Establishing a-bromo-g-butyrolactone as a platform for synthesis of functional aliphatic polyesters – bridging the gap between ROP and SET-LRP†

Utilizing α-bromo-γ-butyrolactone (αBrγBL) as a comonomer with e-caprolactone (eCL) or L-lactide (LLA) produces copolymers with active and available grafting sites, e.g., for SET-LRP, where the choice of the grafting monomers is limited only by one's imagination. This was deduced by utilizing a wide range of different acrylates of varying polarities and was realized with the aid of a fluorinated alcohol, 2,2,2-trifluoroethanol, which acts as a universal solvent for both the hydrophobic macroinitiators and the grafting monomers. Using αBrγBL successfully provides a simple route to merge the two polymerization methodologies, ROP and SET-LRP. αBrγBL inherently meets all of the prerequisites to act as a platform monomer for the synthesis of functional aliphatic polyesters, i.e., it is inexpensive, available, and able to form isolated grafting sites along the polymer chain. The copolymerization of αBrγBL together with two of the most commonly used cyclic ester monomers, e-CL and LLA, proceeds with a high degree of control and a linear relationship between the feed ratio of αBrγBL and its composition in the copolymer. The formation of isolated units of αBrγBL in the copolymer is visualized by the reactivity ratios of the copolymerization reactions and confirmed by 13C-NMR spectroscopy. The incorporation of isolated αBrγBL is the feature that makes this class of copolymers unique, and it can be considered to provide a route to the “perfect graft copolymer” with a degradable backbone.


Introduction
In the early era of polymer science, great emphasis was placed on the mere ability to create polymers, whereas today, functionality is the key.Functional polymers open up applications with endless possibilities, where properties can be tailored, altered, and/or maintained over the complete lifetime of the material.In light of this, the focus today is on conferring function to the main chain of the polymer.One class of polymers that is inherently of great value for many applications is aliphatic polyesters; because of their ester functionality, they most oen degrade within a reasonable time frame.Unfortunately, many of these monomers lack sites that allow alterations and modications of the polymer backbone.
Therefore, a major scientic focus has been on imparting different functionalities to aliphatic polyesters.The routes pursued range from copolymerization with functional monomers [1][2][3][4] and post-polymerization modication 5,6 to functional initiators, 7,8 to name a few.They all have their pros and cons, yet copolymerization with functional monomers offers a high degree of functionality, most oen with the retention of a high degree of control.Important aspects to consider when copolymerizing a functional monomer are its reactivity, maintenance of its function, and how the functionality is spread throughout the polymeric chain.The routes for the synthesis of functional monomers appropriate for ring-opening polymerization are, in theory, innite.In the literature, there exist descriptions of numerous elegant monomers that yield the desired polymeric properties, but they are oen hindered by lengthy synthetic routes resulting in low yields at high cost, thereby limiting their applicability to a large scale.
Hence, we felt inspired to nd an inexpensive and straightforward monomer that can bestow the desired functionality on commonly used aliphatic polyesters.We therefore turned our attention towards a relatively unexploited family of lactones: the g-lactones. 92][13][14] Not considering polymers synthesized under extreme pressure and high temperature or the formation of oligomers, [15][16][17][18][19] this statement still holds true.
Many monomers that are unable to homopolymerize, however, can be copolymerized, as shown with g-butyrolactone, 1,3-dioxane, 1,4-dioxane, etc. 20,21 In 1964, Tada et al. performed the rst copolymerization of g-butyrolactone together with the more ring-strained monomer, b-propiolactone. 224][25][26][27][28][29] The main objective for forming these copolymers includes the use of an inexpensive, exible, and degradable monomer that, together with b-butyrolactone, resembles polymers produced by bacterial fermentation, i.e., polyhydroxybutyrate. [23][24][25][26][27][28][29] Recently, g-butyrolactone's ability to form isolated units along the polymer chain at moderate conversion during copolymerization has been highlighted. 11,13ombining ring-opening polymerization with controlled radical gra polymerization offers the ability to specify the gra copolymer for a specic task.Therefore, much effort has been concentrated on developing monomers with these characteristics, starting in 1999, when 3-caprolactone with an attached ATRP initiator at the g-position was synthesized. 1 Others have realized this feature by synthesizing a-substituted 3-caprolactones with chlorine and bromine thus successfully producing radical gra copolymers with degradable backbones. 2,30,31However, factors that are oen overlooked include how the monomers with radical initiating sites are dispersed throughout the polymer chain and what the useable graing monomers are.
To overcome these shortcomings, we turned our attention towards a scarcely used building block, a-bromo-g-butyrolactone.We believe that this monomer inherently has desirable properties such as being inexpensive and having functionality and low homo-reactivity, together with being twosided, i.e., both having the ability to be ring-opened while at the same time functioning as a SET-LRP initiator.Its anticipated inability to form a homopolymer should result in isolated sites that are susceptible to SET-LRP, thus providing "the perfect gra copolymer".The hypothesis is that this building block will provide an easy way to merge SET-LRP with controlled ROP.Our aim is to establish a-bromo-g-butyrolactone as a platform monomer for the synthesis of functional aliphatic polyesters.This will be achieved by copolymerizing a-bromo-g-butyrolactone with L-lactide or 3-caprolactone, followed by a sequential graing of acrylates of varying polarities: n-butyl acrylate, methyl methacrylate, and 2-hydroxyethyl methacrylate, under SET-LRP conditions with the aid of the universal solvent, 2,2,2-triuoroethanol.

