In this series of articles, the board members of ChemSusChem discussrecent research articles that they consider of exceptional quality and importance for sustainability.T his entry features Dr.P ieter Bruijnincx, who discusses bio-based approaches to new and existing chemicals for large-scale polymera pplications, highlighting that the development of methodologies to obtain key monomers from biomass leads to new chemistry, aids the transition to am ore sustainable chemical industry, and fostersnew interdisciplinary approaches. New and Drop-in Bio-based ChemicalsNature offers many opportunities for the production of renewable platform molecules (i.e.,b uildingb locks for am ore sustainable chemical industry). Of particulari nterest, in terms of impact,a re those biomass-derivedp latformm olecules that can be used as monomersf or large-scale polymer applications. [1] Indeed, world plastics consumption showsaconsistenta nnual growth, reaching 311Mti n2 014. These polymers are unfortunately stilll argely made from fossil resources. Bio-based polymers do find their way into the market and show faster annual growth rates than the fossil-derived ones, yet stillcontribute to less than 5% of the total production volume. Much research and development efforts are currently being devoted, both in industry and in academia, to develop conversion technology and polymer materials to increasethis share. An obviousincentive of using high-value, renewable monomers for large-scale polymer applications lies in the concomitant reduction of the carbon footprint of the chemical and polymer industry.T he use of renewable resources, such as biomass,also offers another,d istinct opportunity:c hemical diversity.T he high functionalgroup density and overall oxidation state of many of the main components of biomass offer versatility in terms of reactivity and access to chemical structures and compounds that can be much less easily and efficientlym ade from conventionalf ossil resources. The building blocks that can be obtainedf rom biomass can then be classifieda se ither drop-in (i.e.,m olecularly identicalt oc urrent petrochemical-derived monomers) or new monomers. The former hold the advantage of access to existing markets and applications, but need to be ablet oc ompete on price (there being no real premium on 'green'). The latter offer ac ompetitive advantage that is based on performance, rather than on price, but require applicationsa nd markets to be developed.

The propagation rate constant for ion pairs (k p ± ) in the polymerization of isobutylene in conjunction with TiCl4 in hexanes/methyl chloride 60/40 (v/v) at −80 °C has been determined using two different diffusion clock methods. The rate constant k p ± was in the range of (0.3−1.0) × 109 L mol-1 s-1, 4 orders of magnitude higher than presently accepted values. The first method involved on-line UV−vis monitoring of the addition of the π-nucleophiles 1,1-bis(4-methylphenyl)ethylene, 1,1-bis(4-tert-butylphenyl)ethylene, and 2-phenylfuran to hydrochlorinated isobutylene n-mers (n = 2, 3, 36). The apparent rate constants of capping, k c K i, and the rate constant of ionization, k i, have been determined. For a given n the k c K i values were identical independent of the nature and nucleophilicity of the π-nucleophile, which was attributed to diffusion-limited addition. Using the diffusion-limited second-order rate constant of k c ∼ 3 × 109 L mol-1 s-1, K i and k - i have been calculated. From the concentration of active chain ends (determined from K i) and the apparent rate constant of propagation for isobutylene (k p ± [active chain ends], determined separately), the absolute propagation rate constant of k p ± = 1 × 109 L mol-1 s-1 was calculated. The second simple diffusion clock method involved competition experiments, i.e., polymerization carried out in the presence of a π-nucleophile, which stops short of completion when all chain ends are capped. From the limiting conversions and number-average degrees of polymerization, k p ± = (3−6) × 108 L mol-1 s-1 have been obtained.

Poly(ethylene 2,5-furandicarboxylate) (PEF) is a polyester from ethylene glycol and 2,5-Furandicarboxylic acid which has gained increasing interest due to its excellent properties compared to chemically similar PET. This paper presents an estimation of the crystallization enthalpy, the crystalline and amorphous density and the crystallization kinetics of PEF. Using Avrami and the Hoffman-Lauritzen theory, HoffmanLauritzen parameters are proposed that relate crystal growth rate of catalyst-free PEF to temperature and molecular weight. Characteristic is a higher activation energy for chain diffusion (U*) for PEF compared PET, which can be attributed to more restricted chain conformational changes. Finally, the crystallization rate of PEF is shown to be significantly affected by catalyst type.

The absolute rate constant of propagation for ion pairs (kp ( ) was determined by the diffusion clock method in the living carbocationic polymerization of isobutylene at different solvent polarity and temperature in conjunction with TiCl 4, Me2AlCl, and BCl3 as Lewis acids. The kp ( ((3.6-5.7) × 10 8 L mol -1 s -1 in hexanes/MeCl 60/40, v/v) was independent of temperature and nature of Lewis acid and increased moderately with increasing solvent polarity to a nearly diffusion-limited value (∼1.7 × 10 9 L mol -1 s -1 ) in pure MeCl. A similar kp ( (∼(5-6) × 10 8 L mol -1 s -1 ) value was obtained in the nonliving polymerization of isobutylene in conjunction with EtAlCl2 in hexanes/MeCl 60/40 (v/v) at -80 °C, indicating that living and nonliving polymerizations proceed on identical propagating centers. The apparent equilibrium constant of ionization (activation) K i app () KiKD0, where Ki is the absolute equilibrium constant of ionization and KD0 is the equilibrium constant of TiCl4 dimerization) was calculated from the apparent and absolute rate constant of propagation. The rate constant of deactivation, k-i was determined from the conversion vs polydispersity plots. From Ki app and k-i, the apparent values of ki (ki app ) kiKD0, where ki is the absolute rate constant of ionization) were also calculated. On the basis of the results, the observed large differences in the overall polymerization rates with different solvent polarity and temperature can be attributed solely to the changes in the active center concentration, which decreases with decreasing solvent polarity and increasing temperature. From the temperature dependence of K i app , the apparent standard enthalpy () ∆Hi°+ ∆HD0°) and entropy () ∆Si°+ ∆SD0°) of ionization, and from the temperature dependence of ki app and k-i, the apparent activation enthalpy and entropy of the activation/deactivation process were calculated.

