Chapter 13
Thermodynamics, Kinetics, and Mechanisms of Cyclic Esters Polymerization Andrzej Duda and Stanislaw Penczek Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, PL-90-363 Lodz, Poland
Recent advances in pseudoanionic ring-opening polymerization (ROP) of cyclic esters initiated with covalent metal alkoxides and carboxylates are reviewed. General aspects of covalent ROP are discussed: first, thermodynamics, particularly of monomers exhibiting low ring strain. Then, structures of covalent initiators, their behavior in initiation, formation of active species, kinetics, and mechanism of propagation. Polymerization with reversibly aggregating species is analyzed in more detail. Mechanism of initiation and propagation, in the polymerizations induced with metal carboxylates (tin(II) octoate (Sn(Oct) ) is discussed. It is shown that the actually initiating species are formed by converting Sn(Oct) into OctSnOR and/or Sn(OR) in reactions with ROH present in the system. Chain transfer reactions (transesterifications, both intra- and intermolecular), taking place in the polymerization of lactones and lactides, are quantitatively analyzed. 2
2
2
Ring-opening polymerization (ROP) of cyclic esters is one of the preferred methods for a synthesis of high molar mass aliphatic polyesters (1-3) and more recently has even been extended to the enzyme catalyzed processes (4-7). Although polycondensation is at the basis of the major industrial aromatic polyesters (e.g. polyterephthalates), ROP has already been used in industrial production of two aliphatic polyesters, namely poly(e-eaprolactone) (1) and poly(L-lactide) (3). These are the ecologically degradable polymers and may play an important role in future polymer industry. Particularly poly(L-lactide), made from renewable starting materials (carbohydrates of various agricultural origin), can become attractive for countries that do not have their own sources of olefins. On the other hand, ROP of cyclic esters became an efficient tool in studies of the mechanism of anionic and pseudoanionic (covalent) ROP. This is because in many cyclic ester/initiator systems the termination could be excluded. There are, however, two well documented chain transfer reactions. Both are based on transesterification, 160
© 2000 American Chemical Society
161 taking place also in polycondensation: back- and/or end-to-end-biting and chain transfer to foreign macromolecules followed by the chain rupture. Intention of the present paper is to review basic principles of thermodynamics, kinetics, and mechanisms of cyclic esters polymerization. Although this paper is based mostly on the work of our own, it describes general phenomena related directly to the controlled synthesis of poly(aliphatic ester)s. Elementary chain-growth reactions: initiation and propagation are discussed from the point of view of the livingness of polymerization. Eventually, the molecular structures of the growing species are compared with their activity in transesterification.
Thermodynamics Aliphatic cyclic esters of various ring sizes provide high molar mass polyesters. In Table I we compare the thermodynamic data of polymerization: enthalpy (AH ), entropy (àS° ), and a monomer concentration at equilibrium ([M] ), at 298 Κ for oxirane and five cyclic esters of various ring sizes (8-13). In ROP, conducted at constant pressure, the change of enthalpy is mostly due to the monomer ring strain energy, if monomer-polymer-solvent specific interactions can be neglected (14-17). The major contributions to the ring strain come from: deviation from the non-distorted bond angle values (e.g.: for cyclic esters: 110.5° (C-C-C), 109.5° (C-O-C(O)) or 110° (O-C(O)-C)), bond stretching and/or compression, repulsion between eclipsed hydrogen atoms, and non-bonding interactions between substituents (angular, conformational, and transannular strain, respectively) (14, 17). Moreover, polymerization of majority of monomers is accompanied by the entropy decrease. Thus, polymerization is thermodynamically allowed when the enthalpic contribution into free energy prevails (thus when AH < 0 and AS < 0, the inequality \AH \ > -TAS is required; cf. eq 1). Therefore the higher is the ring strain the lower is the resulting monomer concentration at equilibrium (eq 2). V
p
eq
P
p
P
P
AG = Atf -TAS p
p
p
ln[M] =Ai/ /RT-^;/i? e q
p
(1) (2)
(where Τ is the absolute temperature and R the gas constant) Although formation of the three-membered α-lactone intermediates in the nucleophilic substitution reaction of α-substituted carboxylate anions was postulated on the basis of quantum mechanical calculations (18), these cyclic esters have never been isolated. Therefore, we decided to give for comparison in Table I thermodynamic parameters for another highly strained three-membered oxacyclic monomer - oxirane. The four-membered β-propiolactone also belongs to the most strained cyclic monomers and its equilibrium monomer concentration is immeasurably low, namely about 10" mol-L" at room temperature. 10
1
162 Table I. Standard Thermodynamic Parameters of Polymerization for some Selected Oxacyclic Monomers (298 K) 0
Monomer
EO
PL
BL
VL
LA
CL
Ring size
3
4
5
6
6
7
lc
lc
lc
lc
ss
lc
-140
-82.3
5.1
-27.4
-22.9
-28.8
-174
-74
-29.9
