1.3. PREGUNTA DE INVESTIGACIÓN
2.2.10. IMPACTO EMOCIONAL DEL TRATAMIENTO DE QUIMIOTERAPIA
Conducting the polymerisation of di/tri-vinyl monomers via a free radical mechanism leads to instantaneous crosslinking of the product, but by utilisation of CCTP the molecular weight and hence the branching of these materials can be reduced to oligomeric products, dependent on the level of catalyst employed. The first published instance of the use of CCTP for the synthesis of branched polymers was by Golokovet al., to which triethylene glycol dimethacrylate (TEGDMA) was polymerised using a cobalt(II) hematoporphyrin tetramethyl ester complex as a chain transfer agent. Although soluble oligomers could be obtained, the reactions were inconsistent and the resulting polymers not fully characterised, containing no structural information.179 Further to this work a patent was filed in 1986 by Abbey utilising the same monomer with a Co(II) catalyst which was generatedin situ. The levels of catalyst used led to the formation of only low molecular weight products such as dimer, hence the majority of polymers yielded were linear.180
In 1998 a further patent was filed by Guan as part of E. I. Du Pont Nemours and Company, pertaining to the synthesis of multifunctional hyperbranched polymers by CCTP of di-or tri-vinyl monomers, including homopolymerisations and copolymerisations of a range of multivinyl monomers with monovinyl monomers.181The polymers produced in this work exhibited low solution viscosities and a high level of vinyl functionality, hence it was postulated that these materials exhibited potential for application to automotive coatings and photopolymerisations. This work was also published in part in 2002.124 Guan used a concentration of cobalt catalyst that he postulated would lead to cascade trimerisation of the monomer. As every trimerisation contains a branching point, with termination occurring predominately via β-hydride
abstraction, branched polymers are formed which possess high levels of vinyl functionality, both internal and external.
Figure 1.21: Mechanism for the CCTP of EGDMA, cascade trimerisation theory postulated by Guan,124leading to the formation of branched vinyl terminated polymers
The intrinsic viscosities (IV’s) of ethylene glycol dimethacrylate (EGDMA) homopolymers synthesised were measured using SEC equipped with viscometry detection. It was found that the IV of branched Poly-EGDMA’s synthesised were significantly lower than their linear counterparts. A further feature observed was that there was virtually no dependence of IV on molecular weight, whereas for linear polymers IV increases linearly with molecular weight in good solvents, following the
Mark-Houwink equation with an α value of approximately 0.7.182 The use of CCTP of
divinyl monomers provides a novel method for the synthesis of branched polymers whereby, in the case of homopolymers, molecular weight is directly proportional to the level of branching therefore providing a tool for the control of polymer topology through molecular weight control.
In 2002, work on the polymerisation of divinyl monomers by CCTP was expanded through a collaboration between the University of Strathclyde, Viscotek and Ineos Acrylics, whereby branched copolymers of MMA and branching monomer tripropylene glycol diacrylate (TPGDA) were synthesised by both CCTP and conventional chain
transfer polymerisation, also known as the “Strathclyde route”, using dodecanethiol (DDT) as the chain transfer agent.114 Approximately 5 wt% of branching monomer TPGDA was incorporated into the copolymerisation with MMA, with resulting polymers retaining high levels of vinyl functionality. It was found that several factors had an effect on the level of branching of the polymers synthesised; by increasing the level of crosslinking monomer, an increase in branching was observed and low molecular weight products were predominately linear, with branching increasing with molecular weight. Reduction in the CoBF concentration led to variable Mn’s, although Mw’s
increased as expected, hence the effect of varying CoBF was found to have little effect on branching and only on molecular weights.
The copolymerisation of acrylates and methacrylatesviaCCTP caused the formation of gradient compositional polymers, as the reactivity ratios are approximately 0.5 and 2.0 respectively,183 therefore at low conversions resulting instantaneous copolymers were found to be methacrylate rich, and at high conversions the formation of acrylate rich copolymers was observed by fractionation and analysis by SEC. Use of acrylates in CCTP also leads to the formation of thermodynamically stronger C-Co bonds due to
the lack of an α-methyl hydrogen for the CoBF species to abstract;184
hence a lower apparent catalyst activity can be expected, this has the likely consequence that towards the end of the polymerisation, when the number of propagating acrylates increases, the system will be effectively starved of CoBF and termination by combination may predominate, leading to increased PDi’s.185 Hence, although CCTP has the benefit over conventional chain transfer of employing a greatly reduced amount of catalyst, the control over composition and PDi for the copolymerisation of acrylates and methacrylates led to the conclusion that conventional chain transfer provided more control in this case.
A publication pertaining to the synthesis of branched polymers by CCTP also arose from the Russian Academy of Sciences by Kurmaz et al., concerning the copolymerisations of styrene and branching monomers EGDMA and TEGDMA copolymers.186 Comparisons were made between the free radical polymerisation and CCTP of styrene and EGDMA, finding that by use of a cobalt catalyst (cobalt(II) tetramethyl hematoporphyrin IX, [CoIIP]) the gel effect was suppressed, leading to the formation of polymers with superior molecular mobility and elasticity, and that by
increasing the level of [CoIIP] the modulus of elasticity and forced elasticity was diminished due to the formation of lower molecular weight products.
Figure 1.22: Structures of di- and trivinyl monomers employed in published accounts of CCTP, in the formation of branched polymers
Further research into the scope of branched polymers by CCTP was carried out by Kurmaz et al. in 2004, whereby copolymerisations of EGDMA and dodecane methacrylate (DDMA) were investigated.187 Kurmazet al. postulated that the steric bulk of DDMA would limit interaction of the polymeric radical with pendant vinyl groups, which would restrict cyclisation and crosslinking reactions. Attempts to carry out these reactions in bulk proved less controlled and the gel effect was no longer suppressed only delayed, suggesting a dependence of the reaction on diffusion control of the catalyst. An important realisation was also made that the resulting polymers could be utilised as macromonomers for the formation of star, hyperbranched, graft or block copolymer systems.
In 2005, a conflicting piece of literature was published by Sherrington et al. whereby a comparison was made between use of CCTP and conventional chain transfer polymerisation, the “Strathclyde route”, using dodecanethiol (DDT) as the chain transfer agent in the synthesis of branched copolymers of EDGMA and MMA.120 It was found that the reproducibility of obtaining soluble copolymers using these conditions was poor, in contrast to results reported by Guan for the homopolymerisation of EGDMA by CCTP. Sherrington surmised that perhaps backbiting cyclisation reactions were responsible for the results seen by Guan, as this would lead to a ‘reduced tendency to crosslink’. Their endeavours into conventional chain transfer polymerisation were more fruitful, with soluble polymers being obtained. Multi-detector SEC was used to characterise the synthesised polymers by measurement of the g’ values, which gives an indication of branching in polymers based on a comparison of their IV compared to a linear counterpart, which will be discussed in more detail later in this work. The general trend observed was that polymers synthesised by the Strathclyde route possessed the lowest g’ values, and hence, were more branched than those synthesised by CCTP. This is not unexpected as the level of crosslinker employed in their conventional chain transfer polymerisation was greater than in their CCTP’s.
Further work on the homopolymerisation of divinyl monomers was carried out by Kurmazet al. in 2006, with a comparative study of the homopolymerisations of butane- 1,4-diol dimethacrylate (BDDMA) and BDDA, whereby soluble polymers were obtained.188 However, although a wealth of examples exists for the synthesis of branched polymers by CCTP; the high levels of vinyl functionality retained have not been exploited, which in part, is the aim of this thesis.