High Throughput experimentation has been well established as a tool in early stage catalyst development and catalyst and process scale-up today. One of the more challenging areas of catalytic research is polymer catalysis. The main difference with most non-polymer catalytic conversions is the fact that the product is not a well defined molecule and the catalytic performance cannot be easily expressed only in terms of catalyst activity and selectivity. In polymerization reactions, polymer chains are formed that can have various lengths (resulting in a molecular weight distribution rather than a defined molecular weight), that can have different compositions (when random or block co-polymers are produced), that can have cross-linking (often significantly affecting physical properties), that can have different endgroups (often affecting subsequent processing steps) and several other variations. In addition, for polyolefins, mass and heat transfer, oxygen and moisture sensitivity, stereoregularity and many other intrinsic features make relevant high throughput screening in this field an incredible challenge. For polycondensation reactions performed in the melt often the viscosity becomes already high at modest molecular weights, which greatly influences mass transfer of the condensation product (often water or methanol). When reactions become mass transfer limited, catalyst performance comparison is often no longer relevant. This however does not mean that relevant experiments for these application areas cannot be performed on small scale. Relevant catalyst screening experiments for polycondensation reactions can be performed in very efficient small scale parallel equipment. Both transesterification and polycondensation as well as post condensation through solid-stating in parallel equipment have been developed. Next to polymer synthesis, polymer characterization also needs to be accelerated without making concessions to quality in order to draw relevant conclusions.

A poly(styrene-b-isobutylene-b-styrene) (SIBS) triblock polymer is employed as the polymer drug carrier for the TAXUS Express2 Paclitaxel-Eluting Coronary Stent system (Boston Scientific Corp.). It has been shown that the release of paclitaxel (PTx) from SIBS can be modulated by modification of either drug-loading ratio or altering the triblock morphology by blending. In the present work, results toward achieving release modulation of PTx by chemical modification of the styrenic portion (using hydroxystyrene or its acetylated version) of the SIBS polymer system are reported. The synthesis of the precursor poly[(p-tert-butyldimethylsilyloxystyrene)]-b-isobutylene-b-[(p-tert-butyldimethylsilyloxystyrene] triblock copolymers was accomplished by living sequential block copolymerization of isobutylene (IB) and p-(tert-butyldimethylsiloxy)styrene (TBDMS) utilizing the capping-tuning technique in a one-pot procedure in methylcyclohexane/CH3Cl at -80 degrees C. This procedure involved the living cationic polymerization of IB with the 5-tert-butyl-1,3-bis(1-chloro-1-methylethyl)benzene/TiCl4 initiating system and capping of living difunctional polyisobutylene (PIB) chain ends with 1,1-ditolylethylene (DTE) followed by addition of titanium(IV) isopropoxide (Ti(OIp)4) to lower the Lewis acidity before the introduction of TBDMS. Deprotection of the product with tetrabutylammonium fluoride yielded poly(hydroxystyrene-b-isobutylene-b-hydroxystyrene), which was quantitatively acetylated to obtain the acetylated derivative. The hydroxystyrene and acetoxystyrene triblock copolymers have acceptable mechanical properties for use as drug delivery coatings for coronary stent applications. It was concluded that the hydrophilic nature of the endblocks and polarity effects on the drug/polymer miscibility lead to enhanced release of PTx from these polymers. The drug-polymer miscibility was confirmed by differential scanning calorimetry and atomic force microscopy evaluations.

The anionic ring-opening polymerization of L-lactide was initiated by dipotassium-polyisobutylene-alcoholate telechelic polymer to yield poly(L-lactide)-block-polyisobutylene-block-poly(L-lactide) triblock copolymer, a partially biodegradable thermoplastic elastomer. The pure triblock copolymer was obtained by gradient column chromatography on silica gel. The molar mass and molar mass distribution of the block copolymer was ascertained by SEC and quantitative ' H NMR spectroscopy. It showed two glass transitions and microphase separation.

The living carbocationic polymerization of 2,4,6-trimethylstyrene (TMeSt) was studied using the 1-chloro-1-(2,4,6-trimethylphenyl)ethane/BCl3 initiating system in CH2Cl2 from −70 to −20 °C. The equilibrium constant of ionization (K i) was calculated from the cation concentration determined by UV−visible spectroscopy. From the apparent rate constant of propagation (k p app) and K i, the absolute rate constant of propagation for ion pairs k p ± = 1.4 × 104 L mol-1 s-1 was calculated at −70 °C. The rate constant of ion collapse, k - i was determined from the polydispersity vs conversion plots. From K i and k - i, the rate constant of ionization, k i were also calculated. From the temperature dependence of K i, and k - i the enthalpy and entropy of the ionization and deactivation were obtained, respectively. Competition experiments i.e., polymerization carried out in the presence of 2-chloropropene as a π-nucleophile, were also carried out at −70 °C to determine k p ± separately. NMR spectroscopy, gel permeation chromatography, and elemental analysis suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting number-average degree of polymerization and the known rate constant of capping k c = 62.0 L mol-1 s-1, k p ± = 8.3 × 104 L mol-1 s-1 was determined. This value is similar to that determined from k p app and K i; thus, the utility of competition experiments is corroborated.