Experimental
Materials 3-Caprolactone (3CL) (Aldrich) was dried over calcium hydride for at least 24 h and subsequently distilled at reduced pressure under an inert atmosphere prior to use.L-Lactide (LA) ($99%, Boehringer Ingelheim) was recrystallized twice from toluene (HPLC grade, Fisher Scientic, Germany) and once from dry toluene (99.8%,Sigma-Aldrich, Sweden) and was dried in vacuo for at least 48 h prior to use.a-Bromo-g-butyrolactone (aBrgBL) (97%, Sigma-Aldrich, Sweden) was dried over molecular sieves (3 Å) and stored under an inert atmosphere prior to use.

Polymerization reactions
ROP copolymerization of aBrgBL with LLA and 3CL.All reaction vessels were dried in an oven at 150 C for 48 h before use.Firstly, the desired amounts of benzyl alcohol and Sn(Oct) 2 were added to the ask followed by addition of the monomers LLA or 3CL together with aBrgBL.The amounts of main monomer (3CL, LLA), initiator, and Sn(Oct All reactions were performed in the bulk under an inert N 2 (g) atmosphere at 110 C. All of the reactants were weighed into a two-neck round-bottom ask (25 mL) under a nitrogen atmosphere in a glovebox (Mbraun MB 150-GI).Each ask was equipped with a magnetic stirring bar and sealed with a two-way valve and a septum.All reactants were stirred at a constant temperature that was maintained (AE2 C) using an IKAMAG® RCT basic safety control magnetic stirrer.
Aer 20 h, the reaction mixture was cooled to room temperature, and the copolymers were dissolved in chloroform and precipitated three consecutive times in methanol.The precipitates were dried under reduced pressure for 4 days.
Graing via SET-LRP.All the SET-LRP reactions were carried out in 25 mL double-necked round-bottom asks equipped with three-way valves and septa under a nitrogen atmosphere.In a typical SET-LRP reaction, the catalyst, Cu(II)Br 2 , the ligand, tris [2(dimethylamino)ethyl]amine (Me 6 TREN), and the monomers, methyl methacrylate (MMA), butyl acrylate (nBuAc), or 2-hydroxyethyl methacrylate (HEMA), were solvated in 2,2,2-triuoroethanol (TFE) with an initiator : catalyst : ligand : monomer ratio of 1 : 1 : 2 : 50.In other 25 mL double-necked round-bottom asks equipped with three-way valves and septa, the different initiator polymers, poly(3CL-r-aBrgBL) or poly(LLAr-aBrgBL), that were chosen based on the highest amount of incorporated aBrgBL, were dissolved in TFE.Both systems were cooled in dry ice, and the air was removed by three freezevacuum-nitrogen-thaw cycles.The initiator polymers were subsequently transferred into the rst round-bottom ask under an inert atmosphere.A 10 cm length of copper wire (Cu(0)) with a diameter of 20 gauge was then added to the ask.The ask was immersed in a thermostatted oil bath at 25 C. Samples were withdrawn at specic time intervals under a nitrogen atmosphere, and the conversion and molecular weights were determined by 1 H-NMR and GPC.The reaction was ended aer 15 h.The resulting polymer was dissolved in chloroform and precipitated in cold methanol and then dried at room temperature until all the solvent had evaporated.

