Polymers from Renewable Resources - American Chemical Society

Downloaded by UNIV OF MASSACHUSETTS AMHERST on September 26, 2015 | http://pubs.acs.org Publication Date: January 15, 2001 | doi: 10.1021/bk-2000-0764.ch008

Chapter 8

Lipase-Catalyzed Ring-Opening Polymerization of ω-Ρentadecalactοne: Kinetic and Mechanistic Investigations 1

Lori A . Henderson and Richard A . Gross

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*Novo Nordisk BioChem North America Inc., 77 Perry Chapel Church Road, Raleigh, NC 27525 Herman F. Mark Professor, Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201

Abstract The bulk polymerization of ω-pentadecalactone (PDL) at 50°C catalyzed by an immobilized lipase from a Pseudomonas sp. (I-PS-30) deviated from an ideal living system. Analysis of the kinetic data indicated that the polymerizing system consisted of propagating chains that were slowly increasing in number and that M is not a simple function of the [monomer] to [initiator] ratio. It was also shown that the M / M (MWD) becomes more narrow with increasing conversion and that only a fraction of the water available in the system was used to initiate the formation of chains. Based on these results, comparative studies between slow -initiation and slow exchanges involving dormant and active chains were made. The current model used to describe these observations is that which governs the slow dynamics of exchange during propagation. Interestingly, the rate at which monomer was consumed was found to vary from first to second order when examining the kinetics at different stages of the reaction (i.e., 3-13% and 13-40% conversion, respectively). The concentration of polymer chains ([R~OH]) increasing with monomer conversion may explain the increased sensitivity of the reaction rate to monomer concentration as the reaction progresses. In summary, the kinetic analyses along with the analysis of products at low conversion indicates that the PDL polymerization using I-PS-30 proceeds by a chain-reaction mechanism with a slower rate of initiation relative to the rate of propagation. An enzyme -activated monomer mechanism was defined for PDL based on the following: (i) no termination, (ii) a Rp that is dependent on [R~OH] , [Catalyst] and [Monomer] and (iii) the formation of 15-hydroxypentadecanoic acid prior to chain growth. n

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© 2000 American Chemical Society In Polymers from Renewable Resources; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

101 Introduction Recent research efforts exploiting enzymes as catalyst in polymer synthesis has lead to significant technological advances . Several studies on designing architectural polymers without tedious protective/deprotective chemistry, well-controlled block copolymers, high optically pure polymers and macromolecules with controlled stereochemistry have been successfully demonstrated in our laboratory. As the advantages of enzymes in polymer synthesis become more evident, it is clear that the rates of chemical catalysis greatly exceed those for similar transformations using enzymes in organic media. Thus, the goals and objectives in our research laboratory also include developing an understanding of mechanistic pathways to enhance reaction rates. Early studies indicate that the reaction rates were strongly dependent on the enzyme-substrate specificity. It is now known that lipases (of similar origins) will ring-open a range of lactone monomers (4, 5, 6, 7, 12, 13 and 16 membered rings) . Pioneering work carried out by MacDonald on the ringopening polymerization of ε-caprolactone (ε-CL) using porcine pancreatic lipase as the catalyst demonstrated that product molecular weight can be controlled as well as the chain end structure by proper selection of the initiator . Kinetic studies were later pursued by Henderson et. al, to develop an understanding of factors that govern the reaction rates and M for the lipase catalyzed polymerization of ε-CL. This latter investigation also demonstrated that by studying the propagation kinetics while utilizing tools developed from classical polymer syntheses, mechanisms for enzyme-catalyzed polymerizations can be proposed. This protocol was subsequently applied to evaluate the ring-opening of ε-CL and trimethylene carbonate using Novzym 435 as the lipase catalyst . It was found that by varying the monomer to catalyst ratio via the concentration of the enzyme, the rate of propagation varied significantly for both polymerization processes. The above protocol defined by Henderson et. al was based on the kinetic models for living, controlled and immortal polymerizations (see definitions below). The fitting of experimental data to these kinetic models, developed on a hypothesized acylenzyme mechanism for transesterification in organic media, was also the premise of this work involving the ring-opening of PDL. Over the past few decades, research in polymer synthesis has been directed towards achieving living conditions to obtain well-defined architectural polymers. It is one of the most versatile methodology used to engineer macromolecules with well-defined topographies. In addition to controlling and producing uniform size polymers, it provides the simplest and most convenient method for the preparation of architectural polymers. Living polymerizations techniques give the synthetic chemist two particularly powerful tools for polymer chain design: 1) the synthesis of architectural polymers by the sequential additions of monomers and 2) the synthesis (in-situ) of functional polymers by selective termination of living ends with appropriate reagents. Examples of the type of polymers that can be prepared include graft and multiblock copolymers, comb-like, star, and ladder polymers. In general, the well-behaved "living systems" need only an initiator and monomer.


