TheT1r13C spin–lattice relaxation time of helical polyguanidines

A.R. Lima,* , J.R. Stewartb, B.M. Novakc

aDepartment of Physics, Jeonju University, Jeonju 560-759, South KoreabDepartment of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA

cDepartment of Chemistry, North Carolina State University, Raleigh, NC 27695, USA

Received 8 December 1998; received in revised form 28 December 1998; accepted 8 January 1999 by A. Pinczuk

Abstract

The solid state dynamics of three helical polyguanidines differing only in their stereochemistry was investigated by13C CP/MAS NMR. From these studies, the structures of the polyguanidines were confirmed, and the13C spin–lattice relaxation timesin the rotating frame were measured. The relaxation times of all the polyguanidines indicated that they undergo fast motions, i.e.motions on the fast side of theT1r minimum. The main chain carbon of polyguanidineI -(R/S), with equal amounts of (R) and (S)chiral side chains, has higher activation energy, 10.7 kJ/mol, than the analogous polymers with enantiomerically pure sidechains (I -(R) andI -(S)), 5.1 kJ/mol.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:A. Insulators; A. Polymers, elastomers and plastics; B. Chemical synthesis; E. Nuclear resonances

1. Introduction

Of the synthetic polymers that exhibit somedegree of conformational order, the helical subclass(e.g., polyisocyanates [1] and polyguanidines [2,3])has been of particular interest. These polymers arebest modeled as single macromolecular chains thatcan possess regions of right- and left-handed screwsenses separated by helix reversals (i.e. chainconformations that change the signs of the dihedralangles defining the helical sense) [4]. The helicalconformation is chiral with the left- and right-handed senses having an enantiomeric relationship.With chiral side chains, the two helical senses nowbecome diastereotopic and one will be thermodyna-mically favored (lower free energy). These chiralityissues can have important and sometimes surprising

consequences in terms of both solution and solid stateproperties [2,3].

In our studies with polyguanidines, we have foundthat when homo-chiral monomers are polymerized,their polymers adopt kinetically-controlled conforma-tions that have been characterized as helical chainspopulated by helical reversals [5]. We have foundthat these kinetically-controlled states can beconverted to their thermodynamics states throughannealing the polymers in solution. Interestingly, wehave also discovered that this annealing process willoccur, albeit slowly, in the solid state at roomtemperature. Prompted by this discovery, we becameinterested in the solid state molecular motions thesechains were capable of undergoing.

In this article, we verified the structure of poly (N-methyl-N0-(1-phenylethyl) carbodiimide) using13Ccross-polarization magic angle spinning (CP/MAS)NMR. Also, the 13C spin–lattice relaxation times inthe rotating frame were measured as a function of

Solid State Communications 110 (1999) 23–28

0038-1098/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0038-1098(99)00038-1

PERGAMON

* Corresponding author. Fax:1 82-652-220-2362.E-mail: aeranlim@hanmail.net (A.R. Lim)

temperature using CP/MAS NMR. From these results,we discuss the mobility, the correlation time, and theactivation energy for each carbons of the polyguani-dines as a function of the chirality of the side chains.

2. Experimental

2.1. Preparation of materials

All starting materials were obtained from commer-cial suppliers and used as received. The carbodiimidemonomers were prepared with slight modifications toliterature procedures [6]. Bischloro-h 5-cyclopenta-dieny-dimethylamido titanium (IV) was prepared bya modification of the procedure of Patten [7] andGoodwin [8].

The three polyguanidines were prepared using thesame method illustrated forI -(R/S). In a dry box underAr atmosphere, a reaction vessel was charged with amagnetic stir bar,N-methyl-N0-(1-phenylethyl) carbo-diimides (10.9 g, 68.1 mmol), and 0.5 ml of apreviously prepared 0.15 M toluene solution ofbischloro-h 5-cyclopentadienyl-dimethylamido tita-nium (IV) (17.3 mg, 0.75 mmol). The vessel wasremoved from the dry box, sealed under vacuumand placed over a magnetic stir plate at room tempera-ture. After three days the polymerization wasquenched by the addition of wet toluene and theresulting polymer purified by precipitation frommethanol and lyophilization from benzene.