-65.0
-25.0
-53.9
13
Conditions
1
klmol'
0
C
&S p l
l
JmoF K~
16
3.6-10" weight fraction 8
Ref
2.8-10 9
-11
2.9· 10 10
2
-2
3
3
3.9-10
1-10'
6-10'
11
12
13
8
EO = ethylene oxide (oxirane), PL = β-propiolactone, BL = γ-butyrolactone, VL = δ-valerolactone, LA = L,L-dilactide, CL = e-caprolactone. Thefirstletter denotes the state of monomer, the second that of polymer: 1 = liquid, c = condensed, s = solution. Standard states: weightfraction= 1 (lc) or 1.0 mol-L" (ss). Calculated from eq 2 (lc) or determined experimentally at the monomer - polymer equilibrium (ss); in order to obtain the approximate value in mol-L" the weightfractionvalue should be multiplied by 10. b
c
1
d
1
However, both the six- and seven-membered monomers, namely L,L-dilactide (LA) and ε-caprolactone (CL) have relatively high equilibrium monomer concentrations, that can not be neglected in the practical considerations: in handling of the final polymer and in studies of polymerization, particularly at elevated temperatures. It has to be remembered, that whenever in the final polymer objects (films, injecting molded pieces) one or more macromolecules are ruptured, the active species may emerge, from which the unzipping will take place until the equilibrium monomer concentration is reached. Fortunately, at least in the case of L,L-dilactide its final product of hydrolysis, lactic acid, is an ecologicallyfriendlycompound. More recently, we have determined the standard thermodynamic parameters for polymerization of L,L-dilactide (12). Its equilibrium concentration appeared considerably high, particularly at elevated temperatures (at which L A is usually polymerized). For a temperature range from 80 to 133°C [LA] changes from 0.058 to 0.151 mol-L" . This six-membered cyclic diester assumes irregular skew-boat conformation, in which two ester groups can adopt planar conformation and has therefore a relatively high enthalpy of polymerization equal to 22.9 kJ-mol" . This is eq
1
1
163 very close to the ring strain of δ-valerolactone and CL, equal to 27.4 and 28.8 kJ-mol' , respectively. Strain comes in these compounds from C-H bonds interactions and from distortion of the bond angles. In contrast, high ring strain in the fourmembered β-propiolactone is mostly because of the bond angles distortion and bond stretching. In the five-membered cyclics ring strain comes almost exclusively from the conformational interactions (14, 16). It is known, however, that the five-membered esters are not strained because of the reduced number of the C-H bond oppositions, caused by the presence of the carbonyl group in the monomer ring. Indeed, for γ-butyrolactone (BL) we have AH = 5.1 kJ-mol" and AS° = -65 J-mol^-K" . These give [BL] » 3-10 mol-L" , whereas the monomer concentration in bulk does not exceed 13 mol-L' . Therefore in the majority of the polymer chemistry textbooks it is stated that BL is not able to polymerize (e.g. one finds in (Man's textbook: "...the 5-membered lactone (γ-butyrolactone) does not polymerize..." (19)). B L is indeed not able to give a high molar mass homopolymer, but this feature sometimes is incorrectly identified with an inability of BL to undergo the ring-opening reaction at all. The equilibrium constants for the first few monomer additions (K , K ...) are not the same, as for an addition to a high polymer (K ) due to an influence of the headand tail-end-groups and usually K and Κχ are larger than K : 1
1
1
p
3
P
1
eq
1
0
u
n
0
n
Ko
X + B L 55=^
X-bl *
Κι
X-bl* + B L
X-bl-bl * (3)
X-(bl)n-bl* + BL =5=^
X-(bl) bl * n+r
(PBL)n
(PBL)n+l
(where X denotes initiator, bl the polyester repeating unit derived from BL, and bl* the pertinent active center) Moreover, the concentration term contribution into the Gibbs energy, usually presented as RTln [M], because -(m)„- * -(m) i-, should be given differently for short chains, when this equality may not hold, namely: n+
AG = AH - T M ° + RTln{[(PBL) ]/[BL][(PBL) ]} n
n
n+1
(4)
n
at the very beginning of the B L ring-opening, when [(PBL) ] » [(PBL) ] may outweight a sum of the enthalpic and entropie contributions (both positive) and eventually gives the negative value of AG . Therefore, even when AH ~ 0 as for reactions 3, formation of short B L oligomers is not thermodynamically forbidden, because apparently -AS° < |Rln{[(PBL) ]/[BL][(PBL) ]} |. n
n
P
n+1
n+i
P
n
164
Al(0'Pr)
3
Q HaÇ'"" \ + η | C=0 H C^ / 2
initiation oligomerization
Ο [| *• Al{[0(CH ) C] OiPr} 2
3
m
3
2
C H
m =1,2,3...
Ο II 3 H-PiCHWsCWOiPr
hydrolysis
(5)
Thus, we studied polymerization (actually, oligomerization) of BL initiated with aluminum /m-isopropoxide (ΑΙ^Ρτ^) (eq 5) (20-22). Both size exclusion chromatography (SEC) (Figure 1 (a)) and mass spectrometry (MS) with chemical ionization (Figure 1 (b)) show, that indeed oligomers are formed - up to decamers as detected by MS, whereas only the tetramer was seen in the SEC traces.
1
1
1 BL
(a)
1
2 n
11 V
=4jJ\]
10
^
20
30
V, mL
200
-1
400
600
800
m/z
Figure 1. Oligomerization of γ-butyrolactone (BL) initiated with Al(0Pr) . SEC trace (a) and mass spectrum (chemical ionization) (b) of the isolated series of the linear oligomers: H-[0(CHi)sC(0)] -OiPr . Conditions of oligomerization: [BL] = 3.8 molL , [Al(UPr) ] = 0.2 mol-L , THF solvent, 80