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In Polymers from Renewable Resources; Scholz, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

102 The only criteria for living is that the chain polymerizations proceed without irreversible chain breaking reactions; that is, termination and chain transfer. This includes any reactions proceeding by slow initiation, undergoing exchanges between species of various reactivities (reversible), reversible deactivation such as the equilibrium between dormant and active species and reversible transfer reactions. The effect of these reactions on molecular properties have been studied by Matyjaszewski for several cationic polymerizations (illustrated in Table 1). Downloaded by UNIV OF MASSACHUSETTS AMHERST on September 26, 2015 | http://pubs.acs.org Publication Date: January 15, 2001 | doi: 10.1021/bk-2000-0764.ch008

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Table I. Models that deviate from an ideal living system and their effect on kinetics, molecular weight and molecular weight distributions Rate Slow initiation slower than ILS* (increasing slope)

Termination Transfer

Slow exchange

MW initially higher but approaches ILS

slower than ILS (with deceleration) no effect

no effect (limited conversion)

no effect

may be equal or higher than ILS

lower than ILS

MWD narrows with conversion and is less than 1.35 broader than ILS usually broader than ILS may be very broad even bimodal or narrow and unimodal 6

"ILS is "ideal living system". *MWD decreases with conversion. Chart derivedfromthe material written by Krzystof Matyjaszewski . 7

In a truly living system where initiation is also complete and fast relative to propagation, molecular weights can be controlled by the [monomery[initiator] ratio and the resulting MWD are indeed narrow (MJM ~ 1.0). However, living systems do not necessarily produce well-defined polymers with control over the molecular weight and narrow MWD. To achieve this, the rates of initiation and corresponding rates of exchange must be greater or equal to the rates of propagation. When chain transfer does exist, and is faster than propagation, the propagating reactions are thus described as "controlled" polymerizations. It is important to note that the terms; living, control and immortal polymerizations have recently been redefined due to controversy and confusion in 0

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103 the literature. As of April 1997, the Nomenclature Committee of the ACS Division of Polymer Chemistry has adopted a description of these polymerizations proposed by Matyjaszewski et. al. Some featujes relevant to this investigation are: living is as previously described above, control represents a synthetic method to prepare polymers where transfer and termination may occur but at a low level such that control of the molecular properties are not altered (narrow MWD expected) and it is suggested that immortal be named as "living with reversible transfer" where narrow distributions may result if these reactions are fast compared to propagation. In this investigation, the propagation kinetics were ascertained from the direct initiation of ω-pentadecalactone using the lipase from a Pseudomonas sp. immobilized on Celite 521. Theoretical and experimental Rp expressions were derived and used to (i) generate analytical solutions for assessing living characteristics and (ii) confirm the proposed lipase- catalyzed mechanisms for initiation and propagation via an acyl-enzyme intermediate. Kinetic studies were carried out to less than 40% monomer conversion based on our preliminary work on PDL ring-opening where the propagation kinetics appeared to be diffusioncontrolled at higher levels. Since these processes are also heterogeneous (respect to the catalyst only), many additional factors can govern the rate of propagation at some point during the polymerization. Therefore, the kinetic data was evaluated for livingness initially from 3-40% conversion and then at two different stages of the reaction; 3-13% and 13-40%. Comparisons were subsequently made to models that deviated from a true living polymerization (i.e., slow-initiation and slow exchanges during propagation). Overall, this chapter represents a minor segment of the dissertation research carried out by the author to develop an understanding of enzyme-catalyzed polymerization mechanisms. Additional scientific adventures like: (i) mechanisms underlying lipase action dining polymerization, (ii) an activated monomer approach to synthesis, and (Hi) rate constants for initiation, propagation as well as the rate of polymerization, have been postulated for both εCL and PDL polymerizations catalyzed by lipases . 4

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Experimental Materials The monomer, ω-pentadecalactone (98%, Aldrich), was used without further purification. The lipase PS-30 was a gift from Amano enzymes (USA). This enzyme is from a Pseudomonas sp. and has a specified activity of 30,000 μ/g at pH = 7.0. The adsorption of this lipase onto Celite 521 (referred to as I-PS-30) was as previously described . The resulting lipase preparation was also assayed according to a procedure given elsewhere . 4

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104 Bulk Polymerization of PDL All reactions were carried out in the bulk at 50°C. The ratio of monomer to IPS-30 catalyst was 2/1 w/w. The polymerization procedure using water from the reaction mixture as the initiator is as follows. The monomer (400 mg) and lipase (200 mg I-PS-30) were transferred into separate 6 mL reaction vials and then dried over P 0 in a vacuum desiccator using a diffusion pump apparatus (0.06 mm Hg, RT, 38 h). The contents of the vials were mixed under dry argon atmosphere, securely capped, and immediately stored in dry ice until all transfers were completed. The reaction vials were placed in a constant oil bath at 50°C and removed at predetennined reaction times. The reactions were terminated by removal of the enzyme. This involved dissolution of the residual monomer and polymer in chloroform-d and vacuum filtration. The insoluble enzyme was washed and the filtrates combined. Proton (*H) NMR was then used to analyze the chloroform-d soluble fraction.


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Solution Polymerization of PDL The solution polymerization of PDL was conducted utilizing the same transfer and drying techniques described above. To a reaction vial containing PDL (400 mg) and I-PS-30 (200 mg), 1 mL of chloroform-i/ (anhydrous) was added via a syringe under argon. The vial was placed in an oil bath at 50°C and removed after 1 h. The sample was then processed as described above and resulted in

Polymers from Renewable Resources - American Chemical Society

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