2.2. Solid state NMR spectroscopy

Solid state NMR experiments were performedusing a Varian 300 NMR spectrometer. CP/MAS13C NMR experiments were performed at Larmor

frequency of 75.46 MHz. The samples were placedin the 7 mm CP/MAS probe as powders. The magicangle spinning rate was 3 kHz to minimize spinningsideband overlap. Thep /2 pulse time was 5ms, corre-sponding to a spin-locking field strength of 50 kHz.13C T1r measurement was made by applying a13Cspin-locking pulse after a 1 ms CP preparation period.The decay of the13C magnetization in the spin-lock-ing field was followed for spin-locking times of up to30 ms.

3. Results and analysis

The polyguanidines synthesized for this studydiffered from one another in the chirality of theirside chains: racemic 1-phenylethyl,I -(R/S); (R)-1-phenylethyl,I -(R); and (S)-1-phenylethyl,I -(S) (Fig.1). These side chains were selected because of theavailability of both enantiomerically pure forms, andbecause we have previously shown that the racemicand enantiomerically pure polymers have differentsolution and solid state properties.

Structural analysis of the polyguanidines werecarried out by NMR spectroscopy. Fig. 2 shows thesolid state13C CP/MAS NMR spectrum ofI -(R/S) atroom temperature. The13C CP/MAS NMR spectrumof the polyguanidinesI -(R/S) sample consisted of fivesignals at chemical shifts ofd � 148.30, 127.84,57.82, 34.58, and 26.43 ppm at room temperature.The five peaks of polyguanidinesI -(R/S) are assignedin Fig. 2. The spinning sidebands are marked with anasterisk. The most intense signal is the carbons in thearomatic ring, and the main chain of 148 ppm reso-nance peak has a relatively small intensity. Thechemical shifts for polyguanidinesI -(R/S) weremeasured at various temperatures, and were foundto be nearly independent of this variable. In the caseof polymersI -(R) andI -(S), 13C CP/MAS NMR spec-trum is similar to those for polyguanidinesI -(R/S) asshown in Fig. 2. The temperature dependence of thechemical shifts for polyguanidinesI -(R) andI -(S) arealso similar to that of polyguanidinesI -(R/S), andindependent of temperature.

The spin–lattice relaxation time in rotating frame,T1r , for each carbon of the polyguanidines weremeasured in the temperature range 25–658C withvariable spin-lock on the carbon channel following

A.R. Lim et al. / Solid State Communications 110 (1999) 23–2824

Fig. 1. Chemical structures of the three different helical polygua-nidines:I -(R/S), I -(R), andI -(S).

cross-polarization. TheT1r values were measuredfrom the decay of magnetization during a13C spin-locking pulse after preparation of magnetization bycross-polarization [9]. All the traces were fitted bythe following exponential function: [10,11]

Mz�t� � M0exp 2t

T1r

!�1�

whereMz andM0 are the loss of the magnetization andthe total nuclear magnetization of13C in thermal equi-librium, respectively.

Fig. 3(a) and (b) shows theT1r for the C-1 and C-5carbons as a function of temperature in case of poly-guanidinesI -(R/S), I -(R), andI -(S), respectively. TheT1r

13C spin–lattice relaxation time graduallyincreases with increasing temperature. The degree ofthe change for all the carbons with respect to thetemperature are similar, and theT1r , correspondingto four peaks except C-5 in polymersI -(R/S) haveshorter relaxation times thanI -(R) and I -(S). Like-wise, the relaxation times of C-1, C-2, and C-4 slowlyincrease with increasing temperature, while therelaxation time of C-3 remains nearly constant withincreasing temperature. In case of C-5, the spin–lattice relaxation times drastically increase withincreasing temperature as shown in Fig. 3(b). TheT1r of the C-5 is 2–6 times longer than C-1, C-2, C-3, and C-4 at room temperature, because of the fact

that dipolar relaxation is more efficient when a carbonhas bound protons [12–14]. The relaxation times ofall the polyguanidines undergo motions on the fastside of theT1r minimum, and all of these carbonswere determined to undergo fast motions on thehigh-frequency side of theT1r minimum, under fastmotion conditionsv1t c p vt c p 1 [11,15]. On thefast side of theT1r minimum, an increase inT1r movesto smaller values oft c. Therefore, the increase inT1r

with temperature represents an increase in mobility athigher temperatures for these carbons [15].