Instruments
Nuclear Magnetic Resonance (NMR). 1 H-NMR (400.13MHz) and 13 C-NMR (100.62 MHz) spectra were recorded with a Bruker Avance 400 spectrometer at 298 K.For measurements, either $10 mg ( 1 H-NMR) or $100 mg ( 13 C-NMR) of the polymer was dissolved in 0.8 mL CDCl 3 in a 5 mm diameter sample tube.The spectra were calibrated using the residual solvent signals, 7.26 ppm ( 1 H-NMR) and 77.0 ppm ( 13 C-NMR), for CHCl 3 .The copolymer compositions and monomer conversion were determined from the 1 H-NMR spectra by comparing the relative intensities of the peaks originating from the monomers with the resonance peaks from the polymers (d LLA ¼ 5.01 ppm, d PLLA ¼ 5.17 ppm, d 3CL ¼ 4.22 ppm, d P3CL ¼ 4.05 ppm, d aBrgBL ¼ 4.55 ppm, d P(aBrgBL-r-3CL) ¼ 4.34 ppm, and d P(aBrgBL-r-LLA) ¼ 4.43 ppm).The 13 C-NMR spectrum was used to qualitatively determine the macromolecular architecture.For the equations used to calculate the composition and conversion of the different monomers, see ESI. † Gel Permeation Chromatography (GPC).GPC was used to determine the number-average molecular weights (M n ) and dispersity indices (DIs) of the polymers during and aer polymerization using a Verotech PL-GPC 50 Plus equipped with a PL-RI detector and two PLgel 5 mm MIXED-D columns, 300 Â 7.5 mm (Varian, Santa Clara).Samples were injected with a PL-AS RT autosampler (Polymer Laboratories), and chloroform was used as the mobile phase at a ow rate of 1 mL min À1 at 30 C with toluene as an internal standard.The calibration was set using polystyrene standards with a narrow molecular weight distribution ranging from 160-371 000 g mol À1 .

Results and discussion
Our aim was to establish a-bromo-g-butyrolactone (aBrgBL) as a platform monomer for the synthesis of functional aliphatic polyesters.The hypothesis was that aBrgBL, an easily accessible monomer, will inherently act as a bridge between ring-opening polymerization (ROP) and graing by single electron transfer living radical polymerization (SET-LRP) (Scheme 1).It was also anticipated that aBrgBL, when copolymerized by ring-opening polymerization, would form isolated sites along the copolymer chain.Although aBrgBL was shown to possess all of these features, it has, to our knowledge, previously been neglected in this context, and it has been shown that the unopened aBrgBL by itself can act as a radical initiator for ATRP. 32To fully elucidate the properties of aBrgBL, we concentrated on two different questions: how does aBrgBL copolymerize with commonly used lactones or lactides focusing on the kinetics and the formed macromolecular architecture, and how does it behave during a graing step via SET-LRP.

