Synthesis and characterisation of hyperbranched polymers

Abstract

Hyperbranched polymers are a subclass of dendritic molecules characterised by a highly branched architecture. Such polymers are usually prepared in an easy and affordable one-pot synthesis and therefore are considered suitable for industrial large-scale applications such as coating and resins formulation.
Hyperbranched poly(ester amine)s (PEAs) and poly(amido amine)s (HPAMAMs) were synthesised by the double monomer methodology (DMM) with multifunctional monomers. The Michael addition reaction was used with and A2 + B4 methodology. The use an A2 + B4 system has the advantage of producing hyperbranched polymers with high number of functional terminal groups but also the disadvantage of (potentially) undergoing gelation due to the use of monomers with symmetric functionalities. Therefore a key project aim was the development of novel, simple, versatile and cost-efficient synthetic strategies to exclusively synthesise soluble (gel-free) hyperbranched polymers via the A2 + B4 system. It was also an objective of this work to (i) further modify/functionalise the chemical structure of the resulting hyperbranched polymers, (ii) understand the long-term (storage) stability of the synthesised polymers and (iii) explore potential industrial applications.
PEAs and HPAMAMs were synthesised by selecting suitable pairs of monomers (A2 and B4) with a molar ratio of A2:B4 which was higher than 1:1 (A:B (>2):4). The reactions studied show susceptibility to gelation and at long reaction times the formation of a cross-linked product or alternatively a sol-gel product was observed in many cases. In this latter case the relative amount of gel and soluble (sol) fraction was found to be dependent on the reaction conditions used such as monomer molar ratio, temperature and monomer solution concentration. A first strategy to produce exclusively soluble branched polymers involved the use of a large stoichiometric excess of A2 monomer with respect to B4 monomer and quenching the polymerisation after a certain time. It was observed that at a molar ratio A2:B4 of 3:1, highly branched and gel-free polymers were formed and after 24 hours (i) for HPAMAMs, a polymer with Mn 620 g/mol, Mw 10550 g/mol, Ð 17.7 and degree of branching (DB) 0.98 was obtained in methanol/water at 40°C while (ii) for PEAs, a polymer with Mn 620 g/mol, Mw 1150, Ð 1.8 and DB 0.45 was formed at 60°C in DMF. A novel alternative strategy was developed with the above-mentioned requirements, that enabled the synthesis of soluble branched polymers without the need to monitor and stop the reaction at a certain conversion of functional groups above which gelation occur. Moreover, a variety of effective and versatile strategies for the modification of the basic polymer skeleton of HPAMAMs were explored.
The stability of PEAs was studied in methanol and evidence of degradation was found both by SEC analysis and NMR spectroscopy. It was shown that degradation occurred via cleavage of the ester group of the polymer catalysed by the amine groups within the same structure. The stability of HPAMAMs towards degradation was instead studied in water and a good long-term stability both in dilute and in concentrated solution was observed by SEC analysis and NMR spectroscopy. However, HPAMAMs underwent chain-coupling in water which did lead in some cases to gelation. Polymers were hence modified in order to reduce the risk of gelation.
The properties of the HPAMAMs polymers synthesised by A2 + B4 system were finally investigated and proved to be of potential commercial utility in certain industrial applications.