To study molecular motions,T1r values must berelated to corresponding values of the rotational corre-lation times,t c [16]. For the spin–lattice relaxationtime in the rotating frame, the experimental value ofT1r can be expressed in terms of a correlation timet c

for the molecular motions by the well-known BPP(Bloembergen–Purcell–Pound) function [17–19].The values oft c calculated from BPP function arelisted in Table 1. All of the carbons of polyguanidinesI -(R/S) have longer rotational correlation times thanthe corresponding carbons in the pure enantiomeranalogs. Thet c of C-5 is 150–400 times longer thanthat for the other carbons.

We know that the dominant mechanism of therelaxation is related to the changes in motion. Thetheoretical temperature dependence of the rate ofrotational motions is a simple Arrhenius expression[20]

tc � t0expEa

RT

� ��2�

whereEa is the activation energy for the molecularmotions andR is the molar gas constant [21]. Thus,a plot of the natural logarithm of the correlation timeas a function of the inverse temperature is linear witha slope that is proportional to the activation energy formotion. Fig. 4 shows thet c of the C-1 and C-5 (mainchain) for these samples. The activation energies forthese carbons, determined via fits of Eq. (2) are listedin Table 2. All carbons of polyguanidinesI -(R/S) havelarger activation energies than the correspondingcarbons in the pure enantiomer analogues. The C-5of polyguanidinesI -(R/S) is 2 times larger thanthose measured for the same C-5 inI -(R) or I -(S).The activation energies of the polyguanidinesI -(R)and I -(S) are nearly same for the correspondingcarbons on these two polymers, and are consistent

A.R. Lim et al. / Solid State Communications 110 (1999) 23–28 25

Fig. 2. Solid state13C CP/MAS NMR spectrum of polyguanidinesI -(R/S) at room temperature.

within experimental error. From these results, themotion of the main chain in the racemic material ismore rigid. Also, the activation energy for C-1 and C-2 in polyguanidinesI -(R/S) is higher than those ofpolyguanidinesI -(R) and I -(S). Note that the C-3 inall the polyguanidines have much lower activationenergies, which indicates a low degree of rigidityfor this carbon.

4. Discussion

Our interest in polyguanidines stems from theirmacromolecular chirality and the influence thereofon such facets as their stiff-chain properties, confor-mational stability and dynamics, as well as their ther-mal properties. The enantiomeric polymersI -(R) andI -(S) have a diastereotopic relationship withI -(R/S).This stereochemical difference manifests itself in theirthermal properties (I -(R/S)) decomposes approxi-mately 408C belowI -(R) or I -(S), and in their solutioncharacteristics. Concerning the latter, static scalingchains, exponents of 0.88 and 1.0 have been measuredfor I -(R/S) and I -(R), respectively. Both are stiffchains, but at identical molecular weights,I -(R/S)has a smaller radius of gyration or a more compactstructure. The racemic chains are thought to adoptdisordered conformations vis-a-vis the regular helixadopted by I -(R). Of particular interest is thedynamics that the polyguanidines can undergo in thesolid state, as this relates to, or places limits onthe potential chiral applications of these materials.

The 13C CP/MAS NMR results presented here indi-cate substantial differences in the solid state dynamicsexpressed by these diastereotopic macromolecules.Each of the three helical polyguanidines were studiedusing the13C CP/MAS NMR. The chemical shifts inall cases were consistent with the structures shown inFig. 1. Unfortunately, because of peak broadening, thedifferences of the various helical conformationscannot be elucidated using this data. Dynamic infor-mation, however, could be obtained by measuring thespin–lattice relaxation times for each carbon using13C CP/MAS NMR. In all the polyguanidines, theT1r relaxations arising from the non H-bearingcarbons are slower than those of the H-bearing

A.R. Lim et al. / Solid State Communications 110 (1999) 23–2826

Fig. 3. Temperature dependence of13C spin–lattice relaxation timein the rotating frame,T1r , for polyguanidinesI -(R/S), I -(R), andI -(S): (a) C-1; (b) C-5.

