Resultados de la Simulación

The allyl-functional carbonate monomer, 2-allyloxymethyl-2-ethyltrimethylene carbonate (AOMEC) was prepared in only one step from a cheap and commercially available precursor, trimethylolpropane allyl ether diol (TMAD). The synthesis of the AOMEC monomer was carried out by the ring closure of TMAD by carbonylation using ethyl chloroformate, as previously reported by He et al. (Scheme 2.2).48

Purification of the crude liquid monomer was performed by vacuum distillation, with the pure AOMEC recovered in 73% yield (Figure 2.2).

Scheme 2.2. Synthesis of 2-allyloxymethyl-2-ethyltrimethylene carbonate (AOMEC)viaring-closure of trimethylolpropane allyl ether diol (TMAD).

Figure 2.2. 1_{H NMR spectrum of 2-allyloxymethyl-2-ethyltrimethylene carbonate (AOMEC) post-}

Initial polymerisation studies were performed in CDCl3at 25 °C with initial monomer

to initiator ratio ([M]0:[I]0) = 30, using a bifunctional organocatalytic system of 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU, 1 mol%) and 1-(3,5-bis(trifluoromethyl)- phenyl)-3-cyclohexylthiourea (TU, 5 mol%), with benzyl alcohol used as initiator (Scheme 2.3). This dual catalyst system was selected as it has previously been shown to have good activity for the ROP of cyclic carbonates whilst still maintaining control over the polymerisation.49,50 _{Monomer conversion for all polymerisations was}

monitored by 1_{H NMR spectroscopy, specifically the change in integral of the}

methylene signals on the pendent ether group of both the carbonate and polymer (atδ = 3.27 and 3.19 ppm respectively (Figure 2.3), with number-average molecular weight (Mn) determined by measuring the integral of the CH2resonance of the benzyl alcohol

against either the integral of the methylene on the ether group (atδ= 3.19 ppm), or the integral of the alkene CH resonance (δ= 5.79 ppm). At 1.0 M AOMEC concentration, the polymerisation proceeded with a linear increase in monomer conversion up to 66% after 45 h, at which point the polymerisation rate decreased rapidly, with no further discernible increase in molecular weight observed by1_{H NMR spectroscopy.}

Scheme 2.3. Synthesis of PAOMEC viaorganocatalyzed ring-opening polymerisation, using a dual DBU/thiourea catalyst system. ROH = mono- or bifunctional alcohol initiator.

Figure 2.3.1_{H NMR spectrum of AOMEC polymerisation taken at 62% monomer conversion. [M]0:[I]0}

= 30, initiated from benzyl alcohol, using a catalyst system of 1 mol% DBU and 5 mol% TU (400 MHz, 293 K, CDCl3; * = CHCl3).

By comparison, the homopolymerisation of MAC under the same conditions proceeds to ca. 80% monomer conversion in just 40 minutes, before rapid retardation of the polymerisation to equilibrium at ca. 90% monomer conversion.49 _{The reduced}

polymerisation rate of AOMEC with respect to MAC is likely the result of both steric and electronic factors - the increased steric bulk of the pendent ethyl group may have some inhibiting effect on nucleophilic attack by the initiating alcohol species or propagating polymer chains at the α-carbon, whilst the pendent ether group of AOMEC should be less electron-withdrawing than the ester group of MAC, resulting

in the α-carbon being less susceptible to nucleophilic attack by the propagating alcohol

species.

In order to increase the polymerisation rate, the concentration of the reaction mixture was increased to 2 M and the catalyst loading increased to 5 mol% DBU and 5 mol% TU. Under these conditions, the [M]0:[I]0 = 30 polymerisation achieved 82%

conversion in only 150 minutes, at which point the polymerisation rate was rapidly retarded. The polymerisation was quenched by addition of acidic Amberlyst 15 ion exchange resin, with purification of the polymer performed by repeated precipitations

into coldn-hexane.1_{H NMR spectroscopy and size exclusion chromatography (SEC)}

analysis demonstrated that the polymerisation proceeds with good control, with the observed molecular weight closely matching the theoretical value, and the polymer possessing narrow dispersity (TheoreticalMn= 5,000 g mol-1_{, observedMn= 5,000 g}

mol-1_,Đ

M= 1.08) (Figure 2.4). The polymerisation study was extended across a range

of degrees of polymerisation (DPs), with a linear correlation observed for both Mn

against [M]0:[I]0, andMnagainst monomer conversion, both characteristic of a living

polymerisation (Figure 2.5 and 2.6). Across the range of molecular weights and conversions, excellent control was maintained over the polymerisation of the monomer, with dispersities ranging from 1.04 for a DP 230 polymer to 1.17 for DP 10 (Figure 2.7).

Figure 2.4.1_{H NMR spectrum of DP 42 PAOMEC initiated from 1,4-butanediol, using a catalyst system}

of 5 mol% DBU and 5 mol% TU (400 MHz, 293 K, CDCl3; * = CHCl3, ** = residual hexane from

Figure 2.5. Plot of number-average molecular weight (Mn) and dispersity (ÐM) against % monomer

conversion for the homopolymerisation of AOMEC. Conditions: [AOMEC] = 2.0 M in CDCl3, [M]0:[I]0=

50 using 1,4-butanediol as initiator, 5 mol% DBU and 5 mol% TU as catalysts.

