introduced into polyethylene.95 Alkene groups in combination with thiol groups were subsequently used in thiol Michael- or thiol-ene-reactions offering a novel reaction pathway for various end-group transformations.95,96 The reaction of di-polyethylene magnesium directly with disulfides of trithiocarbonates leading in trithiocarbonate functionalized PE (Scheme 2-8).97
These examples demonstrate the versatility of the catalyzed chain growth polymerization for the efficient end-group modification of polyethylene.
Several of these strategies were applied and further developed in this work.
Scheme 2-8. Possible end-group functionalization based on CCG.