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Branched polymers by in situ termination with silyl enol ethers Case Study

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Added on: 2023-02-27 05:45:28
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Part 1 : Case for support

Background

Branched polymers, telechelic oligomers and block copolymers produced by cationic polymerizations are usually synthesized by sequential end capping of living macrocarbocations1 and several suitable systems are available.2-5 However, unlike the related anionic polymerizations, cationic polymerizations are more difficult to control.6 For this reason, commercial scale cationic polymerizations, targeted at the preparation of telechelic oligomers, usually rely on termination reactions to provide chain-end functionality. We have recently shown that a third method for adding chain-end functionality is possible7-11 and we propose here that this methodology can be extended to the synthesis of highly branched polymers. Ab initio cationic polymerization takes advantage of the fact that under conditions of fast propagation chain-end capping by a species added to the polymerizing mixture at the start of the reaction (i.e. before initiation) is feasible. Ideally, such a species reacts with the propagating carbocation with a rate constant equal to or lower than the rate of propagation. The usual terminating species (e.g. alcohols, ammonia etc.) are not suitable since they are deliberately selected to react at high rates with the chain-end carbocation. The PI considered that certain silyl enol ethers might be suitable as ab initio chain-capping agents. This group of compounds have also been employed as chain-capping agents in living cationic polymerizations in which they are added in a separate step at the end of the polymerization.12,13 Similarly silyl enol ethers have also been used to cap chain ends formed after atom transfer radical polymerization.14 In our previous work we exploited the well-known fact that alkylation of silyl enol ethers is usually followed by rapid desilation to give ketones. The feature ensures that the kinetic chain is stopped as soon as the silyl enol ether is alkylated. In this proposal it is intended to use the dual reactivity of macro-carbocations to react with both vinyl groups and silyl enol ethers. Thus, monomers with silyl enol ether functionality will act as branching points, as shown in figure 1. The method is similar to the self-condensing vinyl polymerization (SCVP), which was first introduced by

Frechet. et al,15-17 as a route to hyper-branched polymers. Several variants of SCVP have been reported including: cationic polymerizations;15 nitroxide mediated radical polymerization;18 atom transfer radical polymerization (ATRP);19 reversible addition fragmentation chain termination (RAFT) polymerization20-22 and group transfer polymerization.24 Also, cross-linking polymerizations that use transfer to curtail gelation provide an economic route that does not require the synthesis of special branching monomers.25,26
These methods can be regarded as divergent techniques whereas the new branching-by-termination method, described in this proposal, can be regarded as convergent vinyl self-condensation. As far as we are aware, no examples of convergent self-condensation as a route to hyper or highly branched polymers are known. The use of a termination step, which occurs at the ?-chain end,27 in the coupling reaction has three significant advantages:

1) It allows the ?-chain end to provide chemical functionality to the polymer (R’ in scheme 1) via the use of a functional initiator.

2) The use of novel monomers containing both vinyl and silyl enol ether functionality offers the intriguing possibility of tuning the macromolecular architecture by varying the reactivity of the two groups. If the silyl enol ether groups react quickly, then this produces short “macromonomer” chains, which will subsequently be incorporated into the longer chains that grow once the terminating silyl enol ether groups are exhausted. This should result in a comb-like molecular architecture. Conversely, a “hyperbranched” structure (with evenly distributed branching) results when the reactivity of the vinyl group is similar to that of the monomers and the silyl enol ether groups react more slowly.

3) The current divergent routes to highly branched polymers lead to polymers that are extremely polydisperse in molecular weight. Müller et al have shown that, in the extreme case of homopolymerization of an inimer (a vinyl monomer containing an initiating group), this is because there are two different active centres (initiating and propagating centres).28 Müller et al have, both theoretically and experimentally shown that both cationic-SCVP and ATRP-SCVP produce highly polydisperse and multi-modal materials19,28 and we have recently confirmed these results by producing highly branched polymers made via RAFT radical polymerizations.20-23 Müller’s treatment also shows that hyper and highly branched polymers produced by condensation of AB2 monomers, the original system proposed by Flory,29 should have molecular weight distributions that are half as broad as those prepared by divergent SCVP. It is our belief that the convergent scheme proposed here should share some of the advantages of the AB2 system over that of SCVP.

The bulk of this proposal is an experimental investigation aimed at producing branched polymers via the above, and similar, reaction schemes. The above considerations, however, highlight the importance of modelling as a tool to understand and control the progress of the reaction and the nature of the polymeric materials produced. To this end we propose a small and complementary modelling investigation aimed at predicting distributions of molecular weight, branching and polymer architecture from the appropriate chemical rate equations. This exercise, supported as far as possible by experimental data on reaction rates and product, is aimed at checking whether the reaction is progressing as envisaged and, having done this, suggesting optimum reaction conditions for a given product.