Graing of acrylates from the aliphatic polyester main chain
There is a great focus in polymer science on joining controlled radical polymerization with controlled ring-opening polymerization.][35] One method that holds vast potential for graing from aliphatic polyesters is SET-LRP.0][41][42] But the polarity of these solvent restricts the use of aliphatic polyesters such as poly(3caprolactone) (PCL) as a polymeric graing initiator due to its hydrophobicity.4][45][46][47][48] These solvents have been coined "universal solvents" for SET-LRP, as they open the possibility of using hydrophobic monomers, and in our case, open the possibility of using a hydrophobic pre-polymer as an initiator.
The copolymerization of a-bromo-g-butyrolactone (aBrgBL) with 3CL or LLA yielded a macroinitiator with active and available graing sites for SET-LRP.The graing of the different acrylic monomers, methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and butyl acrylate (nBuAc) from poly(3CL-r-aBrgBL), proceeded in a controlled manner with a linear relationship between the conversion and the molecular weight (Fig. 1).However, when using poly(LLA-r-aBrgBL) as a macroinitiator, the graing of MMA was accompanied by severe degradation of the main chain (Fig. 1).The exact nature of this degradation behavior is still under investigation.Most likely it is due to the fact that Me 6 -TREN can act as a transesterication catalyst during graing, which is further supported by the dispersity evolution of poly(3CL-r-aBrgBL) graed with both MMA and HEMA.All selected monomers were successfully graed onto the polymer backbone, thereby highlighting the versatility and ability of aBrgBL to act as a bridge between SET-LRP and ROP, for a wide range of monomers.
Elucidating the copolymerization behavior of aBrgBL with 3CL or LLA In the 1930s, Carothers et al. stated that "the g-lactones and other ve-membered cyclic esters show no tendency to polymerize, and no corresponding polymers are known".This was concluded aer gBL had been heated both in the presence and absence of catalyst for one year. 10Later, in the end of the 1990s, gBL was polymerized using Al(OiPr) 3 as a catalyst, forming short oligomers. 16,18Although oligomers were formed, the original statement is still valid to a large extent, and hence similar behavior is to be expected for aBrgBL.This was also veried in this work when no homopolymerization of aBrgBL was observed aer 20 h with Sn(Oct) 2 as a catalyst at 110 C.
The chemical structure of aBrgBL suggests that the most suitable ROP catalyst would be a coordination-insertion catalyst.This is based on the notion that any catalyst with a slightly basic character, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 4-(dimethylamino)pyridine (DMAP), could lead to an elimination of the Scheme 1 Synthetic outline for the formation of aliphatic polyester graft copolymers.First, ring-opening polymerization of a-bromo-gbutyrolactone with 3-caprolactone or L-lactide forms the macroinitiator, followed by the grafting of different acrylates via SET-LRP.
Fig. 1 The relationship of the monomer consumption with the molecular weight obtained from GPC of the SET-LRP grafted copolymers of poly(LLA-r-aBrgBL) and poly(3CL-r-aBrgBL).The polymerization reactions were conducted with a Me 6 -TREN/aBrgBL ratio of 2 and 10 cm of copper wire.

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bromine moiety at the a-position, hence removing the monomer's ability to further act as an initiator for SET-LRP.Therefore, Sn(Oct) 2 was chosen as a catalyst for the copolymerization of aBrgBL with 3CL or LLA.The change in the chemical shi of the a-proton for aBrgBL of the precipitated copolymers of aBrgBL and either 3CL or LLA revealed that the monomer had been incorporated into the polymer chain (Fig. 2a and b).The difference in the shi of the a-proton upon copolymerization of aBrgBL depends on which comonomer was used, i.e., 3CL or LLA, and the chemical shis of the a-proton of the aBrgBL unit were d P(aBrgBL-r-3CL) ¼ 4.34 ppm and d P(aBrgBL-r-LLA) ¼ 4.43 ppm when polymerized with 3CL and LLA, respectively.This also indicates that there is an absence of homosequences of aBrgBL along the polymer chain, which would lead to the appearance of a peak at the same chemical shi in both copolymers.