Figure 2.6. Plot of number-average molecular weight (Mn) and dispersity (ÐM) against initial monomer-

to-initiator concentration ratio, [M]0:[I]0for the homopolymerisation of AOMEC. Conditions: [AOMEC]

Figure 2.7. SEC chromatograms of polymers initiated from pentaerythritol dibenzyl ether (PDE), with [M]0:[I]0ranging from 12 to 290 to give polymers with DPs of 11 to 232, andĐMvalues ranging from

1.17 to 1.04. Samples measured against polystyrene standards using CHCl3as eluent.

Various mono- and bifunctional alcohols were demonstrated to be effective initiators for the polymerisation of AOMEC, including 1,4-butanediol, 1,4-benzenedimethanol and pentaerythritol dibenzyl ether (PDE) (Figure 2.8). In each case, the polymerisation of AOMEC proceeds with excellent control, producing polymers with narrow dispersities and predictable molecular weights as determined with 1_{H NMR}

spectroscopy, by integration of resonances from the initiating species against the methylene resonance on the pendent ether of the polymer (Table 2.1). Polymerisation was also initiated from a bifunctional poly(ethylene glycol) (PEG) macroinitiator (Mn

= 2,000 g mol-1_,Đ

M= 1.04). The reaction proceeds with good control, resulting in a

PAOMEC13-b-PEG62-b-PAOMEC13triblock copolymer with narrow dispersity (Mn=

9,200 g mol-1_{, Đ}

M = 1.07), further demonstrating the initiator versatility of the

Figure 2.8. Mono- and bifunctional alcohol initiators used for the polymerisation of AOMEC. 1 = Benzyl alcohol; 2 = 1,4-butanediol; 3 = 1,4-benzenedimethanol; 4 = Pentaerythritol dibenzyl ether (PDE). Table 2.1. Polymers of AOMEC initiated from different mono- and bifunctional alcohol initiatorsa

Initiator [M]0:[I]0b Conversion

(%)b Time (h) Theor.Mn (g mol-1₎c DPb _Mn (g mol-1₎b ĐMd 1 25 80 3 4,000 21 4,200 1.10 2 20 84 3 3,400 17 3,400 1.16 3 62.5 82 6 10,200 49 9,800 1.09 4 12 86 1.5 2,000 11 2,200 1.17 4 25 87 3 4,400 22 4,400 1.09 4 50 82 6 8,200 40 8,000 1.14 4 100 77 10 15,400 77 15,400 1.13 4 290 80 24 46,400 232 46,400 1.04

a_{Polymerisations performed in CDCl3at 25 °C, [AOMEC] = 2.0 M, using 5 mol% DBU and 5 mol% TU.}b

[M]0:[I]0, monomer conversion, degree of polymerisation and number average molecular weight

determined by1_{H NMR spectroscopy.}c_{TheoreticalMncalculated from [M]0:[I]0× monomer conversion}

× molecular weight of AOMEC (200.23 g mol-1_). d_{Determined by SEC analysis against polystyrene}

Figure 2.9.1_{H NMR spectrum of PAOMEC13-PEG62-PAOMEC13triblock copolymer synthesised using a}

catalyst system of 5 mol% DBU and 5 mol% TU (400 MHz, 293 K, CDCl3; * = CHCl3).

Figure 2.10. Size exclusion chromatograms of PEG prior to chain growth (Mn= 2,000 g mol-1,ÐM= 1.04)

and PAOMEC-PEG-PAOMEC triblock (Mn = 13,300 g mol-1,ÐM = 1.07). Samples measured against

polystyrene standards using CHCl3as eluent.

Triblock copolymers were also synthesised using PAOMEC as a macroinitiator. A DP 80 sample of PAOMEC initiated from PDE was isolated, purified and dried. The DBU-catalysed ring-opening polymerisation ofL-lactide (LLA) using this PAOMEC

macroinititator was then performed, yielding a PLLA93-b-PAOMEC80-b-PLLA93

demonstrated that the polymer possessed a narrow dispersity (ÐM = 1.12), which indicates that the polymerisation of LLA proceeded with good control (Figure 2.12).

Figure 2.11. 1_{H NMR spectrum of PLA93-PAOMEC80-PLA93 triblock copolymer synthesised using a}

catalyst system of 1 mol% DBU (400 MHz, 293 K, CDCl3; * = CHCl3; ** = residual hexane from

precipitation).

Figure 2.12. Size exclusion chromatograms of PAOMEC prior to chain extension (Mn= 15,800 g mol-1,

ÐM = 1.13) and PLLA-PAOMEC-PLLA triblock copolymer (Mn= 30,100 g mol-1,ÐM= 1.12). Samples

measured against polystyrene standards using CHCl3as eluent.

The activity of organocatalysts which can be applied to the homopolymerisation of MAC is limited by the potential for the pendent ester functionality to undergo transesterification, thus generating branched polymers.12 _{However, the absence of}

ester functionality in AOMEC enables the application of less selective catalyst systems to the polymerisation of this monomer. To this end, triazabicyclodecene (TBD) was also applied as a bifunctional catalyst for the polymerisation of AOMEC across a range of monomer to initiator ratios. Applying a catalyst concentration of 1 mol%, the polymerisations were considerably faster than those catalysed by the dual DBU/TU system, with polymerisations up to DP 100 reaching 80% monomer conversion in under 1 h. Polymerisations catalysed by the bifunctional TBD catalyst remain well-controlled, with dispersities not exceeding 1.20 (Figure 2.13).

Figure 2.13. Size exclusion chromatograms of PAOMEC initiated from 1,4-benzenedimethanol using TBD as bifunctional catalyst, with [M]0:[I]0ranging from 10 to 125 to give polymers with DPs of 9 to

98, andÐMvalues ranging from 1.20 to 1.11. Samples measured against polystyrene standards using

CHCl3as eluent.