Relevance to beneficiaries

Branching has a very significant effect on polymer processing, rheology, mechanical properties and biological properties (e.g. branching can effect the adsorption of proteins and branched polymers have applications in gene delivery). Branching also tends to influence polymer solution viscosities and it has been shown, by one of us, that branching has a significant effect on the lower critical solution temperatures of polymers in aqueous solution. The latter observations will eventually have a significant impact on the emerging nanotechnologies whilst the former effects are already of huge importance to most of the structural applications of bulk polymers, such as polyethylene. The synthetic methods described here are highly amenable to scale up so that it is envisaged that large quantities of material could be prepared in this way; presenting a real possibility for early exploitation. Many applications of hyperbranched polymers are possible but a long-term goal of these research efforts is to produce amphiphilic systems with potential as delivery vehicles. It is therefore highly likely that eventually the pharmaceutical and health care sectors will benefit. For example, surfactant-like branched copolymers, which have hydrophobic inner regions with many polar end groups, might be envisaged. As well as having potential in drug delivery, such materials would have many environmental uses (dispersion of oil slicks, replacements for conventional phosphate surfactants etc.). Thus, industries based on environmental clean up may find uses for materials prepared using this methodology. We are also actively exploring the use branched polymers as bioresponsive elements. Thus the polymers produced in this programme will be of immediate interest to our collaborators and are likely to be the focus of new project in this area in the future.
The analytical methodology developed in this project, coupled with the modelling effort, will also impact on the work of others in field of branched polymers. The particular chemistry provides the possibility of generating accurate data (from FTIR-SEC analyses) on degree of branching as a function of molecular weight, supplemented by our modelling results, both of which will be of direct interest to the polymer reaction modelling community. During this aspect of the work, we be able to validate results derived from triple detection (viscometry, light scattering and refractive index)-SEC by comparison to absolute data on branching distribution derived from FTIR-SEC analyses.

Programme

In this programme it is proposed that the concept of ab initio cationic polymerization and the alkylation of propagating radicals, with silyl enol ethers, can be used in syntheses that will produce hyperbranched polymers as shown in Figure 1. This illustration shows how the use of a silyl enol ether vinyl monomer, 1, in cationic polymerization can be used to produce hyperbranched polymers. However, radical polymerization in the presence of 1, 2, 3, 4 or 5 will also lead to hyperbranched structures.

Thus the branching monomer 1 can be copolymerized in cationic polymerizations and branching is derived from Lewis acid mediated addition of the propagating cation to the silyl enol ether. Radical polymerization in the presence of either 1, 2, 3, 4 or 5 will also progress in the same manner but branching will occur by radical addition to the silyl enol ether. As indicated above, varying the branching monomer will change the form of the molecular weight and architecture distribution because the reactivity ratios are different. The effect of structure on the polymerization will be further studied by using the acrylate analogues 4 and 5.

Aim

To use the alkylation of silyl enol ethers to prepare highly branched copolymers. Our main objective in this section of the work is to polymerize a wide range of monomers, in which the propagating species is predicted to have differing reactivity towards the SEE and to examine a range of monomers with useful functionality in our biomaterials programmes. We will also alter the reactivity of 1, 2 and 3 by changing the structure of R and by using acrylate analogues of 2 and 3; i.e. 4 and 5.

Objectives

(1) To prepare silyl enol ethers suitable for use as branching monomers in both cationic and radical polymerizations: 1, 2, 3, 4 and 5
(2) To prepare highly branched polymers using a variety of monomers and silyl enol ethers via both cationic and radical polymerization.
(3) To characterize the materials prepared in (2) by combined application of SEC, GPEC, MALDI-TOF mass spectrometry, FTIR and NMR spectroscopy, in particular obtaining branching density as a function of molecular weight across the SEC spectrum.
(4) To compare the molecular weight and branching distributions of these polymers to those produced using self condensing vinyl polymerizations
(5) To produce a theoretical model for predicting distributions of molecular weight, branching and polymer architecture, for comparison with experimental results.