Table 1The correlation time calculated from the value ofT1r for the poly-guanidines at room temperature

Assignment t c ( × 10210 s)I -(R/S) I -(R) I -(S)

C-1 5.29 4.50 4.56C-2 10.70 9.22 9.31C-3 8.39 6.93 7.09C-4 19.31 14.07 14.11C-5 1460.00 1613.00 1648.00

carbons. This difference results from the dependenceof the relaxation on the inverse sixth power of theinternuclear separation. The H-bearing carbonspossess short C–H bond lengths, and therefore exhibitan efficient or fast relaxation. For non H-bearingcarbons, the dipolar relaxation mechanism is less effi-cient because the internuclear distances to othernucleii are larger.

The 13C spin–lattice relaxation times,T1r , in therotating frame showed an increased mobility at highertemperatures. The13C spin–lattice relaxation times ofall the polyquanidines undergo fast motions, i.e.motions on the fast side of theT1r minimum. There-fore, the increase inT1r represents an increase inmobility at higher temperatures.

The activation energy is a quantitative measure ofrigidity, and can be obtained for each carbons from thecorrelation times as a function of the temperature. Thebackbone of polyguanidinesI -(R/S) containing equal(R) and (S) chiral side chains has a higher activationenergy, 10.7 kJ/mol, than the analogous polymerswith enantiomerically pure side chain (I -(R) and I -(S)), 5.1 kJ/mol. It is also worth noting that the activa-tion energies of the C-1 and C-2 in polyguanidinesI -(R/S) are distinctly different from those in polyguani-dines I -(R) and I -(S). In this case, the activationenergy for polyguanidinesI -(R/S) is nearly twotimes in magnitude.

The dynamic measurements obtained in this studyshow clear differences between the racemic and enan-tiomerically pure polyguanidines. From a relativeperspective, the backbone mobility ofI -(R/S) ishindered. We speculate that this is due to unfavorablesteric interactions that prevent a smooth precession ofthe chain around the helix axis. This hypothesis iscurrently under investigation.

5. Conclusions

Using side chain chirality as the variable, solid statedynamics of a series of polyguanidines has beenmeasured using13C CP/MAS NMR techniques.Spin–lattice relaxation times in the rotating framewere measured for all of the carbons and from this,

A.R. Lim et al. / Solid State Communications 110 (1999) 23–28 27

Fig. 4. Arrhenius plot of the natural logarithm of the correlationtimes as a function of the inverse temperature for polyguanidinesI -(R/S), I -(R), andI -(S): (a) C-1; (b) C-5.

Table 2Activation energies obtained from Arrhenius plots of the correlationtime as a function of the reciprocal of the temperature

Assignment Ea (kJ/mol)I -(R/S) I -(R) I -(S)

C-1 4.17^ 0.02 3.20̂ 0.05 3.29̂ 0.05C-2 2.26^ 0.08 2.08̂ 0.13 1.88̂ 0.13C-3 0.74^ 0.01 0.76̂ 0.02 0.73̂ 0.02C-4 4.51^ 0.03 4.45̂ 0.08 4.32̂ 0.08C-5 10.71^ 0.10 5.21̂ 0.15 4.94̂ 0.15

the rotational correlation times were calculated. Finally,activation energies were calculated for the variouscarbons. The conclusion arrived at is that the chiralityof the side chain does influence the backbone dynamics.The main chain carbon ofI-(R/S) has a higher activationenergy than the analogous polymers with enantiomeri-cally pure side chains ((I-(R) and I-(S)). As expected,the activation energy of the polyguanidinesI -(R) andI -(S) are the same within experimental error for thecorresponding carbons on these two polymers. This isconsistent with their mirror image relationship (i.e.,they are enantiomers of one another). Current studiesdesigned to elucidate both the nature of these motionsand the underlying structural characteristics control-ling the same are now in progress.

Acknowledgements

This work was supported by the Korea Science andEngineering Foundation through (A.R. Lim) theResearch Center for Dielectric and Advanced MatterPhysics (RCDAMP) at Pusan National University(1997-2000). Bruce M. Novak would like to acknowl-edge the Materials Research Science and EngineeringCenter at the University of Massachusetts and theFAA for financial support.

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