Kinetic features of the copolymerization of aBrgBL with 3CL or LLA
The kinetic features of the copolymerization of aBrgBL with either 3CL or LLA were examined using two main experiments: the amount of monomer incorporation with a varying feed ratio of aBrgBL and the monomer consumption at prolonged reaction times.Although the ve-membered lactone ring is considered easy to open, it is also easy to close.Therefore, the intuitive trend would be that the lower the ratio of aBrgBL to the comonomer, the higher the conversion of aBrgBL would be.In other words, the incorporation of the monomer is a matter of statistics.If there exist more reactive monomers in the vicinity of the newly ring-opened aBrgBL that can react with the active chain end, the probability of aBrgBL to be "locked-in" the polymerizing chain is increased.
To visualize how the conversion of aBrgBL is affected by the initial feed ratio, several reactions were conducted at a constant monomer-to-initiator ratio of the more reactive monomer, 3CL and LLA, [M]/[I] ¼ 400 or 200, where only the ratio of aBrgBL was varied.The notion of a "locking-in" methodology during copolymerization was based on the idea that the most probable addition of aBrgBL occurs at the chain end during the propagation of the chain and not through trans-esterication-based ROP.
The incorporated amount and conversion of aBrgBL during copolymerization with both 3CL and LLA follow the expected trends.That is, the higher the feed ratio of aBrgBL to the comonomer, the more units are incorporated into the main chain, and the lower is the monomer's total conversion (Fig. 3a  and b).The amount of aBrgBL was determined by 1 H-NMR spectroscopy and calculated using the difference in the chemical shis of the a-proton of the monomer and the formed polymer (Table 1).Composition of the copolymers of aBrgBL and 3CL or LLA as a function of varying feed ratios and monomer-to-initiator ratios.
It is possible to incorporate quite a high amount of aBrgBL into the copolymers (up to 12 mol%) (Table 1).This is, however, connected to a low total conversion of aBrgBL during copolymerization (Fig. 3a).The low conversion of aBrgBL would be considered a major drawback if it simply acted as a propertyaltering monomer, but because its main purpose is to act as an initiator for SET-LRP, the incorporated amount is more than enough.The limited degree of incorporation could even be considered an advantage, i.e., if the conversion of aBrgBL is high, the formation of homosequences is more likely.It has been shown that during the copolymerization of g-butyrolactone (gBL) and 3CL when the conversion exceeds 12%, the block sequences of gBL were formed. 13This result was in contrast to what had previously been shown, that is, the formation of isolated monomers even at conversions as high as 22%. 11Although there is some discrepancy in the numbers, it should be safe to conclude that if the conversion is below 12%, copolymers with isolated gBL units are formed.Hence, the polymerization behavior of gBL is used as a template for the anticipated polymerization behavior of aBrgBL.
During controlled polymerization, control over the dispersity of the formed polymers is of immense importance.For the performed copolymerization reactions, it is evident that when 3CL was used as a comonomer, higher dispersities (1.6-1.7) were attained, in contrast to the results for LLA (1.2-1.3).Possible explanations could be either that the aggregation state during bulk polymerization at 110 C is vital, PCL has a T m $ 60 C, PLLA has a T m $ 160 C, or that the difference in reactivity of 3CL and LLA results in a different dispersity (Table 1).To elucidate the mechanism of aBrgBL addition, we conducted kinetic experiments where the dependence of M n , D, monomer consumption, and the composition of aBrgBL and 3CL on time were determined (Fig. 4).The reaction conditions chosen were 110 C, 1 mol% Sn(Oct) 2 , [MCL]/[I] ¼ 400, and The number average molecular weight and dispersity of the aBrgBL and 3CL copolymers show a clear dependence on the  View Article Online polymerization time (Fig. 4a and b).During the initial stage of the copolymerization, the addition of aBrgBL shows a linear relationship to the conversion of 3CL.Hence, the addition of aBrgBL is dependent on having a more reactive monomer, in this case 3CL, acting as an end-capping monomer.If there is no more reactive monomer available in the reaction, aer the addition of aBrgBL to the propagating chain end, it will ringclose again and resume its monomeric form.When the more reactive monomer is fully consumed, the addition of aBrgBL stops, and the molecular weight decreases rapidly, together with an increase of the dispersity (Fig. 4a and b).The molecular weight behavior of the copolymers is consistent with what has been observed during the copolymerization of gBL and LLA, which was attributed to the occurrence of transesterication reactions. 23In contrast to what was found here, they did not observe any reduction in the amount of gBL in the copolymers with time.The reduction observed here is believed to be a consequence of aBrgBL being more easily transesteried than 3CL, resulting in chain ends of aBrgBL that, for thermodynamic reasons, ring-closes to produce the monomeric unit, thus reducing the amount of aBrgBL in the copolymer.