Silyl enol ether preparation

The programme will require the synthesis of the branching monomer, 1, using the previously reported method of Hirao et al.30 Also, the programme will use the branching monomers 2 – 5. We have already prepared, 2 and 3. Our preferred method of synthesis of these compounds is the use of the trimethyl silyl triflate/triethyl amine procedure, which in our hands leads to high yields and high purity following distillation.8-11

Synthesis of highly branched polymers by cationic polymerization

The first aspect of the programme will be the synthesis of highly branched polymers using, in the presence of 1, the cationic polymerization of a representative set of monomers: isobutyl vinyl ether (iBVE); methyl vinyl ether (MVE), 2-methylpropene (2-MP) and styrene (S). We have already shown that poly(vinyl ether) end-functional oligomers can be prepared by ab initio cationic polymerization in the presence of silyl enol ethers.7-11 The monomers will be polymerized at various monomer concentrations, various concentrations of 1 and at a range of temperatures. The reactions will be sampled in order to study the evolution of the molecular weight distribution and monomer conversion with time. The initiating systems are also an important variable and in particular the Lewis acid has a critical effect on the propagation rate constants and the rate constants for alkylation. The activity of the Lewis acids is also controlled, in part, by the solvent. Thus, studies involving systematic changes of the Lewis acids and the solvent conditions will be performed. The effective analysis of these polymers will require the availability of linear analogous polymers, prepared using cationic polymerization. These will be prepared by living cationic polymerization. However, it is unlikely that we will be able to prepare linear polymers, using living techniques that can be used to cover the high molecular weight range. In these cases we will restrict ourselves to either using conventional standards (e.g. polystyrene prepared by anionic polymerization) or comparisons within sets of polymers prepared with various amounts of branching agents. Experiments that involve carrying out sets of polymerizations will be facilitated by using either our carousel reactors (in which we can perform 12 simultaneous polymerizations) or our ChemSpeed synthesis robot.

Synthesis of highly branched polymers by radical polymerization

Methyl methacrylate (MMA) chain end functional oligomers have recently been prepared in preliminary work by us using the radical polymerization of MMA in the presence of 2-phenyl-2-(trimethylsilyl)ethene.8b Therefore, the method outlined above should be also applicable to radical polymerizations. The same experimental design strategies outlined above will be followed for investigations of radical polymerizations of a representative set of monomers: MMA; ethyl acrylate; acrylamide (AA): glycerol monomethacrylate (GMA); Dimethylamino ethyl methacrylate (DMAEMA); S and vinyl acetate (VA). The latter set of monomers covers a wide range of monomer reactivity and radical stabilities and includes two functional monomers (GMA and DMAEMA). The use of each branching monomer 1 - 5 will be studied. The main variables to consider in this section of the work will be: polymerization temperature; monomer concentration and the concentration of branching agent. As in the previous section we will sample the reactions and follow the progress of the molecular weight distributions and monomer conversions over time. In Our previous work, involving the use of RAFT to prepare highly branched polymers by radical polymerization,20-23 we also prepared standard linear polymers with narrow molecular weight distributions. Therefore, we will use RAFT to prepare standard linear polymers in cases where these are not commercially available.

Analysis

It is possible to extract information on the progress of ab initio cationic polymerization by assessment of chain end functionality, obtained by NMR, MALDI-TOF MS, and combining this with molecular weight data from osmometry and SEC (using a combination of detectors). NMR yields data on average end group structures and branching junctions (phenyl ketone groups when 1 is used) and will be combined with measurements of absolute number average molecular weight, from osmometry, to give values for chain end functionality. Also, chromatographic methodology for the analysis of similar block copolymers is applicable and has recently been used by us in the analysis of poly(isobutene-block-styrene)s.31 It is intended to adapt this methodology to preparative separation of the highly branched polymers with respect to composition. This procedure will then allow us to produce full analysis across the compositional distributions, using MALDI TOF MS, SEC, FTIR and NMR. On-line SEC-viscometry measurements provide semi-quantitative data on the degree of branching by providing the Mark-Houwink exponent (?). For example, in our previous work20-23, 32 we produced polymers with ?-values approaching 0.2, which is well below the theoretical limit for linear polymers.
The use of on-line viscometry measurements is well-established in the literature and they are regarded as one of most appropriate ways of assessing the degrees of branching. However, in order to provide maximal information to constrain and inform the simulations it is advantageous to produce quantitative values for the degrees of branching. The synthesis explained here produces a ketone at the branch point. Quantitative analysis of this group across an SEC chromatograph will allow us to produce data on branch point density as a function of molecular weight. Such data constitute a strong test of the simulations and the proposed reaction rate equations. In previous work we used NMR spectroscopy to produce average values for degrees of branching and branching efficiency.32 It would be possible to produce quantitative branching data across the distributions using on-line SEC-NMR. However, the same data can be provided by FT-infra red spectroscopy on-line with SEC and funds are requested to purchase the necessary hard-ware to extend our triple detection SEC by incorporating the latter detector. The technique was originally developed to provide similar data on polyolefins produced on a commercial scale by the petrochemical industry and can produce distributions composed of a larger number of data points than the NMR techniques. It is also much easier to integrate a FTIR red detector, rather than an NMR spectrometer, with our triple detection apparatus. SEC with the FTIR red detector combined with data from viscometry will allow us to produce data on branch point frequency as a function of hydrodynamic volume. This data can then be combined with data from viscometry and light scattering to produce plots of branch frequency, hydrodynamic volume and molecular weight. The use of a spectroscopic determination of branching while simultaneously producing “Mark-Houwink” plots will be of wide application beyond this project and will be of interest to several other groups in the field.
The programme is mainly concerned with the examination of a new synthetic method. However, many of the materials prepared will be new and in order to facilitate collaboration we will determine some key physical properties. Thus the glass transition temperatures will be determined by DSC and solution viscosities will determined.