Architectural features of the copolymerization of aBrgBL with 3CL and LLA
A good way to describe copolymerization behavior is to use the system-specic reactivity ratios during copolymerization.To calculate these ratios, it is vital to keep the conversion low (oen below 35%) in order to obtain accurate values.The restricted ability of aBrgBL to homopolymerize will render a relatively true value of the reactivity ratios even at maximum conversions of the most reactive monomer (i.e., 3CL or LLA).Even so, all calculations were performed at a conversion below 20%.The reactivity ratios were calculated using the Fineman and Ross method, 49 where the reactivity ratios, r 1 ¼ k 11 /k 12 and r 2 ¼ k 22 /k 21 , are given as the ratios of the rate constants between the four different possible copolymerization reactions.The r 1 reactivity ratios of the copolymerization of aBrgBL with 3CL and LLA were determined to be 4.4 and 18.5, respectively, whereas the r 2 ratios were close to zero for both (Fig. 5a).This can mean two things: the non-existence of the aBrgBL chain-end addition to the monomeric form of aBrgBL, or the reactivity for the addition to 3CL is many times higher.If aBrgBL was able to    homopolymerize, the latter explanation would result in the formation of "diblock-like" copolymers.However, aBrgBL's inability to homopolymerize yields a solid conclusion of the macromolecular architecture based on the reactivity ratio r 2 , that is, isolated aBrgBL units are formed throughout the polymer main chain.Although the r 2 value for the copolymerization of aBrgBL and LLA does not rule out the formation of homosequences of aBrgBL, the deviation from zero is interpreted as a function of the line regression rather than the system itself.3CL situated at the propagating chain end has a higher reactivity towards aBrgBL during copolymerization than LLA.This is revealed by its r 1 value being almost four times smaller than that determined for LLA (Fig. 2 and Table 1).This is in line with what was anticipated for the copolymerization.The difference in reactivity of the propagating chain end originates from a primary hydroxyl being more reactive than a secondary hydroxyl, as shown by the difference in the homopolymerization and copolymerization of 3CL and LLA.The rate of homopolymerization of 3CL is much higher than that of PLLA in similar systems, although the ring strain is higher for LLA.During the copolymerization of LLA and 3CL, this results in a gradient copolymer, where LLA is predominant in the beginning and 3CL in the end. 50he existence of isolated units of aBrgBL along the main chain of the copolymers was further veried by 13 C-NMR spectroscopy.The carbonyl group is very sensitive to its neighboring units; 51 this makes it a valuable tool to verify the architectural features of copolymers.Isolated aBrgBL along the main chain of the 3CL copolymer was found as three distinct peaks of equal intensity (Fig. 4b).For copolymers with 3CL, there is oen assumed triplet behavior, meaning that each carbonyl group is affected by its two neighboring units. 18In summation, the copolymers synthesized are shown to be isolated aBrgBL units along the chain, providing excellent sites for subsequent SET-LRP.

Conclusions
a-Bromo-g-butyrolactone (aBrgBL) as a comonomer with 3caprolactone (3CL) or L-lactide (LLA) produces copolymers (poly(3CL-r-aBrgBL) or poly(3CL-r-aBrgBL), respectively) with active and available graing sites for SET-LRP.The different graed acrylates range from hydrophobic n-butyl acrylate and methyl methacrylate to hydrophilic 2-hydroxyethyl methacrylate.These copolymerization reactions were accomplished with the aid of a uorinated alcohol, 2,2,2-triuoroethanol, which acts as a universal solvent for both the hydrophobic macroinitiator and the graing monomers.The graing via SET-LRP from poly(3CL-r-aBrgBL) for all acrylates proceeded in a controlled manner with a linear relationship between the conversion and the molecular weight.This successfully shows that aBrgBL provides a versatile and simple route to merge the two polymerization methodologies, ROP and SET-LRP.
The copolymerization of aBrgBL together with two of the most commonly used cyclic ester monomers, 3-CL, and LLA, proceeds with high control, and a linear relationship between the feed ratio of aBrgBL and its composition in the copolymer is observed.During the copolymerization, the consumption of 3CL and aBrgBL is linearly related to each other, although the rate is lower for aBrgBL.When the most active comonomer, 3CL, is fully consumed, the conversion of aBrgBL stops.We can therefore conclude that the addition of aBrgBL occurs mainly at the active chain end rather than as an effect of trans-esterication.Its inherent inability to form homo-sequences under ordinary polymerization conditions was observed both in the 13 C-NMR spectra, which only displayed peaks originating from isolated aBrgBL units along the polymer chain, and from the calculated reactivity ratios.
We believe that aBrgBL inherently holds all the prerequisites to act as a platform monomer for the synthesis of functional aliphatic polyesters, i.e., it is inexpensive, available, and able to form isolated graing sites along the polymer chain.The incorporation of isolated aBrgBL is a feature that makes this class of copolymers unique and is considered to provide a route to the "perfect gra copolymer" with a degradable backbone.

Fig. 4 (
Fig. 4 (a and b) Conversion in the copolymerization of aBrgBL and 3CL as a function of time (a) and the effects on the number molecular weight (M n ) and dispersity of the copolymer (b).

Fig. 5
Fig.5(a and b) The Fineman and Ross method was used to calculate the reactivity ratios as shown by the linear fit (a).The 13 C-NMR spectrum shows the isolated units of a-bromo-g-butyrolactone (aBrgBL) along the polymer chain during copolymerization with 3-caprolactone (3CL) (b).

Table 1
Composition of the copolymers of aBrgBL and 3CL or LLA as a function of varying feed ratios and monomer-to-initiator ratios