Modelling

We require a model of a batch reaction synthesis of a polymer using the scheme outline in figure 1. Simple solution (numerical or analytical) of reaction rate equations provides information on the conversion of each reacting species as a function of time, and statistical information on which species are chemically linked and the time of their reaction. From such information one can produce a Monte Carlo algorithm to generate computationally a representative set of molecules. Given enough molecules so generated, it is straight forward to obtain distributions of molecular weight, branching, or other measures of the size and shape of the polymers. In particular, the branching density as a function of molecular weight, measured using SEC with an FTIR detector, can be directly predicted and provides a strong test of the model.
If there are typically a large number of monomers between each branch-point, one can save computational time in the Monte Carlo scheme by building polymers on a strand-by-strand, rather than monomer basis. Such methodology was employed by Tobita33 to model the molecular structure of low density polyethylenes, and developed further by us34 for the modelling of the batch reaction of polyolefins with multiple metallocene catalysts.
In addition to the initial reagent concentrations and final degree of conversion, this type of modelling requires the relative rate constants for the different reaction steps as input. With reference to the mechanism shown in figure 1, the ratios of rate constants (k2/k1, k3/k2) are easily obtained from composition data of low conversion polymerisations. k2/k1 can be obtained from ratios of the polymerized residues of M1 (from figure 1) and the added branching monomer (1, 2, 3, 4 or 5) using NMR spectroscopy. k3/k2 can similarly be obtained from the fraction of ketone groups, at low conversion, relative to the total amount of the branching monomer using both NMR and FTIR spectroscopy. Use of this relative rate constant data from low conversion experiments will then be used to predict, at any conversion and initial composition, branching frequencies, molecular weight distributions and chain end functionality distributions, which can be measured precisely by SEC-FTIR analysis. Thus, the work will be significantly enhanced by the availability of accurate experimental data on the distributions of branching frequency facilitated by the data from on-line SEC-FTIR analyses, providing a genuine and meaningful test of the model.
If the model provides a reasonable description of the available experimental data (some fine-tuning may be necessary given the wealth of data available from sampling the reaction at different times, varying reagent concentrations, and analysis of overall branchpoint and functional group densities in the final and intermediate products), it will be possible to use it to infer useful details of the molecular structures generated, and how these can be modified through the reactor conditions. If the model is to be used in such a predictive mode, it is useful to have data on absolute rate constants for the reactions (since this governs the overall reaction time). Absolute propagation rate constants in cationic polymerizations are notoriously ill-defined. However, following a very substantial amount of work in this area we will be able to employ rate constants from previously defined systems; that is by using polymerization conditions define in the previous determinations we can fix the appropriate parameters. More defined data on both propagation rate constants and reactivity ratios are available for radical polymerizations.

Dissemination and exploitation

The results of this work will be of interest to many scientific communities (organic chemistry, polymer chemistry, materials, reactor engineering) and will be reported in the usual international scientific literature. However, it is also very likely that exploitable intellectual property (IP) will also be produced at an early stage of the work. Therefore, publication will initially be delayed until such time that both the university’s and the investigators/research fellow’s IP position has been fully protected. We have already agreed that the majority of the intellectual property lies with the synthesis aspects of the work and as such will be protected and owned by the University of Excellent Research. Following IP protection potential exploitation routes with third parties will be explored (e.g. licensing, out sourcing of marketing and manufacture etc.). The work will also be presented during at least one national meeting of the Polymer Chemistry community and at least one international meeting such as the Autumn ACS meeting 2008. Several other routes are available for dissemination to the industrial community including the regular meetings of the IRC in Polymer Science.

Management

The postdoctoral researcher (Dr. P. Chem), under the direct supervision of Dr F. Applicant will oversee the day-to-day running of the practical side of the project (which constitutes the bulk of the work). The modelling tasks will be performed by Dr S. Fiddel. These three people will constitute the project team. In addition to regular e-mail contact and informal meetings (Dr. Applicant has just begun a fellowship which will ensure he is regularly in University of Outstanding Research), we anticipate meeting formally at least 4 times per year to ensure the transfer of information between experimental and modelling tasks.

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